Waterproofing shower walls and shower pans is a critical step in every shower remodel. But creating a moisture resistant and waterproof membrane around your shower to protect your home from potential water damage is not an easy task, and involves a few traps that can be avoided with the right advice.
In this post, I’ll go over the 7 biggest mistakes to avoid when waterproofing shower walls and pans. And at the end of each section, I’ll give you a quick solution based on industry best practices, and/or a workaround to avoid the mistake.
I’m Steve Gehrmann, with Redblock Industries (Steve’s Bio). I’ve been remodeling bathrooms and waterproofing showers for 15 years.
Whether you take on this responsibility yourself (DIY) or hire a contractor to do it, you need to be aware of these pitfalls to ensure this job is completed skillfully and competently.
Ok, let’s get into it!
Topics Covered
Waterproofing Shower Walls Mistakes:
- Installing Foam Backer Board on an Outside Wall
- Using Liquid Waterproofing
- The Water Vapor Sandwich
- Shower Niche in the Wrong Place
Waterproofing Shower Pan Mistakes:
- Installing Old School Waterproofing
- Combining Two Systems
- Installing the Tray too Early
The Importance of Waterproofing Shower Walls
Waterproofing shower walls is a crucial step in any tile shower to protect the structural integrity of the shower wall and your home in general. It acts as a protective barrier against water and moisture penetration, which, if neglected, can lead to water leaks and/ or condensation in the shower walls and floor.
This, in turn can lead to substantial structural damage, mold and mildew growth, and and costly repairs. The growth of mold can also lead to secondary health risks from inhalation of mold spores, which can compromise your’s and your family’s health.
The 7 Common Mistakes
1. Installing Foam Backer Board on an Outside Wall
When waterproofing shower walls, foam tile backer board panels are always a compelling option. They speed up your shower build and provide decent waterproofing without any extra steps.
But as great as they are in some situations, it’s just a bad idea to rely on them as your only waterproofing layer when your waterproofing an outside shower wall. The waterproofing capabilities of foam backer board panels are simply not adequate for the temperature and moisture changes that occur in an outside wall, regardless of the climate.
I always recommend a perm rating (moisture permeability rating) of around .2 perms (or less) when waterproofing shower walls that are outside facing.
2. Using a Liquid Membrane
If you’re waterproofing shower walls, it’s a mistake to use common liquid waterproofing membranes (like RedGard) for shower walls.
The first reason is because they basically require perfect application. The waterproofing integrity of this membrane relies on an even thickness of material applied uniformly across the entire surface of the shower, and I’ve found that it’s really difficult to achieve this with a paint roller.
In my experience, perfectly uniform application is difficult to achieve even if you’re skilled with applying this type of membrane.
As well, most liquid membrane application instructions suggest that it’s not necessary to apply reinforcing tape in the corners and seams. This questionable advice also contributes to thin areas in the membrane as the liquid tends to recede into gaps and crevices.
There is also a tendency for pinholes to form in the surface of the membrane as it dries. This is mainly due to air bubbling from the backer board when it has not been appropriately sealed (bonded) before application.
Because a uniform thickness is critical to maintaining a good perm rating, and liquid membrane cannot achieve this, they are only appropriate for inside shower walls. This is the second reason why using a liquid membrane is a mistake you should avoid.
The temperature variation on each side of an interior shower wall is generally quite small, so the moisture condensation risk within the wall cavity is low.
Solution
To prevent bubbling when waterproofing shower walls with a liquid membrane, apply a bonding agent to the surface of your cementitious backer board before applying the liquid waterproofing membrane.
I use Weldbond for this purpose (pictured above) because it’s easy to use, non-toxic, economical, and readily available.
You simply dilute it with water (follow the directions for using as a bonding agent), and apply one coat to the tile backer surface using a roller and brush. Make sure you cover the entire surface where you will be applying your liquid waterproofing.
You should also make sure to follow the directions and apply a second coat, or even a third coat to ensure that the thin areas still have adequate moisture protection.
3. The Water Vapor Sandwich
A ham & egg sandwich makes great lunch – A water vapor sandwich on the other hand, makes neither a good lunch nor a good waterproofing strategy…. 🙂
When waterproofing shower walls, it’s a pretty good bet that at least one of those walls will be an outside wall. And outside shower walls need A LOT of extra consideration when you’re planning your waterproofing strategy.
Attached to the stud surface of this outside wall is often a layer of vapor retarder plastic sheeting, designed to reduce moisture transmission into the stud wall. This is perfectly fine if the wall in question is not a shower wall, but not so good if it is.
A big problem is created if this plastic sheet is left in place when you’re also installing a waterproofing membrane on the tile backer board. This creates what is referred to as the “water vapor sandwich“. Although it is important to severely limit moisture from making it into the wall, this two layer approach is not the correct way to accomplish this.
This approach allows moisture to become trapped between the two layers and condenses into water on the vapor retarder surface and quickly rots out your cement board and eventually your wall framing when the plastic sheeting breaches (which it will).
The excessive moisture also causes mold growth, which accelerates the deterioration of the backer board, and framing, and eventually lead to de-lamination of the tile layer and failure of the entire wall assembly.
In other words, a terrible idea.
Solution
During the demo you should remove any vapor retarder plastic sheet or felt from the stud surface on all the outside shower walls. Then install a low perm “sheet” waterproofing membrane like Wedi Subliner Dry, Durock Sheet Membrane, or NobleSeal TS on the backer board surface. This should be the only waterproofing on this wall.
Installing only one waterproofing layer eliminates the water vapor sandwich problem altogether.
If this technique is used, and your outside wall is adequately insulated, any trace amounts of moisture that pass through this membrane can harmlessly dissipate within the stud cavity instead of being trapped by a plastic vapor retarder sheet.
If you don’t trap the moisture, there’s no condensation, rot and mold. Problem solved.
4. Shower Niche in the Wrong Place
When you’re waterproofing shower walls, it’s always a risky move to install a shower niche in an outside shower wall. A preformed foam shower niche can be a used as a great defense against long term leak issues into the shower wall, but they will never eliminate moisture from moving into the wall.
When a niche is installed into a 3 1/2 inch outside stud wall, this problem is compounded because the niche installation requires that all of the insulation be removed to make room for it in the stud cavity.
In an outside wall that’s probably already too thin for an adequate insulation thickness, the moisture moving into the stud cavity behind the niche has an extraordinarily high risk of condensing into water. This means mold growth, dry rot, and other moisture problems in the framing.
Solution
When you’re waterproofing shower walls, and you really want your shower niche to be installed into the outside wall, the best thing to do is to increase the thickness of the wall so more insulation can be installed behind the niche. Maybe not a practical option for some, but certainly the best option.
The bottom line is…. I would strongly recommend that you do not install your tile ready shower niche in an outside wall.
But if you insist on doing so, the best solution would be to install a shower niche made from material that is impermeable to moisture, like a finished stainless steel niche. If moisture can’t get through it, there’s much less risk of condensation behind it.
If you’d like to know more about the mistakes that are often made when installing a shower niche, check out my 7 BIG Shower Niche Problems post.
Shower Pan Waterproofing Mistakes
5. Installing Old School Waterproofing
In the old days, when they learned how to waterproof a shower, the waterproofing layer was always beneath the mortar base in all tiled showers. The objective was to protect the wood framing from water exposure, so it made some intuitive sense at the time.
This system is still referred to as a “water in, water out” because water is meant to flow around the mortar slab, then percolate out of the system via a sloped pan or rubber membrane beneath the base connected to weep holes in the center drain. The fundamental assumption is that water penetration through and around the mortar base is acceptable, as long as it eventually made its way towards the drain.
I can tell you from my own experience that this assumption is not at all acceptable and actually results in some serious problems… not sometimes, but always.
Even with a perfect subfloor “pre-slope”, the water never completely drains away, and gets drawn into the mortar pan via capillary flow. The mortar base does not get completely saturated due to the pre-slope, but this difference is inconsequential. The mortar base still becomes very well hydrated, almost to the point of saturation.
What happens next is pretty much common sense…the wet mortar base breaks down, and mold starts to grow.
I’ve seen this in countless demolitions, even if the subfloor has the perfect slope and the weep holes were not blocked. This result is a rank, toxic smell from the bi-products of the flexible liner membrane and cement breaking down, and mold… lots of mold and mildew growth. I wish the image below could relay the overwhelming smell of toxic mold in the air after I demoed this mortar base. Only a respirator with a gas rated filter can filter it out.
Meanwhile, the saturated mortar slab floats on this flexible layer (hot mop or pan liner), continuously moves and compresses with every step, creating cracks in your tile installation and compromising the integrity of your shower.
It’s utter insanity to think that this was ever a good idea to waterproof your shower floor, especially when there’s a proven, reliable undertile waterproofing system available that’s easier, quicker and cheaper to install.
Don’t make this common mistake!
6. Combining Two Systems
When waterproofing shower walls or shower bases, you SHOULD NEVER assume that since one waterproofing strategy for the shower base is good idea, than two strategies will be even better.
The “undertile” waterproofing membrane systems were created to eliminate the need for a drainage pan below the shower base. Period! Installing a subfloor pre-slope, shower pan liner and drain assembly is simply a lot of extra work with no benefit whatsoever.
In fact, if you’re using a sheet membrane over the shower base with this type of traditional drain assembly, you will be defeating the most important waterproofing element of this system. The two-step drain installed with a traditional shower pan is not compatible with a surface sheet membrane waterproofing system because the drain perimeter is not designed to attach to the membrane.
As such, there will always be a breach in the membrane around the shower drain if you install this type of drain instead of the integrated drain assembly that’s part of a sheet membrane system. So, if you decide to go ahead with this pointless exercise, water will get between the drain and mortar slab, allowing the mortar base to saturate, trapping water under the sheet membrane.
This may not result in any damage to your framing (assuming the under pan drainage is working properly), but it will have some interesting and undesirable long term effects, I can assure you.
Bottom line….. when waterproofing a shower, I would strongly suggest that you abandon the old school approach, and embrace the new under-tile bonded waterproofing approach. If you’re still committed to the old ways, I would suggest that you commit to it. In other words, never use both systems together.
Solution to the above two mistakes
The solution is simply to install a membrane bonding drain assembly into your dry pack mortar base instead of the old-school two-step drain. These new drain assemblies come with a membrane bonding flange around its perimeter designed to attach to the shower floor waterproofing membrane.
Then, when you waterproof your mortar base, you ensure that it overlaps the bonding flange around the drain, and the edges and corners are waterproofed according to the manufacturer’s directions.
That’s how to waterproof a shower pan, if it’s made of mortar, and you’re planning to tile it.
7. Installing the Tray too Early
When you install your prefab foam shower tray too early in the remodel process there can be problems. I’ve made this mistake a few times during the early days, before learning that they’re quite vulnerable to damage. I learned that it’s always best to install these foam pans only after most of the work has been completed in and around the shower walls.
When trades like plumbers, electricians and framers need to get into the shower area to do any modifications, they will stomp all over your delicate foam shower pan and basically destroy it. All it takes is for them to walk on it with there work boots to irreparably damage the surface and the integrity of the waterproofing system.
If you want a tiled shower, one tradesman that you won’t be able to avoid is your tiling contractor. However, it’s always the best practice to install tile on the pan before you install the wall tiles so you can reduce this risk.
And if for some reason the walls need to be tiled first, you’re probably still OK because most tiling contractors are well aware of the delicate nature of foam pans, and are therefore a lot more careful walking on it than other tradesman would be.
I should also be clear that I believe that the cement coated prefab shower pans are very good products and I don’t think they’re too delicate to be installed, even by Diy’ers. I’ve installed the Wedi Fundo Ligno and Fundo Primo dozens of times and I really do prefer them over mortar shower bases in many ways. I have simply learned that a bit more planning and vigilance is required to avoid this mistake.
The solution
First make sure that any framing, plumbing and electrical work that needs to be done in the surrounding shower wall is completed before you install your prefab foam base. This will reduce the number of people stomping all over it.
If you are installing cementitious tile backer board, you could also ensure that these panels were installed before the pan was installed to reduce construction worker traffic on the pan. However, most foam panel backers are installed as part of a shower system, which requires that the pan/tray be installed before the wall panels. As such, this strategy wouldn’t be possible with a shower constructed with prefab foam panel shower.
Regardless, you should always install the wall membrane (if applicable) and the shower floor tile before the wall tile gets installed, just to make sure the bare (untiled) foam pan/ tray gets as little traffic as possible. You can then install the tile on the shower wall without any worries.
Frequently Asked Questions
How do I choose the most effective waterproofing system for my shower?
The most effective waterproofing system will always be a sheet membrane with a low perm rating. You can easily get away with a liquid membrane if it’s an inside shower wall but most popular liquid membranes are not appropriate for outside facing shower walls. For outside shower walls my professional recommendation is Wedi Subliner Dry membrane.
What materials do I need to waterproof a shower wall?
If you’re installing a sheet membrane you’ll need the membrane itself, preformed membrane corners and seam tape, thinset mortar, a bucket, a mixing drill with paddle, 1/8 inch notched trowel, and a grout float or large flat trowel. If installing a liquid membrane, you need the membrane fluid itself, paint tray/ roller or paint sprayer, and some reinforcing tape for seams and corners.
What is the best way to waterproof shower walls before tiling?
As a remodel professional, I believe the best way to waterproof a shower with only interior walls is to install Wedi building panels. Install is fast and easy with “built-in” waterproofing so there’s no additional waterproofing step. If your shower walls are outside walls, the best way is to install a sheet membrane with a low perm rating like Wedi Subliner Dry, Kerdi DS, or Nobleseal TS.
Is a shower membrane necessary for waterproofing, and how is it installed?
Yes, a shower membrane is always recommended if you want to protect your home from moisture and water damage, dry rot, mold, mildew etc. If installing a liquid membrane (LM), you’ll need a paint roller, some reinforcing tape and some careful application. Sheet membrane install requires a bit some skill with mixing and spreading thinset mortar, but application technique is not as meticulous as with the LM.
Are DIY waterproofing methods reliable for beginners?
DIY waterproofing is OK as long as you follow the manufacturer’s instructions carefully. Both sheet and liquid waterproofing techniques require a bit of skill to apply correctly so make sure you practice on a section of scrap backer board before you tackle the project.
