A major cause of failure of tile over decks and balconies is that the structure to which it is applied allows too much deflection. Tile has little tolerance for deflection. Cracking grout lines or tile is the result of deflection. Deflection (the spring or give of the deck surface) is a product of the joist spacing and thickness of the flooring materials. The TTMAC specification for flooring under tile or slate is as follows: 3/4 inch plywood fastened with 2 inch #10 Exterior Wood Screws @ 6 inch oc @ perimeter and 8 inch oc @ joists covered with 1/2 inch cement board secured with thinset and fasteners. (The deck must have slope built in - suggested slope 1/4 inch per foot). The use of a cement board overlay is required/recommended by tile associations and manufacturers. Cement board, unlike plywood, is not affected by varying levels of moisture. We strongly recommend that decks be built to this specification. For the money that is being invested in the whole system, it would seem unwise to scrimp on the subsurface. In case of a failure (cracking), the first thing anyone will look at (and blame it on) is whether the floor was strong enough and provided the necessary deflection resistance. Concrete decks and balconies are suitable for tile application but still require proper waterproofing or roofing protection. We recommend you follow the local best practice guidelines as laid out by the Tile Contractors Association of TTMAC or the building code. There may be regional differences in the suggested use of cement board or a second layer of plywood. As with all the details involved with a tile deck over a wood structure, you should follow the tile and thinset mortar manufacturer's specifications.