Tile Shower Waterproofing Basics

Good Shower vs. Bad Shower, what’s the difference?

Bad Tile!
Not a great sign…

A chain-smoking, whiskey-drinking, world-weary 1950s private detective would say there are two kinds of custom walk-in showers in the word — ones that leak, and ones that don’t.

It is a little more complicated than that nowadays. But, not much more complicated. Shower construction materials are more sophisticated now than in the 1900s. Yet there are still plenty of old (or just old-school) tile installers and renovators who still make showers like it’s 1950. And, unfortunately, there are also plenty of shady installers who specialize in making showers ‘guaranteed’ to last all of a year or two.

So, here’s a taxonomy for four different kinds of tile showers:

  1. Ones that obviously leak badly (either quickly through a subfloor to the ceiling below, or more slowly into adjoining wall cavities).
  2. Ones that obviously don’t drain properly (because of a badly sloped base that leaves pools of standing water on the shower floor).
  3. Ones that do drain okay and don’t obviously leak, but that still always smell a bit musty and grow surface mold.
  4. Showers that drain great, will not ever leak, and that stay bone-dry between uses.

This post gives a detailed description of what the difference is between shower types 1 through 4, with pics and tips.

If you’re looking for a shorter explanation of traditional custom shower construction and types of failure, then here’s a good brief but thorough overview from the website “Angie’s List”:


But if you’re game for the long descriptive haul, then read on!

How to Properly Construct a Traditional Shower

Before the 1990s, there simply weren’t any waterproof materials that you could directly tile onto effectively. Therefore, the only option for making a leak-proof shower base was to put a waterproof PVC vinyl liner (or, even better, a made-in-place fiberglass liner) underneath a layer of packed concrete.

Old school waterproofing goes like this: first a thin concrete layer is put directly on top of the subfloor (either plywood or concrete slab) to create a pre-slope, then a PVC or fiberglass liner is installed, then another layer of concrete dry pack goes on top of that. Then, finally, the shower floor tile and grout is installed.

The final result looks like this:

Cutaway View — a proper traditional shower base!

Understand that tile and grout are not waterproof. Every time you shower (using ~20gal of water total for an 8min shower at 2.5gal/min), some water goes through the tile floor (mostly through the grout) and into the cement mortar bed. The cement mortar bed then (hopefully) gets rid of this moisture before the next use through simple evaporation back through the grout and by wicking and eventually draining through “weep holes” in the part of the shower drain that you don’t see.

Shower Drains — the part you (hopefully) don’t see!

The shower drain opening that you do see when you look down is actually only the very top part of a traditional shower drain. There’s a ~1.5in section between the visible top of a traditional shower drain and the waterproof liner or fiberglass pan below.


This buried section of the shower drain has little weep holes that (ideally, hopefully) allows the moisture absorbed by the shower base concrete to eventually weep into the drain hole.

So, even a periodically very wet cement shower base will eventually (ideally, hopefully) dry out between uses.

How Traditional Shower Construction Can Go Bad

Again, remember that the whole point of traditional shower construction was to work around limitations from the previous century. Before the 1990s, there weren’t any waterproofing materials that you could properly install tile directly onto. And, tile and grout aren’t waterproof. Therefore, the best solution for waterproofing a shower base was to put a waterproofing membrane (PVC or fiberglass) underneath a sloped concrete layer.

Unfortunately, this makes for lots of install steps that all have to be done correctly for the entire thing to work properly. In engineering terms, there are lots of failure points. Here’s a list:

Waterproofing membrane (PVC or fiberglass) has no preslope

Remember that traditional shower base construction is designed for moisture to be absorbed by the concrete layer underneath the shower floor tile. This moisture then evaporates back out through the grout and also drains out through weep holes in the sides of the shower drain. However, if the waterproofing layer isn’t sloped at least a bit towards the drain, then the moisture in the concrete won’t effectively move towards the drain’s weep holes.

This results in a shower base that takes longer to completely dry out, which in turn results in a musty shower that grows mold.

Drain weepholes are blocked

Pretty straightforward. If you block the drain holes that water absorbed by the concrete layer is supposed to drip out of, then your shower base won’t dry out as quickly as it should. Unfortunately, some shower installers will just pack the concrete right up tight underneath the drain which blocks the weepholes.

Keep ’em Open — pack the drain base with pea gravel before packing with concrete!

To make sure the weepholes stay open, the installer needs to pack some small pea-sized gravel around the drain base. This ensures that the dry pack concrete doesn’t block them.

Waterproofing membrane has open seams, tears, or is just a bit old

Ever covered a box in giftwrapping? Kinda annoying to get all the seams right and the final gift looking perfect. Now, imagine trying to seamlessly wrap the INSIDE of a box. Covering a custom shower floor in PVC liner is kinda like that. Here are some pics to illustrate:

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Even a careful PVC liner install will leave folds at the corners and a big circular cut around the drain. Also, most installers will use several PVC pieces to make the install easier. If these seams (and corners, and drain flange opening) aren’t carefully glued shut, then your shower will leak.

And finally, flexible PVC plastic can be folded in the first place only because it’s been made malleable with plasticizing additives. Ever try bending PVC pipe? It doesn’t. Flexible PVC liners are flexible because the cross-linked long-chained vinyl plastic molecular chains have been baked with additives that break some of the molecular bonds to make the material flexible. Problem is, these additives aren’t chemically bonded to the PVC molecules. Like the solvents in paint, they gradually evaporate and leach away over time. This leaves the PVC brittle and prone to cracking with age.

So, after a decade or so, even a perfectly installed PVC liner can simply crack due to shifting from temperature changes or some foundation settling movement. Bad luck that.

Waterproofing membrane is too shallow

Concrete base needs to be several inches thick — it’ll go right to the top of this (NOT)waterproof liner!

Remember that a PVC liner is basically a waterproof tub that needs to cover everything underneath the shower floor and extend at least 6-in or more up the shower wall. If the liner doesn’t cover at least the base of the shower walls, then water will wick up into the wall framing from the concrete shower base.

Think of it this way — would you take a bath in a tub that was only a couple inches deep? That’s kinda the same thing. If the concrete base of a shower floor extends all the way up to or near the top of the waterproofing liner underneath, then moisture absorbed by the concrete will overtop the waterproofing layer and get into the walls. Which will cause a lot of problems.

Waterproofing membrane is installed badly or wrong(ly)

On the subject of wicking, remember again that tile and grout are not waterproof. This includes the tile and grout that gets installed on shower walls. Some water will get absorbed by the wall tile grout and migrate into the wall tile substrate. That’s why it is important to waterproof the shower wall substrate too!

Three things wrong with this install — NOT how to do it!

But even a nicely waterproofed shower wall substrate won’t help if the shower base liner isn’t properly installed behind the shower wall substrate.

Think about it — would you take a shower in a tub with your shower curtain flapping around outside the tub? Of course not. You’d tuck the bottom of the shower curtain inside the tub so as to not end up with a big puddle of water on the bathroom floor.

If the shower pan liner is installed on top of the shower wall substrate, then that’s exactly the same as leaving your shower curtain flapping around outside the tub.

Water will wick down behind the liner and into the wooden framing. Eventually, you’ll get a moldy dry-rotted mess like the shower framing pictured here:

Demo reveals this shower base was actually a MUSHROOM farm!

Waterproof with MODERN Materials!

There are some other things that can go wrong with a traditional construction shower install, but hopefully you get the point. Making a ‘waterproof’ rube goldberg -like contraption with a half-dozen possible points of failure is not the best strategy for a leak and mold -proof shower that will last. Instead, it would be best to put a continuous waterproofing layer directly behind all the tile in a shower.

This is easy to do today using two materials that came on the market in the 1990s — Kerdi waterproofing fabric and RedGard (or AquaDefense or HydroBan depending on the brand name) paint-on waterproofing.

Kerdi waterproofing

Kerdi is a polymer sheeting that will adhere to just about any surface (concrete, cement backerboard, drywall, even plywood) using plain unmodified thinset. More important, tile is installed directly on Kerdi. And, when Kerdi fabric is overlapped by a few inches, it creates completely waterproof seams since the fabric is actively hydrophobic.

This means you can “wallpaper” the interior of a shower space in a continuous layer of waterproof Kerdi and then tile directly on it. With the Kerdi bonded directly to the shower drain, any moisture that seeps through the tile and grout simply runs down behind the tile directly to the drain.

RedGard waterproofing

RedGard is just one common brand name (other brands like HydroBan, etc, are basically the same thing — like Coke and Pepsi are both colas) for a paint-on waterproofing material that also bonds to concrete, cement backerboard, drywall, and even plywood.

The technical term for this material is “elastomeric waterproofing membrane”, which is basically a fancy way to say “rubbery paint that dries to form a plastic-like waterproof barrier”. It looks like this when dry and ready for tiling:

Now THAT’s some good wall waterproofing!

Just like Kerdi, tile goes directly on top of RedGard waterproofing. So, it has the same advantages of putting a truly waterproof barrier directly behind the surface tile and grout.

While elastomeric paint-on waterproofing is technically code-approved to use for waterproofing shower floors too, most tile pros recommend using it only for walls. As the paint-on waterproofing dries and cures, it contracts just a bit which can cause pinholes or small cracks. This is why a good installer will put two coats on shower or tub enclosure walls, even though some small pinholes or cracks aren’t a big deal for shower walls made from concrete backerboard. But even a small chance of pinhole moisture leaks is unacceptable for a shower floor.

Elastomeric paint-on waterproofing is also technically code-approved for waterproofing plain drywall for shower or tub surround walls. Again though, most tile pros would only use cement backerboard for shower and tub surround walls intended for RedGard waterproofing. A small amount of moisture absorbed into a small patch of cement backerboard isn’t a huge deal. Even a small amount of moisture periodically absorbed into drywall is a big deal though. Cement backerboard doesn’t fall apart when wet, and cement isn’t food for mold. Drywall does fall apart when even moist, and drywall is fantastic food for mold.

The best of both

RedGard is easy to apply and perfectly safe to use for waterproofing the concrete backerboard underneath shower wall tile but not 100% foolproof for waterproofing a shower floor. Kerdi fabric, on the other hand, is a bit of a pain to hang on walls but absolutely guaranteed bulletproof for waterproofing shower floors.

So, why not combine both:

That’s exactly what I do for shower installs with great results. Using Kerdi for the shower floor waterproofing gives great peace of mind, and using RedGard to waterproof the cement backerboard walls saves time over hanging Kerdi throughout.

Final Tip — Don’t Panic!

Shower construction can seem complex and confusing at first. But it really just boils down to commonsense and knowledgeable installation. For example, don’t hold back on the questions when you’re interviewing potential shower installers. Ask ’em what kind of waterproofing strategies they use, and why. If they respond with something that’s confusing or that simply doesn’t sound right, ask ’em to explain.

A knowledgeable and conscientious shower installation expert should be able to clearly explain what materials they use and why. A dopey sloppy installer, on the other hand, will usually fold pretty quickly under questioning.

Just remember — if you’re talking to a tile installer who can’t outsmart water, then they’re probably not going to seem very smart to you!