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.