UPDATE: My idea of tiling our deck got killed by City Planning today as they said they wanted it engineered. Well, I don't have time or money for that now. I can always tile later So, I need a walkable, waterproof coating. The subfloor/floor is going to be 1.25 T and G ply in case we decide to tile down the road. It will be sloped 1-inch for every 12 feet. The deck size is 14 x 20 and is over a garage (occupied space). We're in So. Cal. so it is not as wet or humid as other parts of the country may be but we do get a few heavy rains during the winter. There will be no freezing where we are. Thanks. ------------ I need to know the recommended amount of ext. ply for a 14 x 20 exterior deck over occupied space (enclosed garage/workshop). I plan to tile over the deck. The joists are double 2 x 8s, 16 inch centers running parallel with the 14 foot dimension. My moisture barrier will be Redgard by Custom Building products. So, is one 3/4 ply enough with a 1/4 inch WonderBoard over? Or two 3/4s w/ or w/o the WonderBoard needed? Thanks.