Q: When folding the corners of a shower-pan membrane, how do you keep the built-up material from pushing out the backerboard?

A: Tom Meehan, a tile specialist from Harwich, Mass., and co-author of Working with Tile, responds: As I mentioned in my article “Preventing Leaks in Tiled Showers” (Oct/16), the corners of the membrane need to be neatly folded so that the material lies as flat as possible. But even when you fold the membrane into “hospital bed” corners, there are still at least three layers of it in the corner. Depending on the membrane you use and how neat your folds are, this can amount to 1/8 to 1/4 inch of thickness behind the backerboard. As a result, the backerboard will flare out near the floor of the shower, which can wreak havoc with your tile layout and require you to custom-cut the first few courses for the shower walls.

To make room for the folded shower-pan membrane in the corners, first make shallow cuts in the corner stud about 6 inches from the shower floor,
To make room for the folded shower-pan membrane in the corners, first make shallow cuts in the corner stud about 6 inches from the shower floor,

To make room for this extra material, I create a notch in the bottom of the framing at each corner. Before the first step of installing the shower pan, I make a relief cut in the stud on one side of the corner about 6 or 7 inches up. (Being right-handed, I naturally fold the membrane to the left, so I make the cut on the left stud of the corner). Using a reciprocating saw, I make the cut about 1/4 to 3/8 inch deep.

Break out the wood below the cut with a chisel or screwdriver.
Break out the wood below the cut with a chisel or screwdriver.

With a chisel or a flat-bladed screwdriver, I break out the chunk of wood below the cut. I don’t try to be super neat at this point, and it’s not imperative that the surface of the notch be really smooth. I just make sure that nothing is sticking out of the notch surface (especially an errant framing nail) that could puncture the membrane. After cutting all the notches, I clean up all the sawdust and debris on the subfloor of the shower with a vacuum and proceed with adding the slope to the shower floor.

After the membrane is placed in the shower, carefully fold the excess in the corner, keeping it as flat as possible, and secure the folds with a nail at least 2 inches above the threshold.
After the membrane is placed in the shower, carefully fold the excess in the corner, keeping it as flat as possible, and secure the folds with a nail at least 2 inches above the threshold.

After cutting the membrane (being sure it will extend 6 inches up each wall), I position it evenly in the shower. To fold the corner, I push the right side so that the membrane fits squarely against the corner framing. Then I fold the excess neatly behind the left side so that fold is flat and the leftover flap of membrane extends out evenly to the left, fitting under the top of the notch. I secure the fold in place with a nail at the very top of the notch, well above the top of the threshold of the shower. With the built-up layers of membrane now tucked into the notch behind the plane of the studs, the backerboard will lie perfectly flat and the tile layout will stay the same from the floor of the shower all the way up the walls.

Note, too, this is exactly why I do not follow the NTCA’s suggestion to install blocking between the studs at the bottom edge of the shower. Such blocking would make it much more difficult to cut the notches.

Photos by Roe Osborn