We get called to look at other people's failed showers a few times a year. The conversation almost always arrives at the same sentence: but they used cement board. And cement board is genuinely good stuff. It just does not do the job people think it does.
Here is the layer-by-layer version, from the framing out, and what each layer is actually for.
The framing carries load, not water
Studs and joists hold the room up. That is their entire job in this conversation. What matters about them for a bathroom is flatness, because a wall that bows a half inch over eight feet will show every joint line in a stack-bond tile, and blocking, because a grab bar fixed into anything other than solid timber will pull out of the wall under load.
We block for grab bars in every shower we build whether or not one is going in. Blocking during rough-in costs a few dollars of lumber and twenty minutes. Blocking after the wall is tiled costs a demolition.
The backer board is a substrate, not a barrier
Cement board, fibre-cement board and foam backer board are all there to give the tile something stable to bond to that does not disintegrate when it gets wet. That is the distinction: cement board survives water, it does not stop water. Pour a cup on a sheet of it and the water goes through.
This is why gypsum board of any colour, including the green and the purple sort sold as moisture resistant, does not belong in a wet zone. Moisture resistant means it takes slightly longer to turn to porridge.
- Joints get alkali-resistant mesh tape filled with thinset, never drywall compound
- Screws at 8 inch centres, set flush and not overdriven
- Board held a quarter inch off the pan so it never wicks from the bottom edge
The membrane is the layer that keeps water in the room
The bonded waterproof membrane is the one that matters, and it is the one nobody photographs. It comes as a liquid you roll on or a sheet you bond in thinset, both to ANSI A118.10, and it goes over the whole wet zone with reinforcing band at every inside corner, every change of plane and around every penetration.
With a liquid membrane, thickness is everything. The manufacturer specifies a dry film thickness, usually achieved at twenty to twenty-five wet mils, and you cannot judge that by eye. We gauge it with a wet-film comb as it goes on and log the reading. Twelve mils of a twenty-mil product looks identical on the wall and is not waterproof.
The thinset is a bond, and 95 percent is the number
In a wet area the mortar coverage behind the tile needs to reach about 95 percent. Voids behind tile hold water, freeze-thaw at an exterior wall, and eventually debond. The only honest way to check is to set a tile, pull it back off and look at the back of it.
We do that at least once per wall and photograph it for the project file. It costs one tile.
The tile and grout are finishes
The tile is the last line, not the first. Porcelain is effectively impervious, ceramic less so, natural stone considerably less so, and grout is porous in every version except epoxy. None of them is the waterproofing. If the membrane behind them is right, that does not matter. If it is wrong, no grout sealer on earth will save it.
Which is the whole argument for the flood test. Twenty-four hours of standing water over the drain, before any tile goes down, tells you whether the layer that matters actually works.
