Tiling a Shower Wall: Cement Backer Board, Membrane and Thinset

Behind the old shower wall in this bathroom remodel was plain drywall, which the demonstrator says was how it used to be done. It is no longer done that way because drywall can get wet and grow mold. Drywall is acceptable above the showerhead, but anything under the showerhead must be cement backer board, so all of the backing was ripped out and replaced.

Demolition and inspection

Removing old tile or a fiberglass shower wall is dusty. The video calls for plastic over doorways, a dust-rated respirator, a drop cloth on the floor and plenty of ventilation. A hammer and chisel work across the wall, and a reciprocating saw cuts out screws. Because all the backing board is being replaced, damaging it is not a concern.

With the wall open, the studs should be in good condition and the cavity mold free. If there is mold, the advice is to stop, call a professional and have an expert remove it. This is also the time to reroute plumbing or install a new tub or shower pan; here the shower valve was updated and PEX pipe added.

Backer board and waterproofing

The board chosen matches the thickness of the drywall so the transition between them is seamless. To cut it, mark it, score it with a knife and snap along the cut. Shims at the base set a quarter inch expansion gap. The board is fastened to the studs with concrete backer board screws, not drywall screws. Seams get backer board tape pressed into thinset mortar with a putty knife.

There are two ways to make a waterproof barrier: a vapor barrier behind the board, or a paint-on membrane over it. Using both carries a small risk of trapping moisture between them. The exception is a bathroom on an exterior wall, where local building codes may require a plastic vapor barrier. Here the membrane is brushed onto all seams, rolled over the exposed board, and given a second coat once the first is dry.

Planning the tile

To estimate quantity, multiply width by height for each wall, add the areas and add 10%. For ceramic tile, boxes with the same dye lot number are best; if they don't match, mix tiles from different boxes during installation so color differences are less noticeable.

For layout, center a tile horizontally along the wall and dry fit so no tiny tiles end a row, shifting to either side if needed. Vertically, the pattern starts at the top so full tiles sit there and any cut tiles land at the bottom, where the tub or shower pan may be slightly out of level. Installation therefore begins with the second row, using a board attached to the wall as a guide.

Setting, cutting and finishing

Thinset should have a creamy peanut butter consistency. It is spread in small sections and textured with the notched side of the trowel, then tile is pressed in firmly with spacers between. Notch size depends on tile size and shape. Pulling one tile off after setting shows whether thinset is sticking to the whole tile; if not, use a trowel with a larger notch.

A tile cutter scores and snaps tile, while tile nippers or a tile hole saw handle curves. For bullnose or edge pieces, mortar piped from a zip-top bag with a half-inch snipped corner works like frosting. After removing the guide board, its screw holes are filled with waterproof membrane, and the bottom row leaves a quarter inch expansion gap.

After the thinset dries 24 hours, spacers come out. Grout is mixed in small amounts and worked in diagonally with a rubber float. After about 10 minutes, a wet sponge wipes off excess without removing grout from the joints. Haze remover cleans the tile faces once the grout is dry and may take several applications. After a few days, grout sealer goes on the grout and silicone sealant on corners, edges and joints.