Shower leaks. The last thing anyone wants when they build a new shower. And yet I hear the following questions quite often:
- Is cement board waterproof?
- The tile and grout will keep the most of the water out, right?
- My contractor didn’t put any waterproofing in our shower. Will this cause shower leaks?
- We didn’t know we were supposed to have a shower moisture barrier. Should we start over?
What inspired this post was that recently, on the John Bridge forum, a member had asked a simple enough question: Is a moisture barrier required? He had hired a contractor to install his shower and the contractor said that it wasn’t necessary. But as more information came out- and photos were posted- it was obvious that there were going to be some shower leaks. I would encourage you to click the link and skim over the post.
This might be news to some but let’s get a couple of things out in the open:
Cement board is not waterproof. Hardibacker is not waterproof. Tile and grout are not waterproof.
Cement board will not fall apart when exposed to water. But like a concrete walkway, it can soak up water. It can also dry out and be perfectly fine.
What is a moisture barrier?
A moisture barrier is essentially a waterproof barrier. Its purpose is to keep moisture from getting through the backer board and into the wall. Acceptable forms of waterproof barriers for a shower are:
- 4 mil plastic sheet behind the tile backer board
- liquid waterproofing on the surface of the tile backer board
- a waterproof sheet membrane on the surface of the tile backer board
Shower Waterproofing Crash Course
I have a hard time calling a 4 mil plastic sheet behind backer board a waterproof membrane. Why? Because you attach this membrane by stapling it to the studs then puncture it further by nailing (or screwing) backer board in front of it. But it is an acceptable, and minimal, way to keep moisture from getting into a wall cavity.
So here’s where I draw the distinction between a moisture barrier and a waterproofing membrane:
- Waterproofing for vertical walls is a moisture barrier
- Horizontal surfaces, even those that slope, need a waterproofing membrane.
Moisture Barrier vs Waterproofing
Now I’m not the only one that makes a distinction between waterproofing vertical and horizontal surfaces. Custom Building Products requires two coats of their Redgard liquid waterproofing on all waterproof surfaces. However, they also have this technical bulletin which says that only one coat is needed on the vertical surfaces of the backer board.
It’s sort of like: All bourbons are whiskey but not all whiskey is bourbon. Waterproofing can serve as a moisture barrier but a moisture barrier doesn’t necessarily qualify as a waterproofing barrier.
A moisture barrier is all that’s needed to keep water out of the water cavity. But more extensive waterproofing is required for the horizontal surfaces if you want to prevent a shower from leaking.
Where you are likely to have shower leaks
The horizontal surfaces in a shower usually include the shower floor, half walls, benches, curbs, and recessed niches. These need special attention to ensure that they don’t leak. Acceptable ways of waterproofing these types of surfaces are 2 coats of liquid waterproofing, a waterproof sheet membrane, or the many different foam waterproof products that are available now. They make foam recessed niches, corner benches, floating benches, curbs and shower pans.
Here’s the Point
The whole point of this post is to get to this bottom line:
Water containment on shower walls is important and a requirement. Waterproofing on horizontal surfaces is absolutely critical.
If your shower doesn’t have a moisture barrier failure isn’t necessarily imminent. But I wouldn’t consider it to be a well-built shower. If your horizontal surfaces in the shower aren’t waterproofed a leaking shower is nearly guaranteed!
Mary Sue Butch says
My shower has a regular sheetrock ceiling, which developed black mold. I washed it with a mixture of vinegar and borax, which cleaned the surface but left a stain. Can I tile over this to prevent future mold? Or do I have to replace the sheetrock with backerboard?
DIYTileGuy says
You shouldn’t have any issues sticking the tile to the sheetrock on the ceiling.
Bill White says
In simple terms please. Redoing bathroom. Shower/tub alcove is cement board duralok. Behind cement board I put plastic what next. On other wall do I need plastic first then cement board. Now do I need to do more on walls with tiling half up not near shower. Also floor. Redid subfloor what goes on before cement board thin set and tiles.
DIYTileGuy says
I’m having a hard time understanding what’s being asked. Please check your questions and ask again.
Chris says
I am putting tile down in my kitchen and will be putting Durock and tile in the dishwasher alcove. The alcove is part of the center island that has the sink as well. The subfloor is exposed next to the alcove. My concern is that if the dishwasher should ever develop a leak and water gets out, or the sink over-flows, etc… if water gets in the alcove and thus to the exposed Durcok, will this cause a warping or mold issue with the Durock and subfloor? Should we put a liquid water barrier like Redgard down on the Durock and sub-floor before the tile is applied and the dishwater reinstalled? Thanks for the input.
DIYTileGuy says
It certainly wouldn’t hurt but it’s only effective as long as water is contained in the area of the dishwasher. So it’s very likely that water would simply run off of the waterproofed area unless more of the floor is waterproofed and it’s flashed up any vertical surfaces.
Rob says
Can green board be used anywhere inside shower area including ceiling?
DIYTileGuy says
You can use it on the ceiling.
barrett says
I am remodeling an old house and am in the middle of re-doing the master bathroom shower. The original mud deck and mortar layer on walls are over chicken wire but I doubt it has any moisture barrier or waterproofing layer. I have removed the old tiles and want to install new large tiles with CBP LFT modified mortar. I would like to use RedGuard over the original mortar bed and wall layer to provide an improvement to what is currently there. Do you see any issues with this strategy?
DIYTileGuy says
For whatever reason, I’m reluctant to say that it’s OK but I can’t think of a reason that it wouldn’t be. The walls can be coated with Redgard but I wouldn’t put any on the shower pan.
The shower pan is either waterproof right now, or it’s not. There’s no way to tie Redgard into the drain with the way the old shower drains are designed so covering the pan with Redgard doesn’t fix anything. If anything, it can have an adverse effect by trapping moisture between the two waterproofing layers- the layer below the mud bed and Redgard above. But doing the walls is a good plan, I think.
One word of caution: the old mud beds were usually hot mopped or maybe it has a copper or lead shower pan. It’s not unusual at all for those to be compromised at this point in their life. So the best course of action would be to remove and start over.
Rafael Alvarez says
Hello.I recently renovated my kids shower/tub and only did one coat of Redgard on the vertical walls above the tub.Will this be enough or will I have an issue?
DIYTileGuy says
It’s more about thickness than coats but for something like that I think you’ll be ok.
Keith says
Hey Tile Guy,
Noon question—I recently finished building a tile shower floor and foolishly did not first lay a barrier. Is there anything I can use after the fact to seal it up? Anything like caulking the grout lines helpful? 🤞
DIYTileGuy says
If you don’t have a waterproof membrane in the shower then the best thing is to start over. All that caulking the gaps would do is slow down a leak and when it comes to water damage the slow leaks do the most damage.
Nancy says
Hello Tile Guy,
First, many thanks for all the time you put into your very helpful website. I’ve learned lots and my home renovation projects are better for it.
Second, our contractor agreed to prep a new walk-in shower (I will be doing tile and finish plumbing). The space was formerly a tub/shower combination. Contractor did a presloped pan, plumber did rough plumbing and rubber membrane, then contractor finished the pan, installed a curb, and hung Denshield, taping the abutting joints.
My concern is: his installation of the Densheild doesn’t look much like the manufacturer’s diagram I’ve since seen. In my shower, the Denshield comes down to meet the horizontal plane of the sloping floor base (withing 1/8″). In the Densheild diagram, they show the sides of the pan as an elevated rim around the perimeter (similar to the height of the curb). The diagram shows how this rim elevates the bottom edge of the Densield a couple inches above the horizontal plane of the floor. This isn’t what I have to work with and I’m concerned about longevity of the backing material.
I know Densield is less prone to wicking and I know I can/should seal the small gap at the bottom of the backer boards, but I’m hoping I can add an insurance policy. I’m using redgaurd with a crack isolating fabric to fully waterproof the curb and areas more prone to leaks like the windowsill (vinyl window). Could I use this same method where the Denshield meets the floor?
DIYTileGuy says
In my opinion, Denshield is a pretty minimal product, quality-wise, to have in a shower like this. Consequently, now there’s a sort of “belt & suspenders” fix so that everyone feels better about things.
Here’s the main issue: the shower pan method that was used is designed to work in a certain way. Trying to patch things up with more waterproofing, although it sounds advisable, can actually be counter-productive and trap water in.
Water will get into the mud bed (top layer) and then travel so that it’s underneath the corner that you are wanting to install the waterproofing. Same thing with the shower curb (hopefully your contractor didn’t screw Denshield onto the curb). As you can see, waterproofing these corners isn’t going to accomplish what you want.
So, at this stage, the shower pan either works or it doesn’t. If it doesn’t work, then it needs to be torn out, or repaired, until it functions properly. Hopefully, a water-test was done on the shower liner to make sure that it doesn’t leak. But I don’t recommend a band-aid approach to the shower at this point.
With that being said, the Denshield needs to be waterproofed at all seams and penetrations and I would definitely use the Redgard waterproofing fabric around the window.
Zach says
Hello Tile Guy,
After my contractor finished and left I realized he did a couple things wrong with the shower:
1) the pan was sloped however after I turned on the shower I realize there is a low spot that collects water. I can just sweep the water up and into the drain, so this is more of an annoyance, but so you see a major issue with this? I’ve contemplated having just the area tiles ripped out, slope corrected, and tiles reinstalled. I think this might work bc the waterproofing is redgard uncoupling mat which probably wouldn’t rip (more concerning question on redgard mat below).
2) more concerning issue. My contractor used redgard uncoupling mat. It’s the thicker “waterproof” barrier fabric. After I did some research (after this was finished of course), I realized this is meant for general tile floors or even shower walls, but not the shower floor. It’s clearly too late to do anything about it. The material is waterproof after all. Do you think I am doomed to have a failure/ leak/ mold? We have small hex marble tiles. I’ve applied two coats of floor sealer and all seams are caulked. Would of course prefer to not have to have the floor ripped up as it is expensive tile, our contractor is from out of town and not coming back, and based on labor costs in my area ripping out the floor and doing it again would be a couple thousand.
DIYTileGuy says
I would definietly fix the corner and hopefully it won’t hurt the waterproofing below. But by repairing the tiles in the corner you can see how the corners of the waterproofing were done.
One of the things about the uncoupling membrane is that it’s dimpled and that would provide a little bit of an impediment to attaching waterproof seam tape to it and running it up the wall. I’m assuming that’s how it would be done- the mat on the floor and seam tape folded in the corners.
I’m not saying it couldn’t be done but it would have to be fussed over a little bit during the installation.
Hopefully, everything with the shower is waterproofed OK and there’s nothing to worry about.
Zach says
Thanks for the reply! That is somewhat a relief, about the uncoupling membrane being usable. The spot that isn’t sloped properly/ puddles is actually in the middle of the shower floor near the side of the shower step (the drain is all the way on the right side). So if we popped those tiles out and built up the floor it would not affect the seam, but would be just the uncoupling membrane may only. I’m thinking the membrane would be fine if we popped those tiles.
Zach says
Also, unfortunately I already have a leak. The grout on the shower curb has cracked and water seems to have gotten behind the tile and around to the side of the step of the bathroom floor. Water leaks out of the bottom of the curb at the bottom of a grout joint. At this point I’m thinking we’re going to have to completely redo the step, and some of the shower floor tile, as a best case scenario. Re-grouting and caulk would just be a bandaid. Our originally contractor just used pressure treated wood for the step with no waterproofing. I wish I had been more educated while he was doing this but now I know. He made some major mistakes.
DIYTileGuy says
That is unfortunate but at least you have caught it before it does major damage. As an FYI, you don’t want to use pressure treated wood in a shower. It seems like a good idea on the surface but what happens is it dries out and twists. Normal wood is the way to go.
You might look for a contractor that has taken training from one of the companies that sells a complete shower system. Laticrete, Wedi, and Schluter are probably the three most popular names.
As a rule of thumb, the shower should be waterproof before tile is installed. You never rely on tile and grout to keep water out.