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Q.We've been asked to build a semicircular bathroom with tiled walls. What should we use as a substrate to cover the curved wall, which will have about a 12- to 18-foot radius, and what should we use to waterproof behind it?

A.Tom Meehan, a tile-setter who owns and manages Cape Cod Tile in Harwich, Mass., with his wife, Lane Meehan, responds: There are two ways to approach tiling a room with radiused walls. I think the old-fashioned way — a mud job done with wire and cement — is best. With this approach you need a layer of felt paper, galvanized wire mesh, a scratch coat of cement, and then a full coat (around an inch) of cement; it requires the skill of a professional tile-setter experienced in mud-bed jobs.

The easier method is to apply two layers of 1/4-inch cement backerboard to the framing over a layer of felt paper, staggering the boards at the joints and bonding them together with a thin coat of nonlatex thinset mortar spread with a 3/16-inch V-notch trowel.

I recommend using USG's Durock 1/4-inch-thick cement board (800/874-4968, www.usg.com), which seems to bend more easily than any of the other brands of backerboard. Even so, you'll need all the help you can get to hold it in place, so I would use one of the special cement-board screws instead of nails to fasten it to the framing.

To get a smoothly curved surface, good framing is important, too, preferably with the studs placed no more than 6 inches on-center.