Installing tile over a wood subfloor
Installing tile over a wood subfloor is a lot easier today than it was when I started my remodeling career. The old-fashioned mud method -- floating a thick mortar bed in preparation for the tile -- has given way to using cement backerboard. The cement board is inexpensive ($10 or less for a 1/2-in.-thick, 355-ft. sheet), easy to cut and install and sufficient for most installations. Among the trade names you'll see are Durock, made by U.S. Gypsum Corp., and PermaBase, from National Gypsum Co.
Manufacturers provide clear installation instructions with their products, but there are a few other important points you need to know to guarantee a long-lasting installation. For example, I always like to remove the old flooring material so I can check the subfloor for any water damage. Years of water seeping along the edge of a bathtub or shower can cause areas of rot that need to be repaired.
If everything is in good shape, my carpenters drive corrosion-resistant screws through the plywood and into joists below to ensure a firm foundation for the tile. We set the heads of all screws and any existing nails slightly beneath the wood surface. And we use a lightweight gypsum leveling compound to fill and level voids or low spots. We've found that if you take this kind of care with the prep work, the finished tile won't crack with use.
The proper fasteners, mortar, tile adhesive and joint tape are all specified by the manufacturer. Don't use drywall screws -- ever. Another mistake I often see is grout wedged between the last row of tiles and the tub, cabinets or doorsill. Grout in these transitional areas will crack as the floor goes through expansion and contraction. These areas should get a bead of acrylic caulk so the joint remains flexible.
I'm often asked by homeowners if it's possible to lay down new tile over an existing tile floor. This is not my favorite way to install tile, but it can be done, as long as the existing tile and subfloor are in good condition and the old tile gets scuffed to let the new mortar get a grip. The combination of newly applied mortar and tile will raise the level of the floor by at least 3/8 in. Trimming the bottom of the bathroom door and building up the doorsill usually hide the fact there are two layers of tile instead of one.