Cement board installation is not something that is incredibly difficult.
I think most people worry about how to cut it and once they have a plan for that then they think it’ll be a breeze.
But I’ve seen these screw-ups for several years now. It’s the same ones over and over.
Estimated reading time: 7 minutes
Which products are we talking about?
Mainly, the products I’m talking about are considered to be cement backer boards for tile installation.
These come in two categories: cement board and fiber cement board.
Examples of cement board are: Wonderboard, Durock, and Permabase
Examples of fiber cement board: Hardibacker and Fiberock,
Let me throw in one disclaimer here: These are general guidelines. Each manufacturer will have their own instructions for their own products and they will vary a little bit from each other.
So the moral of this story is to read the instructions and not simply this post.
Finally, if cement backer board isn’t your thing, be sure to investigate other tile underlayment options.
Messing up cement board installation is not just limited to these 5 ways
Now there are more ways to screw things up than just these five things. In fact, this is already assuming that your wood subfloor meets the minimum requirements.
Hint: cement board should never be counted on to stiffen up your floor. The floor needs to meet minimum requirements for tile BEFORE any tile backer board is installed.
However, these are the five that stood out in my unofficial memory as being the most common.
Here they are:
1. No Thinset underneath the backer board
Just the other day, I saw a video featuring someone fastening Hardibacker directly to the subfloor with nothing underneath it. I scrolled down to look at the comments and they did not disappoint. What were his replies, you ask?
“The thinset goes ON TOP of the cement board to stick the tiles!”
A simple perusing of the instructions would show otherwise. But most people don’t look at them- not even the pictures.
Also, I know some are wondering but, no, Liquid Nails does not count as a thinset replacement. We have to remember that this isn’t the same as installing plywood. Cement board is a different product and it needs to be installed in its own way.
Additionally, screws don’t take the place of thinset either. I talk more about screws in #3.
Which type of thinset do I need for underneath the cement board installation?
Here’s the bit of good news: it doesn’t really matter which brand or type of thinset mortar you purchase. I know the manufacturers will want you to use their own brand and may even specify a type. If so, then you should purchase what they require.
But the system works by having a bed of mortar underneath holding the board up and fasteners from above holding the board down. You don’t need any fancy mortar to accomplish that task and many will use an unmodified mortar in this situation.
Additionally, when mixing thinset for installing backer board, it doesn’t hurt to add a bit more water to make it a little looser.
2. Not Staggering the Seams
Now, this is an area that the plywood installers will be familiar with. You don’t want four cement board corners to all come together in one intersection.
You get around this by staggering the boards during installation. One nice thing about tile backer board is that it doesn’t matter which way that you run the sheets.
You can run them in one direction or another but as long as the corners don’t all come together it’s OK. Additionally, you don’t want the cement board seams to line up with the subfloor seams.
Those are the two things to watch out for with cement board installation.
Most cement board manufacturers require galvanized roofing nails or cement board screws.
But this doesn’t stop people from trying to use ring shank nails, drywall screws, or who knows what else?
The problem is that those fasteners don’t have the holding power that cement board screws and roofing nails have.
Additionally, people also mess up fasteners by not using enough of them and driving them too deep.
Typically, it’s every 8 inches throughout but there is some variation between brands on that. Some backer boards will have the fastener locations printed on the board.
4. Not using Mesh Tape on the Seams of your Cement Board Installation
This is more of a little-known fact but most cement board manufacturers want fiberglass alkaline-resistant mesh tape embedded in thinset mortar over the backer board seams.
I remember a time when this wasn’t a requirement but it has been required for the last several years.
5. Installing cement board over a concrete subfloor
If you have a concrete subfloor then you should not be contemplating installing cement board over it.
I know it seems like a smooth surface and it seems like it may fix some of the issues with the concrete floor but don’t do it.
Not only is thinset a critical component underneath cement board but the other critical component is the fasteners. And you won’t have the fasteners if you are trying to install it over concrete.
On the bright side, there are some options for concrete subfloors. One of which is self-leveling underlayment and other floor flattening products.
The other is foam backer board which is OK to install over concrete. This can be beneficial if you are wanting to insulate concrete from an electric floor heating system above it.
Finally, uncoupling membranes, like Ditra, can be installed over concrete. In fact, uncoupling membranes have become popular replacements for CBU in the cement board vs Ditra debate.
I know, there are more ways to screw up cement board installation than just these five.
I could list wrong trowel size, thinset mixed too thick, thinset allowed to skin over, and others.
You tell me in the comments: which ones am I missing? Which mistakes do you see?
FAQ about cement backer board underlayment
Most commonly, cement board underlayment comes in two sizes: 1/4 and 1/2 inch. While either size can be used on a floor the 1/2 inch is the only size recommended for walls.
Further, 1/4 inch typically works best for most flooring transitions. In fact, the 1/4 inch thickness works particularly well when transitioning from hardwood to tile.
1/4 inch thick cement board is typically the thinnest size that you will find. However, foam backer boards can be found as thin as 1/8 inch in some brands.
You definitely do not want to install cement board over a concrete subfloor. However, there are alternative underlayments that you can use such as an uncoupling membrane or perhaps a foam backer board.
You don’t need to use a backer board to install tile on a floor but you definitely need to use a tile underlayment is you are installing tile over a wood subfloor.
Cement board is an appropriate underlayment for tile being installed over a wood subfloor.
You are not required to waterproof cement board installed over a floor unless it is considered a wet area. Bathroom floors outside of showers are not considered wet areas.
Hardiebacker, and other fiber cement boards, are appropriate underlayments for tile floors over wood subfloors.
Julie Chambers says
Hi there I am tiling my mudroom and I have a built in bench with drawers leaving only about an 8 inch gap between the bottom shelf and the floor. The space goes back to the wall about 20 inches. I am trying put hardibacker in and realized there is absolutely no way to get my drill back 20in with only 8in clearance. There will never be foot traffic back there, it won’t even be seen. To do it right tile does need to go all the way back. Can I just use thinset to secure the hardibacker and no screws?
If that area gets tile then I would try to get some sort of fasteners back there. You might try heavily tilting the drill and driving a couple of screws in at severe angles. It’s not really the right way but it’s about the only way you’ll get anything in there. Otherwise, just do the best you can. That’s all that you can do.
Doesn’t a moisture barrier need to be between the cement backboard and floor. The cement will be fine with water but wouldn’t the wood underneath rot? If so, how do you put thin set mortar mix down with the moisture barrier in place?
No, a moisture barrier isn’t necessary as it’s not expected that floor would see the amount of water that would warrant a barrier or some sort. Typically, that’s only in showers or exterior applications.
However, you could apply a waterproof membrane over the cement board and use a waterproof seam tape on the seams andaround the edges if you were trying to protect it. But unless there is a way to drain water then the water would simply run out the door.
I am doing the tile for my master bathroom which is about 18ft wide and 12ft deep. After mixing the mortar I couldn’t find my 1/4 Inch trowel so I just used my 1/2 inch for 3 of the backer boards. I only got 3 backer boards down with one bag of mortar, then I found my 1/4 trowel. Could I use the 1/4 inch for the rest of the backer board or would there be too much elevation change between the backer on the 1/2 inches troweled thinset? Should I do the rest with 1/2 inch, or should I rip up what I already did (which would be a pain). Thanks
You’ll run into an elevation change by switching trowels. I’d probably just keep going with the bigger trowel at this point.
I have a wood pine floor can I put tyvek on it then put leveling cement on top of it before I put the hardibacker down?
Negative. I’m assuming the pine floor is a finished floor? If so, it’ll have to come out and you need to get down to the subfloor. From there you can use Ardex Liquid Backer Board or Custom Building Products LevelQuik fiber-reinforced self-leveling. Or simply install cement board or hardibacker.
Frank i Stannard says
I am looking for the best way to cover and build up over a membrane waterproof deck roof. So I have a waterproof barrier in place as long as I dont put screws or nails through it.
It has no finish. So it is ugly black and shows the over lays where the membrane was welded together. I does not leak. So I am looking for a system that I can go over the standard rubber based waterproof membrane. I want it to look like a nice finish. I considered a mortor cementuous mixture but it is 900sq. I thought cement board may be a good answer and then but an epoxy slurry on top. I would also like to ad some slope/drop to in…the roof is 2 sections that are 12ft by 38ft. I do not like the raised decking systems and they are expensive. I would prefer to avoid a tile finish but will use that if that is my only alternative. It is under an aluminum awning/roof. So thinking out of the box…got any ideas?
From a tile perspective, if you don’t want to put any screws in it then the only way is a mud bed over the top. Also, drainage should be built in below the waterproofing so the water runs the right way at the waterproofing level.
Outside decks are nothing to take lightly especially if they are over a finished space. I recommend picking some sort of system and following the instructions to a “T”.
I laid my cement board on top of plywood using a 1/2” trowel instead of a 1/4” trowel. My thought process at the time was I used a 1/2” trowel for 12”x24” tile so it made sense I’d need a larger trowel for cement board. I didn’t give it a second thought until was researching a separate project. Will this cause major issues? Or is it more that I probably put more weight and height on the floor than necessary?
You’re definitely going to have more height and it could result in some lumpiness of the cement board.
John O. says
I’m working on a small 1/2 bath. I’ve removed the old wood floor and down to the plywood subfloor. There are some high/low spots so i’ve purchased a self-leveler material. I’m not sure if i’m supposed to then apply a thin set over the self-leveler (after it has cured) then the concrete board, then the tile?
Good question and I’m working on a self-leveling post right now.
Hopefully, you’ve purchased the correct self-leveler. There are a very limited number of them that are OK for going over a wood subfloor with no additional reinforcement. So hopefully, you’ll get one of those or install either metal or plastic diamond lath over the floor prior to the leveling product.
Ardex Liquid Backer Board is an example of a self-leveler that doesn’t need reinforcement. Custom’s Fiber-reinforced Levelquick is another.
Secondly, once the self-leveler is dry you can adhere tile directly to it. You shouldn’t add any backer board over the top of it. You could, however, install backer board first and then self-level on top of that. No matter what you do, don’t forget the primer.
Joe Hardee says
I have what seems to be a “unique” situation and have shared very detailed information about my failed exterior raised Tile deck 2nd story(17 years old but leaked from the start) that is being replaced and both installers and companies like Protecto Wrap / Schluter / Merkrete / Ardex / Laticrete… (both the area sales rep and their engineering support staff – some of which have served on the TCNA in previous life) have all pointed me in different directions and given me different reasoning on why each one of them is wright. They were provided with pictures, drawings, and technical details so there really should not have been anything to assume or have to factor in for the proverbial C.Y.A.
While I can’t upload pics i can describe in detail.
First, the house is located in the ATL GA area (we do see some freezing but not extreme and not for long periods). Under the deck is a 6″ thick concrete patio with footers. The deck is a covered deck with an open straight gable roof. The Deck is enclosed on 3 sides by the brick house and only open on the 4th. that side faces west. Both the bottom (under the deck ) and top (on the deck) have a brick column and archway system supporting the deck structure. All banding and ledgers rest and are tied to the brick (poured wall) with large carriage bolts 24″.
The span and framing of the deck – width (house to edge of brick off deck) 18′ middle support is a 24″ triple laminated beam. floor joist are 2×12 on 16″ O.C. The 2×12 in joist hangers are about 8′- 7″ on each side of the beam (accounting for the brick ledge and beam width).
The only time water ever gets on the deck is in a blowing rain and then only the first 3′ or 4′. The reason it failed was: 1) the finished deck sloped toward the house not away 2) bad lipage where tile pocketed water in patches across the deck 3) nothing under the durarock but 1/2 plywood – rusty plain nails – did not tape – did not stagger joints 4) regular sanded grout with no sealing
Leaked profusely for years until I contacted a company called Stone Creations who said they could remedy the situation by creating a slope on top of the tile then adding their product a waterproof limestone that they would make to look like the tile (pattern and color). wound up with 3,800 lbs of concrete and another 700 lbs of their product that absorbed water, swelled, then cracked… Jack-hammered everything off down to the plywood that has de-laminated and deteriorated over 17 years of water log.
Now having the plywood removed, proper slope angel shot with survey laser and sister floor joist installed next to existing joist. Then 2 layers of 3/4″ marine grade plywood installed (staggered joints and laid opposite). Glued and screwed.
NOW comes the knock down drag out between tile installers / TCNA / and Manufacturers… Here is what I am getting:
– You have to have 1/4″ per foot slope per TCNA (on this deck that would be 4-1/2″ – look awfully – and you would feel the tilt…. Several Tile engineers and TCNA member have brought the fact that 1/4″ per foot was established because” …when a tiled surface such as a deck or patio is open to the elements above it serves as a roof… the minimal acceptable pitch or slope for a roof according to the N.R.C (National Roofing Council) which is also the minimal set forth across all U.S. building codes and supported by manufactures of roofing materials. The TCNA accepted the 1/4″ per foot under the grounds that all exterior installed tile would be considered for drainage purposes a “roof”.
– Now I have a fully covered roof with gutter system over my deck (just like the rest of my house) and it is enclosed on 3 sides. I have been told that while I might require 1/4″ slope based on the type of system I install (example those that allow water to pass through like Schluter) other solutions that keep both water and moisture from ever penetrating the surface would require very little slope 1/8″ or even less). I would plan on having 1 3/4″ to 2″ of slope.
– The ceiling below the deck is now finished for an outdoor kitchen under the above tile deck. Pass through systems like schluter all require air vents in every floor joist cavity at both ends if your are finishing the ceiling below. I deck that is 24 foot long with a beam in the middle and 16″ OC joist = 18 pockets on each side of the beam = 36 and vent on each end = That would require 72 vents placed in the ceiling below… that now has finished 4″ wide plank cypress T&G installed on it.
– Some say use cement board others like Protectowrap (with protectodeck) engineering stated to me after reviewing my application with 1 1/2″ of plywood to roll their primer then install the protectodeck membrane on top of 2 courses of plywood (or one course of plywood and one course of cement board) then tape seams and edges with their product, and apply their elastomer liquid membrane
– Then follow TCNA ej 171 an make sure to have soft joints for expansion every “X” feet and where the tile deadend’s into a brick wall or column cut the tile 1/4″ short for soft joint. Best for soft joint is Dowsil 795 or 791 but is limited in colors.
– they also recommended Ladicrete custom / thinset 258 / fusion custom… as most of those offer a waterproof thinset (often referred to as modified or fortified) with both an epoxy grout and industrial silicone rubberized caulk to match the grout.
– of course use a quality porcelain tile < .5% porosity and high friction for low slip and there will be a bullnose (1-1/2" high like a backsplash) where the floor tile comes to the brick to the prevent weeping from the brick (the brick is already flashed).
– Then I have other manufactures that are telling that doing that will lead to failure.
– Please give me your thoughts on the matter… I can send picture if your like
It sounds like you’re going about it the correct way. That is you are getting all the parties involved and coming up with a solution that is geared specifically towards your outside deck with a finished ceiling underneath.
I don’t have the ability to know more than the installers and company reps who have seen your application firsthand and know which of their products to specify for it. But here’s what I know:
I have a little bit of experience with the Protecto Wrap product and it’s not one that I would use again for this type of situation. It’s a peel & stick and you only get one shot at it. While it’s simple enough to get the product installed correctly on straight runs with 2-3 installers, it’s much more difficult to do the folds and wraps. And the folds and wraps matter.
Now, maybe an experienced crew can overcome those hurdles but, for me, I would look into a different solution.
Additionally, it’s been a few years since I’ve take any training at Laticrete but I seem to remember that they don’t have a product that they will guarantee to be the primary waterproofing membrane. So if it leaks, it’s not on them since it’s not their membrane. Personally, I would want one manufacturer to specify the whole system.
Schluter has a system but they don’t publish it anymore as they want to specify for your specific project. But if they write something up for your project and it’s a complete system then I would definitely consider their products for the install.
I don’t know much about Ardex and Merkrete when it comes to their exterior systems but, for what it’s worth, I’ve been impressed with most of Ardex’s products that I’ve tried and they have great support in my local area.
Additionally, I would take a look at Noble Deck EXT if you can get that product and a rep in your area to come out and specify it for your exact situation.
But, to sum up, you have more experts there that can provide better information than me in the opposite corner of the U.S. I think ultimately that you’re going to have to figure out which system makes sense to you and who you think you can trust. You’re doing more research and will be better informed than the average consumer so I think you’ll make good choices.
All the best!
Ok i know after reading this article i technically did the wrong thing but let me explain and you can tell me if you think the tile will be ok or not. I’m almost done with laying 300 sqft of 4″ x 47″ porcelon ỉn a kitchen. Thr floor was not level and the làdy just didnt wsnt to go thru all that so i later 1/4 ditra mat down instead of backerboard. So far (75% done) it seems like the tile is good in place. As im towards the end.. i have had to build up wuth thinset alot. So much that i decided to put down three 36″ x 42″ cuts of bb so that i dont have to build up so much. So i have backerboard over ditra in just a 4′ x 12′ area. I put some thinset under but mostly screwed it down. Do you thìnk it will be ok?
I have no idea what’s going to happen to that floor as you’re in uncharted territory. I would rather have seen you install another layer of Ditra over the Ditra. I think Schluter will actually provide a warranty if you had done it that way (not 100% sure on that).
edit: They probably wouldn’t provide a warranty in your situation but they did some testing of doubling up Ditra and were pleased with the results.
I had tile laid in my kitchen in November (7 months ago). It’s already popping and some sound hollow when you tap on them. Also, grout is starting to break up. We are on a crawl space, and the floor has had extra piers added to it, but you can feel a bounce if a big guy jumps up and down hard. I know the installer laid hardie board down under the thinset, but I don’t know what he put between the hardie board and the subfloor, if anything. I’m hoping to get him out to look at it and diagnose the problem, but he’s not getting back to me. Shocker! It seems to be getting worse by the day, perhaps because we’re now headed into the heat of summer. The floor is definitely not insulated, in case that matters. Any thoughts? If we have someone else tear it all up (later, can’t afford to do it 2x in one year), what should we ask them to do differently? Thank you so much!
Your floor should be under warranty from your original installer. If they didn’t thinset underneath the Hardibacker then they didn’t do the installation correctly and it will have to be redone.
There are several possibilities for a failed floor installation. It’s possible that the structure of your home won’t support it. It’s possible that there wasn’t enough mortar coverage under the tiles. But no thinset under the Hardibacker would be the number 1 thing I would look at.
Thank you. I’ll ask him. I bet he didn’t and I’ll have to get someone else to tear it out and do it right. Lesson learned. Thank you for your help.
I took my tub out to install a tiled shower. I’m on a crawl space so took the sub floor out to move the plumbing to the opposite wall.
I plan on replacing the removed subfloor with what was there, 3/4 plywood, but have room to use something thicker if it’s advisable.
I plan on thinsetting and screwing cement board to the subfloor, then RedGard, then thin set and tile.
I need to match the height of the existing tile in the bathroom. Would this calculation get me to the right height…
Compressed thin set on top of sub floor…1/8″
Compressed thin set on top of cement board…1/8″
Selected Subway Tile…1/4″
Total thickness above subfloor…1″
Does that sound right?
I may not understand fully but it sounds like you removed your tub and will instead build a shower? It sounds like you want the shower to be curbless and even with the existing floor? If both of those are correct then I don’t understand how water will find it’s way to the drain. There’s no slope.
Check out my post on waterproofing systems but if you want to build a curbless shower in that space then I recommend the Wedi Ligno shower system. But even then you’ll have to tie the shower pan into the flooring and than involves removing at least some of the tile floor.
David McKenna says
Hello, I am installing 12×24 porcelain floor tile in my bathroom.
I have 1″ not 3/4″ osb as the sub floor and 24″ O.C. joist with1/4″ Luan over it with peel and stick vinyl tiles on top of that. Should I remove the Luan or can I use thin set on top of the luan?
Also if it is OK to use the Luan, what spacing should the hold down screws be?
Thanks for your time.dwm
David McKenna says
Ps. I forgot to mention I will be using 1/4″ hardie backer under the tiles.
Thanks for your time dwm
Sorry, I didn’t explain it well. My wife has MS. She still gets around quite well, but I want a curbless shower so she doesn’t have to lift her lazy leg over a curb. It will slope away from the entrance toward the back wall where I’ll put a linear drain. I bought an Oakley ProLine linear drain. At 1/4″ per foot slope, I only need 1/2″ drop for water flow.
The main tiled area in the bathroom has been leveled and has a heated floor.
I plan on securing 2x6s with construction adhesive and screws to the existing floor joists to get the slope I need. On top of the 2×6 will be 3/4″ subfloor, thinset, cement board, RedGard, thinset, tile. I’d like to get as close as possible to matching the height of the existing tile.
If I thinset the cement board down using a 1/4 x 1/4 trowel and screw it every 8″, will the thinset end up being 1/8″ thick?
The cement board, both walls and floor will get RedGard before tile
You definitely want to remove the Luan and thinset and fasten the Hardibacker to the subfloor directly.
You’ll be under 1/8 inch with the thinset layer under the backer board. Probably under at the tile layer also.
Could you perhaps recommend a different trowel size to set the cement board and/ or tile?
Maybe a modified procedure, like set the cement board with a trowel to give slightly thicker thunder, then take a measurement and adjust the thinset thickness for the tile to get a match with existing tile?
I’ve pulled up an old tile floor in my bathroom, 52sf. The old tiles are 4″x4″ attached to a plywood underlayment nailed down to the subfloor. There is no mortar between the backer boards and subfloor (it was still brutal to tear up). My intention is to put down 1/4″ Hardie board, using the recommended procedure, then lay 12″x24″ porcelain tiles on top of that.
The old tiles extended underneath the edge of the tub which is slightly flexible (vinyl ?), providing some support for the outside edge. Everything that I’ve heard so far says to stop the new floor 1/8″ from the edge and fill with caulk. If I do that, what should I do to support the edge? Or would you recommend that I extend the tiles underneath like the old floor was?
I’d keep everything with the 1/4 x 1/4 square notch. I don’t think that you will have trouble meeting up. You can always flat trowel the back of the tile or hold your trowel at a straighter angle. If you are just too low then set the subway tile with a 1/4 x 3/8. Or mix your mortar stiffer.
Hi. I have a question. I want to apply a tile to the bottom of a wood post which is outdoors.
Will it work if I install some cement board first the size of the tile I want to use? Will this hold up to the moisture? and what is the best adhesive to use if so. Thank you so much!!!!
Hi I want to use large porcelain tiles over my hardwood floors in my kitchen, dining-room and living-room because we have lost of traffic and a dog. my house was built in 1959. I was wondering in the installer used hardibacker board over the entire floors and screwed them down, would that be sufficient enough to use thinset and then use the porcelain over it? Thank you for your comment.
Yes, you can use cement board and it should be waterproofed also.
You’re not supposed to go over the hardwood floors, even with Hardiebacker. It needs to come out and then you can build over the subfloor
G G says
I’m installing 1’x2’ 3/8” thick slate tile in my 6’ x 8’ mud room. Is it okay to use 1/4” thick cement board as a backer, or is 1/2” preferred? I’m attempting to match the height of the adjacent 3/4” thick hardwood floor. Thank you.
1/4 inch is just fine.
I am redoing a bathroom and installing cement board for the first time. I put down 7/16″ Wonderboard Lite per manufacturer’s direction, which included thin-set in the joints with mesh tape embedded. Everything looks great, however all of the seams are slightly raised due to taping. Do I need to sand these down a bit or is the difference too small to make a difference? (I’m installing 16″ x 16″ porcelain tile, with thin-set being applied with 1/2″ trowel).
It’s probably fine as is. If you can feather out the humps that might work better.
Thanks for the reply. I sanded the high points down with a brick (a GREAT trick BTW) and things are pretty smooth now. This is a great site for DIYers.
Thank you and that seems like a very effective way of doing it.
Is it recommended to put a wedi board or schulter membrane in the front door vestibule on cement floors. The current tiles are all cracked. Will the membrane help reduce the tile from cracking.
It’s a good idea but I don’t know that it will solve the cracking issue. There are a lot of reasons that tiles and grout joints can crack. They may not be installed with proper exterior mortars. Maybe the concrete doesn’t accept a bond very well. Maybe there is no room for movement. It’s hard to say.
Katrina Thorne says
I have 7/8 inch thick by 5 inch wide pine tongue and groove boards as a subfloor. They are solid, but not super tight together due to the age of the house. I don’t think I can thin set over those boards to put the cement board down and if I add plywood my floor will be way higher than the floor that it is meeting up with. Will just screwing the cement board down make the tiles crack? Should I use some thin set on each individual subfloor plank? Using a 3×12 porcelain tile for a small entryway. Thanks!
The only way to do this right is to go over the t&g planks with 1/2 inch AC plywood. Then you can use a thin membrane over that if you want. The other way to do it is to remove the planks and install 5/8 or 3/4 subflooring. Anything short of those two options is taking a risk. Cement board doesn’t add to the structural strength of your floor.
I am going to pour a concrete counter top in place over plywood. Can I install backer board on top of the plywood before pouring the concrete?
Yes. Probably a preferred way of doing it, I would think.
Chad Nolte says
After applying the thinset and laying the cement board down do I need to stay off it till it sets?
No. You want to get on it right away to get the fasteners installed while the mortar is still wet. Once the fasteners are installed you can continue to walk on it all you want.
Hi! I’m using hardie board over my bathroom Subfloor and I do have the joint tape but I am not certain that I understand when to apply this… do I put this on right away after I have all fasteners in or do I apply just prior to tiling? Does it need to dry for a specific amount of time before tiling over it?
You can put it on right away if you want. That’s what I usually do. I prefer to have it on and dry before the tile goes in so it doesn’t come up while you are installing the tile. That’s really annoying. It’s dry when the mortar turns completely light gray (or white if you are using white mortar). Good question!
Ok, so we had 603 square feet of porcelain tile installed through the first floor of our home. The tile runs from our utility room through the kitchen, hall, bathroom and foyer. The installers tore out the old tile which was installed over plywood. They were supposed to rip out the plywood, put down Hardie board and then tile. But they said the plywood was glued down to the subfloor and thought tearing it up would cause damage to the subfloor. So instead they left the plywood. Now the layers of floor are subfloor, plywood,1/4 hardie board, porcelain tile. And they did use thinset between plywood and hardie back and also between hardie back and tile. It did raise the floors higher and although I don’t exactly love that, it isn’t my main concern. I have two questions… is the way this was installed ok? I allowed them to leave the plywood because I didn’t want the subfloor damaged. Also is the weight of this ok? I know that might sound a little crazy, but I always get nervous about the structure of homes. Can my subfloor handle the weight of plywood, hardie back and tile on top of it?
The weight should be no problem and it sounds like a pretty well-built floor.
Thank you so much! That makes me feel so much better!
Uh oh… I spoke too soon. My previous question shouldn’t have been my main concern since we now have ran into a bigger problem. In the last question I asked I explained how the floors are higher than they used to be. Even though I don’t love it higher I thought it was something I could deal with. Until I went to open my front door and realized I am unable to have any rugs since the doors won’t clear them. I have been trying to figure out how to correct this and I’m not having any luck. These are metal doors. I don’t think they can be trimmed at the bottom. But even if they could that would cause a problem with the weather stripping being able to meet the threshold. I don’t want to mess with the frame work of the door, making that higher isn’t an option. Any suggestions on how to fix this?
I’m not sure about how to trim metal doors. My guess is that there is a way and you can raise the height of the threshold. Maybe someone in the community has a better answer?
I removed the carpet of my master bedroom and cure the cracks on the floor with two layers of RedGard. I notice that the corridor tiles were set on top of the old tiles which leveled up the flooring to 1 1/4 inch. So, in order to lay the tiles in the bedroom I need to raise the floor level by at least 3/4 inches. I was going to do it with siroco over the base floor. But you said that it is not a good idea.
It’s 290 square foot and I am laying 6 x 23” tiles. What is the best way to do it then?
I think the best way would be with a self-leveling cement. You might take a look at this post: https://www.diytileguy.com/self-leveling-underlayment/
Ron Green says
I have a 1″ subfloor of 1/2″ plywood and 1/2″ particle board.
I am replacing my back entrance door which currently sits on the 1/2″ plywood above the exterior rim.
Can I remove the small top layer particle board that is directly in front of that door ( about 3′ x 5′ ) and replace it with hydrodefense HardieBacker but cut it so that it extends and lays under the new back entry door on the exterior rim.
The door would then be installed on top of the waterproof cement board which also has the 1/2″ plywood below it.
I want to eliminate the issue where moisture destroys the particle board that normally butts up against the entry door.
The height will be different but I can add a thin luan sheet in-between the 2 to make up the difference.
I would only do this for the 3×5 piece in front of the door.
Everything elsewhere would remain 1/2″ plywood and 1/2″ particle board.
If this is not recommended as described above, then should I try the same process using a 1/2″ piece of plywood with the luan if needed in order to cover the thresh-hold area?
I would think you could do that and they make Hardibacker in 1/2 inch thickness so I don’t see why there would be a height issue. I don’t really understand what you are doing and if this is even a tile question though.
Ron Green says
not really a tile question as much as it is a structural question.
The 1/2 waterproof hardi is .42 thick so the flooring difference can be closely matched by filling with thin luan to match the height of the internal flooring.
Do I need to use any thin set morter for the hardi if I intend to have click vinyl plank installed as the finished layer on top? I would use the cement special screws to attach the hardi to the plywood.
I don’t know if click vinyl plank would require Hardibacker to be thinset down but it certainly wouldn’t hurt. The other thing is that a little mortar underneath may make the heights work out perfectly.
Anderson Cielto says
Please explain WHY is thinset necessary? What does it add to adhering/fastening the backer board that screws cannot. Why are screws alone not adequate? What is it about backer board that necessitates thinset as an additional agent in its installation? I am not asking to be a jerk, it just does not make sense to me. Seven years ago I did a large format tile project (320 sq ft) using screws only, as of today it still going strong, no cracks, no anything? What benefit or capability does thinset add that without it the installation is compromised?
It takes away movement. Screws (or nails) hold the backer down and thinset holds it up. Once dry, it takes away any gaps and any further movement in the backer board.
Please help, I’m about to put Hardie backer board on my cement floors to build up a section that was previously carpeted. I do not trust any that the thinset will hold the boards down long term. I am tiling over existing tile, using mapei eco prom grip. Should I use self leveling cement instead. If so, which brand do you recommend?
You can use a variety of products but Hardibacker shouldn’t be one of them.
Foam board panels, like Wedi board or GoBoard, come in a variety of sizes and have a procedure for installing over cement. Additionally, self-leveling underlayment works fine and would give a flat floor, if done correctly.
The easiest solution, as long as the heights will match up, would be to use an uncoupling membrane like Ditra or Ditra XL
My kitchen tiles became loose one by one. I lift the wonder-board and I see a straight crack. What could be the reason?
There are a lot of causes of loose tiles but you said “I lift the wonder-board…” which is something you shouldn’t be able to do without a lot of work.
So, that tells me that there is no mortar underneath the cement board which could be the cause of your loose tile issue.
isabelino muller, says
what is the reason that you don’t recomend using cement board over concrete floor? if i use thin set and top cons i think it will work ok is just more work, and it take longer to do the job.
If you want to use tap cons and install them per the fastener schedule then maybe that will work. I’m not sure how big the heads are on those. That seems like a lot of work to me and I’m not sure what the benefit is other than you raise the floor up.
Great forum thanks a lot. I am getting ready to install tile in a small upstairs bathroom. I first poured self leveling concrete to make up for a substantial slope, I then cut Hardi backer which I will install with thin set underneath and finally screw down the Hardi backer through the self leveling concrete and into the plywood sub floor. I will probably pilot drill holes into the backer and through the floor leveler so I don’t crack it. Does this seem like an ok idea or do I need to scrap the Hardi backer for a different product over the floor leveler? Thanks a lot, Jon
First thing, is I would check if you installed the correct leveler. There are only certain levelers that are approved for going over wood substrates and one of those types would need to be installed. Alternatively, some levelers require metal lath to be installed over a wood subfloor prior to installing the leveler.
Here’s some more information about self-levelers and how to install them over wood subfloors. https://www.diytileguy.com/self-leveling-underlayment/
Assuming, the correct leveler was installed correctly then you should simply re-prime and add more leveler on top of what’s there. If you wanted to install cement board then that should have been installed first thing and directly over the wood subfloor. You could then add a self-leveling compound on top of that and you wouldn’t have to use the ones that are specifically for wood subfloors in that case.
Bob Geremia says
I am in the process of installing a new ceramic tile floor over hydronic (viega) radiant heat. I will use the viega climate panel, which the pex tubing snaps into. The viega installation instructions say to glue and screw the backer (hardie) board down. Your suggestion is to use thinset and screws. What method should be used and if it is glue, what type? Or if thinset, what type issued over the pex tubing? Thanks for any information.
Hardibacker wants their product thinset and attached if it’s going to have tile installed over it. If there is another company that wants you to install that product differently then you’ll have to check with the radiant floor company as to why, how they do want it installed, and make sure you understand the warranty.
Hardibacker will not warranty their product if it’s installed differently than they require.
Brandon Fontaine says
Can you put thinset mortar on OSB subfloor? This is not advantech OSB just the Lowes cheap stuff. I’ve read multiple conflicting articles and can’t find a straight answer.
It depends on what you are installing. If you are installing cement board or an uncoupling membrane then, yes, you would thinset those down and sometimes to OSB.
But I don’t think that’s what you are asking. What is it that you want to stick to the OSB with the thinset? You definitely don’t want to adhere tile directly to OSB.
Appreciate this page. I’m planning to install tile on my bathroom floor. I have 1 inch thick by 4 inch wide pine boards, with no subfloor underneath. The boards fasten right to the beams underneath, thus removing them is not really an option I believe. They are solid as only about 10 years old, but not tongue and groove so not super tight in all spots. I have a feeling I need to use plywood above before I put down thinset and backer board but want to confirm that. I understand that the backer board does not add structural strength, but do I really need that if the floor itself is strong? The joists underneath are 12×4 and about 21 inches apart. Thanks!
My home has OSB as a subfloor like alot of homes now. I’m wanting to put down tile however I’m not liking what I read about OSB. Is it safe to say that I can treat it like a plywood subfloor and place mortar, cement board, mortar then tile or is it not even worth doing with having OSB?
It sounds like you have an older home and the floor isn’t ready to accept tile. At least not yet.
The best way would be to remove the boards, add some joists with joist hangers in between the beams, and then install a layer of 3/4 inch plywood over the top of everything. The other advantage of this is that it keeps the overall height of the floor down.
Otherwise, you could probably go over the floor as-is with 5/8 inch AC plywood and maybe even 1/2 inch. You could then add 1/4 inch cement board, an uncoupling membrane like Ditra, or even just paint on a liquid waterproofing like Redgard.
Cement board is a great way of going over OSB and the way that you outlined should work perfectly.
hello and thanks for this forum, you’re doing a great service! we are about to tile over a vinyl kitchen floor (in great condition, though ugly). it is near perfectly level and free of any blisters, peeling, etc. am i understanding the right approach is thinset first, then backerboard, then thinset followed by the tiles? can we leave the vinyl floor beneath? we had a suggestion that we could tile right onto the vinyl floor but i’m dubious.
second question – any words of wisdom on deciding whether to lay the tiles in a diagonal pattern rather parallel to the walls?
You can tile over vinyl but it’s not always a good idea. The vinyl has to be well-bonded and can’t be the cushioned kind. Then you’ll need to clean it off to make sure there’s no wax residue or anything else that can inhibit a bond. Additionally, it helps if you can scrape, or sand, the surface to promote adhesion.
What I don’t like about going over vinyl is that it’s oftentimes installed over particle board which is not a good underlayment for tile. If anything happens then the particle board will swell and ruin the tile.
If it is installed over particle board, or plywood underlayment, then it’s typically a simple thing to remove. Here’s a video from my Instagram that shows a little bit of the process. You can rent a toe kick saw if you need to get under the cabinets. An oscillating multitool will get under there too but it’s a little more tedious.
Link to instagram video about removing vinyl subfloor
You can also install backer board, like you mentioned, but then the floor height is going to be a lot higher than it currently is and you could have issues with door heights, etc.
Thank you for a great forum. I am replacing 20 year old 8X8 ceramic tiles in our 80 sqft kitchen installed on gypsum. When I removed the old tiles and gypsum, OSB board revealed random spots of dried mold especially under the refrigerator. Apparently the refrigerator had small leaks over the years but the compressor heat evaporated the water while causing minor mold therefore we did not have any extensive smell of the mold. I used an oscillator cutting/sanding tool to remove the strips of rotted OSB since hitting with a hammer on the OSB did not damage the structure of the subfloor. After sanding, I used two coats of KILTZ to seal the OSB floor. After that I wanted to install Hardibacker 1/2″ cement board and Schluter DITRA on top of it to waterproof before I install tiles. Since I had a hip fracture 2 years ago, doing floor work was extremely difficult and expensive especially thin-setting multiple times between layers of the underlayments. (I experimented laying DITRA first under the sink cabinet (4 sqft area) and realized that 80 sqft will be a huge pain. I then discovered Daltile’s Quictile or RevoTile) floating porcelain tile system during research which does not require thinset underneath. I quickly compared costs of traditional tile ($2.59/sqft) versus Quictile ($4.49/sqft) and with all underlayment, thinset, materials Quicktile was $27 dollars cheaper then traditional porcelain tile minus no thinset and mess. So without labor cost, it will cost $630 to lay Quictile in a 80 sqft kitchen every material included down the penny.
However, my enthusiasm has been curbed when I read Hardibacker installation instructions and your recommendations that I still need to have thinset under the cement board. I prefer DITRA for waterproofing but 1/8″ thickness of DITRA requires two layers to maintain 1 inch thickness required to level the kitchen floor with living room and door heights. That means I am back to where I am with thinset mess and pain and more expense.
My first question is that can I just use the 1/2″ Hardibacker without the thinset between OSB and cementboard (just screws) since Quictile is a new technology and expansion/contraction of the floor can be compensated with this product?
Second question is, Hardibacker 500 is not waterproof unless I use Hardibacker Hydrodefense for Walls.
Can I use Hardibacker Hydrodefense for floors?
For me waterproofing and protecting 20 year old OSB from mold is more important. If you recommend DITRA over Hardibacker Hydrodefense, I would hire someone to install 2 layers of DITRA on OSB. or do I have any other option? Really appreciate your advice.
It sounds like you need to come up 1 inch with the underlayment. I think the best way to go would be to add plywood over the floor as it is right now. You can go with 5/8 or 3/4 inch. This will add quite a bit of sturdiness to the floor and it doesn’t require a layer of mortar underneath it.
Then you could add Ditra, Hardibacker, or Ditra XL (1/4 inch thick) over this.
I’ve never installed the Quictile or Revo tile so I don’t know what they require. You would have to check with the manufacturer to see if mortar underneath Hardibacker is necessary. I know Hardibacker wants mortar underneath but they are also assuming that you will be installing tile with a mortar adhesive. You can waterproof over the Hardibacker on a floor if you want.
margaret byrd says
Is there a down side to using 1/2 Hardibacker over the plywood subfloor floor rather than 1/4 inch Hardibacker? I know they say 1/4 inch is OK on the floor and it is easier to cut.. I have 1/2 inch on site.
Also, on one side of the tub against the wall they installed green board. At the back of the tub there is the Hardibacker where it joins into the open shower. I’m only using a narrow band of tile in the green board area. I had thought to put Redguard over that section which will not receive a lot of water. Am I cool? Or should I demand it be cut to the height of the tile installation and hardibacker installed. The shower is separate from the tub.
Either thickness of Hardibacker works just fine over a floor. The caution with 1/2 inch on a floor is that it may make the bathroom floor higher than the flooring in the adjacent rooms. But if that’s not an issue then 1/2 inch thick is perfectly fine.
Tiling over green board is just fine as long as there is no shower head above the tub.
IUsing thinset under cement board depends on your subfloor. Over plywood I agree.. over advantic you can’t. They spray a wax waterproofing on the floor. Thinset will not bond to it. Most new houses have advantic in them. You have to sand the whole floor down.. if not the thin set will not bond and cause movement.
For installing cement board over a wood subfloor, the mortar doesn’t need to bond to be effective. That’s why it’s still important to use it.
I had the same question, thank you! About how long does it take the thunder to dry?
You’re safe if you figure 24 hours. It usually dry before that and you can tell by the color change of the mortar. It’ll be a lighter color
We’re renovating our guest bathroom and tiling the shower walls. We put the cement board smooth side out, because for reason reason I had it in my head that thin-set was a mastic and not mortar. Am I doomed if I use thin-set on the smooth side?
Smooth side is fine
james k says
I have a laundry room that had a laminate sheet flooring in it. I removed the laminate to find they had put down 3 layers of it glued in various places. It all looks to be the same so I guess it was done at the same time. The last layer was glued down 100%. I finally got this up and find the glue is not coming up. Can I use a cement board to give me a clean level surface to lay my tile on?
Yes, you can, as long as the layers aren’t over concrete. However, if you have a wood subfloor then you may not be down as far as you need to be. Typically, the flooring is over an underlayment that is on top of the subfloor. That underlayment should come out as well.
james kitchen says
It is on a concrete floor. As I said I am down to just the remaining glue. It is hard and not coming up. I talked with a contractor and he said we could do the backer board or a leveling compound. Not sure what to do as I now have a cement floor with the glue remanence on it.
Over concrete, you don’t want to use cement board. You could use a foam board like Wedi or a leveling compound.
The best way to get rid of the glue is to grind it off with an electric grinder and a diamond cup wheel. Alternatively, you could prime over it with a product like Mapei Eco Prim Grip and then use a leveler or even adhere tile directly to that.
I’m in awe not just of your knowledge, but the grasp of products and lingo held by your questioners–and then there is me. I hope I have enough information to make an answer possible.
We are renovating the mudroom and lav of our home. I have a contractor, but was expecting to lay tile ourselves. We’ve never worked with large tiles, nor in as large a space. In total, it is about 100 sq feet: the la,v is about 5*6, and sits in the crook of the “L” shaped mudroom.
The contractor has removed the previous tiles, which have been cracking. If I understand correctly, they had been set over vinyl flooring with no cement board. I had been concerned that there could be rot issues, but he assures me that there’s no such problem, that it was just that the floor wasn’t stiff enough.
This probably where I should mention that he is a great guy, does beautiful work, but isn’t that great a communicator. In general, though, I trust him. I’m not sure, though, that he is immune from getting in over his head.
The tile we are planning to use is a 3/4 inch thick bluestone in pieces as big as 16*24″. What is making me freak out is that the tile company’s recommended practices are geared to exterior installation, and direct setting the tile on 4″ beds of concrete. Our house is 100+ years old, in New England, and certainly doesn’t have concrete subfloors.
Do I need to find another flooring material? Should I limit myself to significantly smaller, or even mosaic, tiles? Are there specific boxes if ticked other than 4″ of concrete that should mean that we are okay-and I can just take a chill pill and worry about something else, instead?
I’d be inclined to listen to the company that sells the tiles. They are trying to tell you the best way to install them. Additionally, I don’t think those are the best beginner tiles to work with. Plus, do you even have room for a 4-inch bed of concrete +3/4 for the stone? What about door swings?
However, if you decide to go away from the bluestone that doesn’t mean that you are stuck with mosaics. Bluestone is very thick and heavy, probably has some variance in the thickness of the product, and is going to be difficult to cut and install. So you could choose another tile that isn’t as difficult to install and work with as bluestone.
If you do decide to move forward with the bluestone installation, I would recommend getting a professional installer. I don’t mean your general contractor but a company that is skilled with that particular type of installation.
Hi there! I’m renovating my 100 year old house and have discovered that I had many layers of flooring glued onto to bathroom and kitchen floors. I was planning on using 1/4″ hardiebacker on the floors. I have removed all the vinyl flooring in the bathroom but there is quite a bit of old black adhesive which has been almost impossible to remove. Is that something I can leave and cover with thinset or do I need to use a self leveling product? Thank you so much!
I have a bathroom floor to tile. I have installed (screwed) a 5mm underlayment plywood on top of the 5/8 inch plywood subfloor. I am told that the 5mm underlayment is luan plywood.
I am ready to lay 1/2 inch durarock. The thin set bags I have seen at the big box store say not to use on luan plywood. Do I need to replace the luan with an exterior plywood or is there a thinset available for use on luan?
Good question! Unfortunately, the black adhesive probably contains asbestos so caution needs to be taken when this is encountered.
Probably, the path forward would be to purchase a bonding primer such as Mapei Eco Prim Grip or Custom MBP. These can be painted over the cutback adhesive. Then you can proceed with Hardibacker or whatever underlayment that you are going to use on your floor.
Unfortunately, the luan plywood needs to be removed and replaced with an exterior plywood with a face grade of ACX. The thinnest that you can get this plywood is 3/8 inch but the 1/2 inch version is much more common. So you could go with 1/2 inch AC plywood and 1/4 inch Durock cement board and be at the same height as you were planning.
Can I put cabinets over cement board right away or do I have to let it dry just want my sink hooked up as soon as possible
Yes, you can walk on the board immediately after installing & fastening and install your cabinets over it as well.
I was considering using DryPly coated plywood under my HardiBoard installation. Am I correct in assuming this is OK as the adhesion of the thinsett to the plywood is not necessary? Thanks
PS – reason I’m considering this is the bog box stores around me have crappy plywood except for this DryPly which is flat as a pancake.
You don’t want to use any treated lumber underneath your tile. Treated lumber has high moisture content and warps when it dries out. This product looks like it falls into this category. What is recommended is exterior grade ACX plywood. Once it’s installed it will flatten out.
Hello, My husband and I just installed hardiebacker last night over two layers of super thick plywood. We used thinset under the boards, the proper screws, meshtaped the seams, and all boards were cut to meet properly. As it’s drying ~ 12 hours later there’s some popping sounds coming from that room. The only thing I can think of is that maybe the thinset was too thin. The screws were installed deep and evenly. Any thoughts? Does that mean it won’t be a solid floor?
It’s hard to say what the crunching is. If it’s creaking and it sounds like it’s down in the floor then there probably isn’t much that you can do and it’s probably harmless. But if it is a crunching sound, it wasn’t there before, and it sounds like it’s near the surface then that’s probably not a good sign.
It could be too little mortar underneath that backer board and/or it could be that the mortar dried out before the board was securely fastened. You might try tapping on the Hardibacker with a hammer handle, or something, and see if it sounds hollow in some spots. If so, it’s best to remove the board, scrape up the mortar, and start over. It’s important that this part of the floor is done right.
Hello, I ripped up an old flag stone hearth in my house that the previous owner installed OVER Carpet! At least they had plywood and cement board (just screwed together w/o thinset in between) on top of the carpet, but still…CARPET!! A lot was wrong with what they did, but my question is after I ripped it all out and got to the sub floor it looks to be a double sub floor with 3/4 inch by 3.5 inch tongue and groove planks on top of various widths (8inch average) by 1 inch thick planks running diagonally. The floor is very sturdy and no bounce. My intention is to replace the hearth with brick veneer, so my question is, is it alright to place cement board right over the second subfloor and brick on that, or should I install ply first then backer board and brick?
It should be mentioned that the rest of the floor is a plank laminate that thankfully doesn’t have carpet underneath, so removing the second subfloor isn’t really an option without extensive reworking.
Thank you in advance, your insight does not go unappreciated!
The right way is probably to install the plywood first and then the cement board. I don’t think cement board is approved over planks by themselves even if it’s the second layer. It doesn’t mean that it wouldn’t work but I’m pretty sure a layer of plywood would be the proper way.
Laid hardie in my 5’x5′ bathroom and totally forgot to screw it. Will be using 12″x24″ tiles. Assuming I need to tear out and redo, but curious what the harm/risk of not having it screwed down (besides voiding warranty). I get that it’d be bad screw after the thinset has cured. Thoughts?
The risk is that it won’t stay down and peel up after it’s tiled. It might work to screw it down after the mortar has cured but I don’t think I’ve ever run into this. You might check with the people at Hardibacker and see what they recommend. If they say it’s OK to screw it down now then you’ve saved yourself a lot of trouble.
Hi. I’m a tenant. The landlord is making repairs from afar using a contractor who did not stagger all of the seams in the kitchen. There is one long seam over 8’ long which concerned me, so I looked up this article to give her the info so thanks your post! Now we get to see if he uses mesh tape and thinset.
The long not-staggered seam was actually the 3rd thing I noticed. I’m trying to stay out of these things as I have already come to realize this “helping” has little value to her, but it’s tough because I love this old house. I’ve been here 15 years. There are style choices being made in the renovation which are negatively affecting my color pallet and pattern choices giving me stress but the foundation issues are worth fighting for.
The two possible issues I noticed first were: on the brand new subfloor tger laid, the piece of 1” plywood is nearly 1/2” away from the sidewall of the bathtub and so is the 1/2” backer board ! Meaning, I can look down and see the ground. I believe this is why the contractor is trying to steer her away from the classic classy penny tiles most iconic of the 1920’s era! Still a problem if he chooses 2”? I don’t know but seems plain wrong!
The other issue is that the 2” quarter round molding were the only part of the moldings removed. (Plaster walls so I don’t blame him. However, on a 4’ stretch of wall, the backer board is only 1 1/2” wide! Such a narrow strip. Doesn’t seem like it will have the same structural integrity using two large pieces of backer board would. Seems like he is putting the pieces on the edge in structural jeopardy. No? I really hope not. I am trying to figure out if any of this is worth speaking up about.
Thanks for the article.
I am having my two upstairs bathrooms in a 1979 home remodeled. The contractor mentioned he would put down thinnest and cement board on the floor before laying the tiles. I did ask if lose or weak plywood needs replacing as one piece when you step it dips down. He said no as the cement board will solve it.
Should the plywood be replaced ?
Thanks everyone for your guidance.
Unfortunately, when the home is a rental, the contractors that get hired tend to be low bid and it’s not uncommon for corners to be cut. It’s hard to say what is going to be a problem. The hole where you can see the ground should obviously be patched or filled with something. Additionally, mesh tape and mortar would help to strengthen the thin piece of backer board. Hopefully, the contractor and landlord will take the right steps forward.
Technically, it’s a matter of how much it moves and it’s not straightforward how it’s determined. There’s no tool or device that I’m aware of that will measure how much deflection your floor has.
However, it should be mentioned that cement board has no structural value. So, your floor will have to be structurally sound before cement board is installed. In my opinion, if you’re the homeowner and you feel the floor needs to be patched before moving forward then I would pay the contractor to do it.
The floorboard dips down about a quarter to half an inch when pressure is applied (eg when stepped on).
Thank you, that’s what I’ll do. Cheaper to do it now than having to replace the floor.
Not sure if this was asked previously but do I need to predrill before screwing into fiber cement board? I’m using 1/4 for a tile entryway and there are some weird shapes and small pieces just due to the odd configuration of the space. I’d hate to have to recut a piece in the middle of installation just because I should have predrilled. Conversely, I’d hate to predrill everything only to find out it wasn’t necessary. Thanks
Not usually, but it’s possible that you may have to do certain corners, etc that may be prone to cracking and breaking. It’s not unusual to install 1/4 inch on floors without predrilling anything. But 1/2 inch on walls where you can only put a nail or screw in certain spots because of the wall studs would be a reason to predrill 1 or 2 spots.
Hi, I’m in the middle of tiling my kitchen floor and realized halfway through that I had picked up the wrong mesh tape and it isn’t alkali-resistant. I’m going to grind the thinset off and redo the tape, but I am wondering if I need to grind the mortar out of the joints as well and refill? Or will it be ok grinding flush with the board and retape from there? Thanks :)
Would it be ok to apply a 1/16 to 1/8″ layer of concrete floor resurfacer (Sakrete Flo-Coat or Quikrete fast-setting self-levelling floor resurfacer) or quikrete vinyl concrete patcher skimcoat over a layer of cement board floor as a final floor wear surface (industrial look)? …i.e. would cement board layer provide similar bond as a concrete surface?
Would such a wear surface provide some level of water resistance?
I think you’re fine if you just redo the tape. Alkali-resistant is supposed to last longer but it’s not like drywall tape is completely ineffective.
You can pour levelers over properly installed cement board. I’m not sure which ones are approved to be the finished surface. I’ve installed them with that aim in mind.
I have a living room floor comprised of 2×6 floor joists on 12″ center, topped with a 3/4″ tongue and groove plywood screwed down every 6 inches and sub floor liquid nails.
The floor joists span appox 6 feet apart on 3 concrete footings running lengthwise of the 14′ x 20′ room.
My plan is to use thinset on top of the 3/4 plywood and then install 1/2″ cement board before installing Saltillo tile.
Do i have enough structural support for this?
It certainly sounds like a well-built structure to me
My tile installer is installing Durock on the floor of my bathroom. He is placing it over terrazzo. I saw your comment warning against placing durock over concrete. Should terrazzo be treated the same?
Also, what would be the point of installing durock over the terrazzo when he can just install the tile right over the terrazzo?
Thanks in advance!
Durock is designed to be used with fasteners. If he’s not using fasteners (which it doesn’t sound like he is) then he’s not installing it appropriately. Your second question is a good question also. You might ask him that.
I just bought an old house, and I want to install ceramic tile on the 2nd floor. Its an osb subfloor that had the old asbestos tile ripped off. Its pretty sturdy (no big creaks when I’m walking around up there). There are some slight low spots in the floor. What’s the best way to go about leveling and sealing it before I put the backer board on
Probably to use something like Ardex (or Henry) Feather Finish or Mapei Planipatch. Alternatively, those can also be used on top of the cement board
Hey Jim… You’re definitely my new hero! I’m new in the tile world, and was wanting to put marble down on my 2nd floor bathroom floor.
The 9″ Joists are 16″ spaced spanning 14 feet. The bathroom is 8 feet deep.
I’m wanting to put down mosaic 2″ squares netted marble tile.
From what I’ve read, marble needs a 3/4″ t&G plywood subfloor, and 1/2″ plywood underlayment.
That seems like overkill for 2″ tile.
Are they talking about larger marble or all marble?
Would it be possible to have less underneath the tile?
The original was 1/4″ plywood, wire mesh and a cement bed – 1960s.
Industry standards call for 1/2 inch plywood over the 3/4 inch subfloor. You can choose to not do it that way and it doesn’t mean that your floor will fail. Also, there’s a product called Permat from Blanke that can be installed instead of cement board. This product claims to strengthen the subfloor so that you don’t need the additional 1/2 inch plywood.
But, if you had a mud bed with wire mesh before then I would think you would have plenty of room for 1/2 inch plywood?
I’m replacing tile in 50s era rambler bathroom. The original tile had a 1/2” of mortar with wire mesh under it. I’ve ripped it all out and the subfloor is cedar plank, running diagonally across the joists with 1/4” to 1/2” gaps between each plank. You can literally see through to the unfinished basement below. The cedar subfloor planks are 3/4” and in good shape but I don’t feel like I can spread thinset over the cedar with all those gaps. Someone suggested to tape over the gaps. Not sure about that. My question is if I can get away with 1/4” plywood, followed by a layer of thinset and then 1/4” durock so that when I add tile it’ll be close to flush with the hardwood in the hall. Like I said it’s pretty solid, structurally, I just need to cover all the gaps. Any insight on this is most appreciated.
These types of floors, assuming the planks are tongue and groove, need a layer of plywood over them and the plywood needs to be 1/2 inch minimum. The reason is to make them structurally sound for a tile installation and this involves removing deflection. You might consider switching over to 1/8 inch Ditra instead of cement board. That way you’re only 1/8 inch over the original thickness and Schluter has a detail in their Ditra Handbook for this exact type of floor.
Joe Keegan says
Hi – i have a 3 season “pool house” in Massachusetts. I would like to use cement-board over the plywood floor and put an epoxy floor on top of that. Do you see any issues with this setup? How thick should I go with the cement board?
The cement board doesn’t count as structural so you can go with whatever size works for you. As far as the epoxy coating goes, you’ll have to check what their suitable substrates are.
Thanks man, much appreciated.
Hi, I am renovating my half-bath on the first floor currently. I am removing the existing 2×2 porcelain tile floor to install a penny tile.
1. The existing tile was installed approx. 30 years ago when the house was built, it seems like it was installed with a mastic instead of thinset over 1/2″ underlayment. The plan is to flatten/remove as much of the old mastic as possible. It is sort of flexible but feels like plastic, not sure what it is.
2. I purchased Ardex X5 and the Wedi anti-fracture waterfproof membrane. I was going to install the Ardex X5/Wedi directly on top of the underlayment after flattening/removing the old mastic. Was going to let that cure then install the penny tile and grout.
Couple questions I have are:
Does that seem like it will work and do you think the Ardex X5 will adhere to the existing mastic?
Any help is greatly appreciated!
This would probably work. A better way of doing it would be to remove the underlayment and install new 1/2 inch cement board or 1/2 inch plywood with the Wedi membrane over it.
I would recommend at least coating the mastic with Custom MBP or Mapei Eco Prim Grip to enhance the bonding capabilities. X5 might stick without but why risk it?
Thanks so much, I appreciate the quick response. I was thinking about replacing the underlayment with a cement-board, if it comes up easily enough that’s probably what I will do.
If not, and I just prime the mastic, should I be concerned about different manufacturers products working with each other? Do you think the MBP or Mapei will work without issues with the Ardex?
Again, thanks for the help!
I don’t think Ardex makes a bonding primer like MBP so there’s not a lot that you could do about that other than to change to Mapei mortar. Obviously, manufacturers want you to always use their line of products but it doesn’t always work to do it that way.
What type backboard should I use. Building a house and tiling the entire floor tung and groove OSB subfloor.
Whichever one you can get your hands on and feel comfortable using. Wonderboard, Durock, Permabase, and Hardibacker are all common brands of traditional backer boards.
Hi I have 1″x12″x12′ planks as subfloor. I want to put 18″x18″ tile on my kitchen floor. The joist are 24″ apart. Do I need to put plywood down before putting in the durock for the tile or will the durock directly on the planks be ok.
I think you’re going to have to not only put 1/2 inch exterior plywood down but I think you might have to go with a product like Ditra for the 24-inch spans. Are the planks tongue & groove? This might be one where it’d be a good idea to find out from an uncoupling membrane manufacturer like Schluter to see what they say.
So I’m doing my first DIY tile job and I’ve made some mistakes. I’m trying to figure out whether I’ve completely ruined the project and I should start over or whether its ok to keep going.
First mistake: When I installed the cement backer board, I found a pre-mixed container that said ‘thinset mortar’. After installing the cement backer board, I looked more closely at the product and it said it was not meant to be used for installing cement backer board onto plywood but only for tiles onto cement backer board.
Second mistake: I staggered the cement boards so they didn’t fall along the plywood lines, but I have four cement boards that almost meet at 4 corners. There is about a 1 to 2 inch stagger so they don’t meet exactly
Third mistake: I incorrectly mixed the thinset for taping the seams of the cement board together. I mixed the thinset and then applyed directly to the cement board without waiting 5 minutes as recommended by the manufacturer. I also did not add the polymer additive and only used water to mix.
Do you think I should scrap my project and start from scratch or I can keeping going and these mistakes aren’t too bad?
Thank you in advance!
I think you’re probably OK. At least one backer board brand says it’s OK to use organic adhesive underneath the cement board. Additionally, I’m not sure modified mortar is necessary for taping the seams.
I can’t predict whether something is going to fail, or not, but you may be OK with how you’ve done it so far.
With your help, I was finally able to complete tiling 1,000 sq ft of basement with large format wood-plank porcelain tiles:) So thank you!
I’ve now moved on to a number of floor-tile projects on the ground floor – small bathroom + entrance – as well as three bathrooms on the 2nd floor. And I have three questions:
From what I can see when I pull back carpet, the sub-floor is plywood of unknown thickness (I assume 3/4″ as the house was built in 1986), screwed into joists 16″ apart, centre to centre.
Having read the comments above, my understanding is that I need to apply thinset (I’ll use my large format Mapei thinset) to the sub-floor with a 1/4″ square-notch trowel, then screw in 1/4″ cement board (Wonderboard in my case) immediately and apply alkali-resistant mesh on the seams with thinset. Then I’ll be ready to apply my target format thinset with a 1/2″ square-notch trowel and my 5/16″ thick large-format tiles.
1) Can I use Mapei Large-Format Tile mortar to secure the cement board to the sub-floor?
And if I’ve done the math right, the tile surface should be <1/8" (compressed thinset) + 1/4" (cement board) + <1/4" compressed thinset + 5/16" (tile thickness) or about 7/8" in total height above the sub-floor.
2) Does that mat sound correct to you?
Assuming that's correct, my only concern is the issue of deflection and whether the floor meets the minimum requirements for being structurally sound to carry the additional weight of all the thinset, cement board and tile. I mean the house isn't that old, we haven't had any issues with bouncy floors or gaps, and I understand that there are no tools for measuring deflection. But I'm not sure what the sub-floor thickness is.
3) So do we need to add a 1/2" of plywood on top of the subfloor? My preference to minimise height disparity with surrounding wood flooring obviously is not too:)
Thank you in advance!
James Upton says
Hi Kevin and sorry for the late reply
Everything that you’ve spelled out sounds good. The steps with installing cement board, the thinset is just fine for installing cement board, and the math is correct.
In regards to the floor height, sometimes a wavy floor will dictate that the tile needs to be raised up higher in certain spots and one of those spots could be where it transitions to the next floor. But you won’t know any of that until you get going with the project.
Finally, I have no way of knowing if your floor meets structural minimums for tile installation. You can go over to the John Bridge forum where they have a deflection calculator and enter your floor information into that. But, if you have I-joists the calculator doesn’t work.
From what you’ve mentioned so far, plywood with 16 centers would be in the category of ‘so-far-so-good’.
If your floor meets the minimum requirements for ceramic or porcelain tile (L/360) then there is no need to add 1/2 inch plywood underlayment. However, you can always add it if you like.
Kevin Aoran says
I installed the cement board in a small bathroom with thin set underneath and screws. But I seemed to have messed up by leaving the old toilet flange in place and cutting the cement board so that it sits around the existing flange, instead of removing the flange, putting cement board up to the pipe, so that I can tile up to the pipe and have the flange rest on the tile.
Any suggestions as to how I can fix this without removing the cement board and starting all over again?
Sorry upon further research, it sounds like I could use a closet flange spacer to make up any height issues. But wanted to confirm that as long as the flange is above or below the finished floor by about 1/4″, the wax gasket should be ok.
Also there is a gap between the flange and the cement board. Do you have any suggestions as to how to prevent water getting between the flange and the cement board/mortar/tile?
James Upton says
You’re on it! The spacer, or extender, is what you need for that. With a renovation, it’s pretty common to have the flange directly on top of the subfloor.
For the gap, you could fill a space with thinset mortar. The spacer should provide a seal so that water doesn’t escape past the flange. Then when you silicone the toilet to the tile, that will prevent water from getting in underneath the toilet.