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Shower Waterproofing Guide: Pans, Sheet and Liquid Membranes, Foam Board, Bonded Drains, the Flood Test, and the Six Places Showers Actually Leak
Flooring & Tile
Shower Waterproofing: Tile Is Not Waterproof, Grout Is Not Waterproof, Cement Board Is Not Waterproof, and Here Is What Is
Every shower I have torn out of a house built before about 2005 had the same thing behind the tile: damp, and under the damp, rot. Not because the tile failed. Because tile and grout were never the waterproofing, and nobody had put any in. Water goes through a grout line the way rain goes through a sweater: slowly, and completely. The membrane behind the tile is what keeps it out of the wall, and the sloped pan under the floor tile is what gets it to the drain. This page explains the systems that work, what each one costs, and the six places where even a good shower is likely to leak.
The job in one paragraph
A shower has to do two things with water: keep it out of the walls and floor structure, and get what lands on the floor to the drain. The first is a continuous waterproof membrane, over the whole floor, up every wall to above the shower head, over the curb and into every niche and corner, with no holes in it. The second is a floor sloped 1/4 in. per foot to a drain that is bonded to that membrane, so water that gets through the tile and grout runs down the membrane and into the drain instead of into the subfloor. That is all. The standards for bonded membranes (ANSI A118.10) and the Tile Council handbook details for showers exist because it is simple to describe and easy to get wrong.
The systems, compared
| System | How it works | Cost on a 36×60, walls | Notes |
|---|---|---|---|
| Traditional mud pan with vinyl liner | A sloped mortar preslope, a PVC or CPE liner clamped into a two-piece drain, a second mortar bed on top, tile on that. Walls: cement board with a poly or felt behind it, shedding to the liner. | Pan about $900; walls $0 to $2 if you count the poly | Fifty years of showers were built this way. It works when the preslope is there. When it is not, the mortar bed over the liner stays saturated and the grout grows mould. Dying out because it needs a mud man. |
| Sheet membrane (Schluter Kerdi, Mapei Mapeguard, Laticrete Hydro Ban Sheet) | A polyethylene sheet with fleece faces, thinset to the walls and floor over any solid backer, seams overlapped 2 in. and banded, pipe seals and corners preformed, drain with a bonded flange. Tile goes straight onto the sheet. | $6.00 per sq ft of wall, $570; pan $900 | What most careful setters in Ontario use now. Thin, testable, no cure time to wait on. The seams are the skill. |
| Liquid membrane (Mapei Aquadefense, Custom RedGard, Laticrete Hydro Ban) | Painted or rolled on over cement board or a foam tray, two coats to a specified thickness, fabric in the corners and over screws. Dries to a rubber skin. | $6.00 per sq ft of wall, $570 | Easy to see where it is and is not. Easy to apply too thin; the mil thickness on the pail is the whole product. Needs 24 to 72 hours before flood test. |
| Foam board system (Schluter Kerdi-Board, Wedi, GoBoard) | Extruded foam panels with a waterproof face replace the cement board; screws with sealing washers; seams and screw heads sealed with the maker’s sealant or band; a matching foam tray with a factory slope for the floor. | Within the $6.00 wall rate; pan $900 | Backer and membrane in one layer, light, square, fast. What I use on my own houses. The sealant at every seam and screw is the whole system; miss one and the foam does not care but your joists will. |
Any of the last three, installed per the manufacturer and flood tested, is a good shower. The vinyl liner done properly is too; done without the preslope, which is most of them, it is a sponge in a bag.
The drain decides the pan
A clamping-ring drain pinches a vinyl liner between two cast pieces with bolts. The weep holes in the ring let water that soaks into the top mortar bed drain down; if the preslope is missing or the weep holes are mortared shut, that bed never dries. A bonded-flange drain has a wide flange that the sheet membrane or liquid membrane sticks to directly, so the waterproof layer and the drain are one continuous surface with tile glued to the top of it. There is no mortar above the membrane to stay wet. That is why the pan can be flood tested before the tile goes on, and why the floor tile sits in a quarter inch of thinset instead of two inches of mud.
Linear drains are bonded-flange drains stretched into a trough, and they let the floor slope in one plane toward a wall, which is what makes large tile and curbless floors possible. They cost more and are part of the curbless shower conversation.
The flood test
Plug the drain with a test plug, fill the pan to within an inch of the curb top (or to a marked line on a curbless floor), mark the water line with tape, and leave it 24 hours. Come back. If the line has not moved and the ceiling below is dry, the pan holds. If the line has dropped, something is leaking and you have found it for the price of a day instead of a bathroom. The test is done before the wall tile and before the floor tile, because afterward there is nothing to look at. Any setter who tells you a flood test is unnecessary with his system is telling you he has not been back to his old jobs. The inspection checklist has the photograph you want of this.
Where showers actually leak
- The curb. The top of the curb gets screwed through to hold the tile backer, or the membrane stops short of it, or it slopes toward the bathroom. The membrane wraps over the curb, no fasteners through the top, and the top slopes 1/4 in. toward the shower.
- The niche. A hole cut in a waterproof wall, with four inside corners and a shelf. Preformed niches or foam board wrapped and banded; the sill tiled with a slope out so water does not sit.
- The drain flange. A clamping ring not tightened, weep holes clogged with mortar, or a bonded flange with thinset skipped under the membrane. The one leak you cannot see from anywhere.
- Inside corners and the curb-to-wall joint. Where three planes meet, a sheet membrane needs a preformed corner; a liquid membrane needs fabric. A pinhole here is a leak.
- Penetrations. The valve, the shower arm, the grab bar screws, the glass installer’s anchors into the curb. Pipe seals on the first two, blocking and sealant on the third, and tell the glass company to drill the wall tile, not the curb top.
- The wrong backer. Green board or regular drywall behind the tile. It was common until the 1990s and it is still turning up on quotes. Why green board belongs outside the shower, not in it.
What it costs and what it saves
On the $3,800 mid-grade shower, the waterproofing is $570 on the walls and most of the $900 pan. Roughly $1,300 of a $3,800 shower is the part you will never see. The alternative is a $2,500 shower that costs $12,000 to $20,000 in year eight when the subfloor, the joists below and the ceiling under it come out. I have priced that job more than once, and the homeowner always has the same look.
Questions people ask
Is cement board waterproof?
No. It is water-tolerant: it does not rot or swell. Water goes straight through it to the studs. Cement board needs a liquid or sheet membrane over it, or a poly sheet behind it draining to a pan liner the old way.
Is grout waterproof?
No. Cement grout absorbs water and passes it through within minutes of the shower starting. Epoxy grout is close to waterproof as a material but the joint between grout and tile is not. Sealed grout slows it down. The membrane behind the tile is what keeps water out of the wall.
Sheet or liquid membrane: which is better?
Both work when installed to the manufacturer’s spec. Sheet is more consistent in thickness and can be flood tested the same day; liquid is easier in odd shapes and around existing plumbing but depends on getting the thickness right and waiting for cure. I use foam board, which is a sheet system in a stiffer form.
How long should a shower flood test take?
24 hours with the drain plugged and the water level marked. Some manufacturers accept less; none objects to more. Do it before any tile goes on.
Does the Ontario Building Code require shower waterproofing?
The code requires a watertight finish in a shower and a backing that is not damaged by moisture; it does not pick the product. The tile standards (TCNA handbook, ANSI A118.10) and every manufacturer’s warranty do. Your municipality may or may not inspect it; assume no one will and photograph it yourself.
Price the shower with the waterproofing in it
The tile and shower cost calculator includes the pan and the wall membrane as their own lines so you can see them in the total. If a quote you already have does not show them, the $99 review will tell you what to ask.
Sources and further reading
- Tile Council of North America: the TCNA Handbook methods for tile over wood, concrete and in showers, and the ANSI lippage and grout standards a tile setter is held to.
- Schluter Systems Canada: the Kerdi shower system and Ditra uncoupling membrane installation handbooks, with the drain, curb and flood-test details.
- Mapei Canada: thinset, grout, self-levelling compound and moisture mitigation product data: what each one needs from the floor under it.
