· by Michael Blair, Nashville's Tile and Stone
Water goes through grout. It goes through the hairline gaps at every change of plane, through the caulk when it ages, and through the tile body itself if the tile is porous. A shower stays dry because of the waterproofing layer behind and beneath the tile, and that layer is invisible the day the shower is finished. Knowing what it should be is the only way to know whether it is there.
What tile does and does not do
Tile is the wear surface. Cement grout absorbs water and passes it through to whatever is behind. On a properly built shower, what is behind is a waterproofing membrane that carries that water down to the drain, and the small amount that gets through the tile every day drains away and dries out between uses. On a shower without one, it reaches the framing.
Cement backer board, the gray board that tile is set on in wet areas, is not waterproof either. It does not rot or swell the way drywall does, which is its virtue, but water passes straight through it. Backer board alone behind tile, which was standard practice for decades, keeps the wall from falling apart while it leaks.
The two ways it is done now
Sheet membranes are a thin polyethylene or similar fabric bonded to the backer board or directly to drywall with thinset, with the seams overlapped and sealed, and with preformed corners and a matching drain. They are the most consistent method because the thickness is fixed and every seam is visible before tiling. Liquid-applied membranes are painted or rolled onto the backer board in several coats to a required thickness, with a reinforcing fabric at corners and seams. They are faster and rely on the installer applying enough.
Both are made into a system with the shower pan. Either a preformed foam pan with a slope built in and a matching drain, covered with the same membrane, or a mortar bed sloped to the drain with a liner beneath it and the membrane above. The old-style vinyl liner under a mortar bed, with no membrane above and a mortar bed that stays wet, is what most showers built before about the last decade have, and it is the source of a large share of the mildew that never goes away.
- Sheet membrane bonded to the walls, seams lapped and sealed
- Or liquid membrane at the specified thickness, with fabric at corners
- Sloped pan with the membrane continuous from wall to drain
- Bonding flange drain that clamps the membrane, not a standard drain
The details that fail
Corners and the curb. Every inside corner is a seam, and the top of the curb is where the membrane most often stops short. Niches are a second set of corners and a horizontal shelf that holds water, and a niche cut into the wall after the membrane was done is a hole in the waterproofing. The drain connection, where the membrane has to be clamped or bonded to the flange, is the third.
Fastener penetrations through the membrane for a grab bar, a door, or a bench are a fourth, and the honest answer for those is blocking planned before the membrane goes on and a sealant rated for the membrane at the penetration. A shower that leaks within a year almost always leaked at one of these four places.
Checking it before the tile
A flood test: the drain is plugged, the pan is filled to just below the curb and left for a day, and the level is marked and checked. Any drop, or any dampness below, means the membrane is not continuous. It costs a day and it is the only time the waterproofing can be checked without removing tile. Ask whether one will be done and whether you can see it.
Photograph the membrane before the tile goes on. The photographs show the seams, the corners, the drain and the niche, and they are the record if anything happens later. A tile contractor who is confident in the work will not mind; one who is not will tell you that too.