A.Tile consultant Michael Byrne responds: There may be advantages to tiling over some existing flooring materials that contain asbestos, provided that the existing structure and its subflooring can be identified and confirmed as adequate for the additional weight of a tile installation. More and more communities are giving the okay to covering existing asbestos-bearing flooring materials with ceramic or stone tiles, provided the structure is up to the task of bearing the weight. If so, the common practice is to install 1/4-inch-thick tile backerboards over the asbestos material, perhaps add awaterproofing­crack isolation system over the backerboard, then install the tiles over the boards with latex thinset mortar and a grid of movement joints filled with a resilient sealant instead of grout. The movement joints should extend from the top surface of the tile to the bottom of the underlayment, but they should not extend down into, or penetrate, the suspect material. The width, number, and placement of movement joints (which are necessary in every tile installation, without exception) are too complicated for a brief explanation, but the TCA Handbook for Ceramic Tile Installation contains a thorough discussion, under Detail EF171 (Tile Council of America, 864/646-8453).