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Q.My clients want to update their bathroom by installing new tile over an existing tiled tub surround. The old tile is in good shape, but it stops about 18 inches from the ceiling; my clients want their new tile to completely cover the walls and ceiling. Is it possible to just pad out the upper wall sections with backerboard and then install the new tile over the old, rather than remove the old tile and start from scratch? If so, how should I prep the walls to ensure that the new tiles will adhere to the old?

A.Contributing editor Michael Byrne, a tile-setter and consultant in Los Olivos, Calif., and moderator of JLC Online's tile forum, responds: If the old tub-surround tiles are uncracked and in good shape and you're confident they've been installed over a sound mortar bed or cement backerboard substrate, you can install new tiles directly over the old. But be sure to verify that the faucet valve stems are long enough to accommodate the old escutcheons, and that your client won't mind the slightly diminished ledge around the tub.

You'll need to abrade the surface of the old tile before installing new tile (for proper adhesion, 95 percent of the tiles' surface should be abraded). First, though, thoroughly clean the tile and grout with a tile cleaner containing phosphoric acid to remove soap and other films; rinse with clear water. Then mask off the tub ledge with heavy paper.

To abrade glazed wall tiles, I usually use a coarse hand-rubbing stone designed for rubbing down concrete. For unglazed porcelain tiles, which have a much harder surface, I prefer to use a diamond cup wheel mounted on an angle grinder; it works fast without digging too deeply into the tiles. For a small area, I've found that you can get excellent results using the side of a regular dry-cutting diamond blade.

To shim out the wall above the existing tile (thinset mortar should never be used as a filler), the best approach is to nail up galvanized diamond lath, float cement mortar over the mesh, and use the original tile surface as a screed guide for removing the excess mortar. You could also use backerboard, plywood, or another hard composite to achieve the desired thickness.

Industry specs require an isolation membrane where there is a change in backing materials, to prevent a crack from appearing between the old and new fields of tile. In your case, I would specify a reinforced membrane that offers both crack isolation and waterproofing properties — like Nobleseal TS (www.noblecompany.com, 800/878-5788) — to cover the whole area to be tiled. When installing the membrane, pay particular attention to the movement joints between the tiles and tub and in the two vertical inside corners. Hard grout in those areas can cause a membrane system failure.

At the ceiling, my guess is that there is moisture-resistant drywall (greenboard) fastened to framing installed 16 inches on-center. This is inadequate for even one layer of tile, and should be replaced with a suitable substrate. In fact, if any of the old wall tiles are installed over greenboard, your best bet is to remove all the old tile and the greenboard, and replace the entire installation.

In 2006, because of its very poor performance in wet areas, moisture-resistant gypsum board was removed from the list of acceptable substrates for tile. It's possible that an older installation could still be in good shape, but only because the tile-setter applied an organic mastic using the two-coat method (which resembles application of the surface-applied waterproofing membrane systems used today). Solvent-based organic adhesives have little adverse effect when applied to greenboard, but most of today's waterproofing membrane systems and thinsets are latex-based, which means they hold moisture long enough to destroy the bond between the paper and the greenboard's moisture-resistant core.