Tom Meehan, a tile installer and owner of Cape Cod Tileworks in Harwich, Massachusetts, responds: There are a couple of methods I use to detail a shower-pan threshold or curb when a membrane has been installed. The first way is to wrap all three sides of the threshold with same-thickness cement backerboard as the shower walls.
In this scenario, I apply the top piece of backerboard first, letting it overhang the inside and outside faces of the threshold by about 1/2 in. I nail this top piece through the membrane into the threshold, but I keep the nails as far to the outside of the threshold as possible to minimize the potential for leaks.
I slip the outside piece of backerboard up under the top piece, and because there is no danger of leaks on this side, I nail it in place. Nails obviously cannot be used to fasten the inside piece, so I start by troweling thinset mortar up under the overhanging lip of the top piece as well as on the backerboard where the inside threshold piece butts. I then push that piece into the thinset.
I detail the threshold before I set the mortar bed for the shower. This way, the mortar bed locks the inside piece of backerboard in place. If I’m not doing the mortar bed right away, I sometimes bond the inside board piece to the pan liner with elastomeric caulk.
Another method I’ve used successfully is dropping a prefolded piece of galvanized expanded-metal lath around the threshold. Again, the only nails used to secure the lath are on the extreme outside top edge of the threshold. I then scratchcoat the lath with cement and sand and let it sit overnight before pouring the mud base.
Be careful; the wire lath and the use of a trowel in the second method can puncture the membrane.With either of these methods, it’s important to remember to pitch the top of the threshold slightly to the inside of the shower for drainage.