Prepare the Substrate
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There are many types of floor tile, ranging from ceramics to clay to natural stone, and most can be successfully installed over various types of substrates, including existing tile, a mortar base, plywood subflooring or cement board.
If you lay new tile over old tile, the original tile and grout must be securely attached. Use a patching compound to fill in broken or missing tiles and any spaces in the old grout. Scuff the old tile surface with sandpaper to provide a better grip for the new adhesive or mortar. Before you begin tiling, wash the floor with a commercial detergent such as TSP (tri-sodium phosphate) to remove dirt, soap film and other contaminants that could prevent adhesion.
Tiling over a mortar (also called “mud” or thinset) base is preferred by professionals because it prepares the original floor with a solid, level base that results in an extremely durable finished floor. Working with mortar is more difficult, however, and requires experience to properly mix and level it on the floor.
Tile mastic, or thinset, is a premixed adhesive that is easy to use right out of the can, and it bonds well to almost any surface. It is commonly used directly on plywood or on cement board. Cement board is a highly stable, cement-based, sheet material typically reinforced with fiberglass (Image 1). It is installed with special screws recommended by the manufacturer. If you tile on plywood, a double layer of plywood with overlapped seams is recommended. The bottom layer should be a minimum of 3/4-inch thick. Use screws to attach this layer to the floor structure (Image 4). The top layer can be 1/4-inch plywood, or a commercial substrate made specifically for flooring underlayment. This layer also must be adequately attached with screws, or according to the product directions.