What Goes Behind Shower Tile, and Why Does It Matter?

Short answer Grout and cement board both let water through, so a waterproof membrane goes over the backer board, and the shower floor slopes to the drain. Before tile, we flood-test the shower pan and hold water in it for 24 hours. A failure in these layers usually shows up years later as rot, not as a drip on day one.

The layers, from studs to tile

Layer	What it does
Wall studs and subfloor	What it doesThe structure. It has to be solid, flat and dry before anything goes on it.
Cement backer board	What it doesA stable base for tile that does not fall apart when wet. It is not waterproof.
Waterproof membrane	What it doesSheet or liquid, rated ANSI A118.10. This is the layer that stops water.
Thinset mortar	What it doesBonds the tile to the membrane.
Tile	What it doesThe surface you see and clean.
Grout	What it doesFills the joints between tiles. It absorbs water.

On the floor, the base under the membrane slopes toward the drain at about 1/4 inch per foot, and the membrane is joined to the drain, so water that gets through the grout has a way out instead of sitting under the tile.

What sits behind shower tile, and the floor sloped to the drain. Only the membrane stops water; grout and backer board let it through.

Another common system uses a foam backer board that is waterproof on its own. The seams, corners and screw heads still get membrane strips and patches, and the floor is joined to the drain the same way. That is what the photos below show.

A shower built this way: waterproof board with a sealed prefabricated niche (left), then every seam and corner covered with membrane strips (right). Tile goes on only after the flood test.

Why grout and cement board are not enough

Cement grout is porous. Water soaks into it with every shower and dries out between them. Cement board behaves the same way: it does not rot or swell, but water passes through it. Without a membrane, that water reaches the studs and the subfloor. Sealing the grout slows the absorption. It does not make the wall waterproof.

The flood test

Before any tile goes on the floor, we plug the drain, fill the shower pan with water and leave it for 24 hours. If the level drops or there is any sign of water below, we find the leak and fix it while it is still a membrane repair, not a tear-out. Our technical supervisor checks the result before tile starts.

What failure looks like years later

A shower with poor waterproofing can look perfect for a long time. Then the signs start:

Grout lines stay dark long after the shower has dried.
Tiles near the floor or the valve sound hollow or come loose.
Paint or baseboard outside the shower bubbles or peels.
The floor next to the shower feels soft.
A stain appears on the ceiling of the room below.
The room smells musty even with the fan running.

By the time these show up, the fix is usually removing the tile, repairing the framing and doing the waterproofing again. That is why I do not recommend saving money on this layer. See also Signs of water damage in a bathroom.

Questions to ask any contractor

Which membrane do you use, and is it rated ANSI A118.10?
Does it cover the whole shower, including the floor, the niche and the bench?
How is the floor sloped, and how is the membrane connected to the drain?
Do you flood-test the pan before tile, and for how long?
Can I see the waterproofing before the tile goes on?
Who checks it, and what happens if the test fails?

A contractor who does this properly can answer each of these in a sentence or two. If the answer is "the grout is sealed" or "cement board is waterproof", keep looking.

Is a tub surround waterproofed the same way?

The walls are. They get the same backer board and membrane. The tub itself takes the place of the shower pan, so there is no flood test, but the membrane has to overlap the edge of the tub correctly, so water runs into the tub and not behind it.

Can a membrane be added without removing the existing tile?

No. The membrane goes under the tile. If an existing shower leaks behind the tile, the fix is to remove the tile and backer board down to the studs, repair whatever is wet, and rebuild the layers in the right order.

What goes behind shower tile, and why does it matter?