You wouldn't need cement board if the surface is somewhat even and if the thinset sticks. I would spot grind the surface in a few places to help adhesion if there is no exposed concrete. You can get tile for a buck or less per sq ft. With thinset and grout you may be at about $1.50 per sq ft if you do it yourself. There is a good chance you would be fine doing nothing but you would not have peace of mind. My wife would not eat anything grown in tar but she worries about everything. If thinset sticks you could probably skip the tile and just stucco. I use Rapid Set stucco mix available from Home Depot sometimes as a sealing coat. It is not a portland cement based product. It has almost no shrinkage, a short work time and is very waterproof (resistant). I doubt it would react with the tar coat as most sealing paints would. Zeuspaul

Not a tile pro, just an interested amature that's never done any outdoor tile setting, but I don't *think* you need to do anything but clean the surface well. Cement board is used to provide a surface for the thinset to adhere to, but it should stick to the cement just fine. UNLESS its been sealed or painted, then you may need to mechanically remove some of the surface. Ditra is wonderful: it replaces the thinset in horizontal applications, and helps keep cracks in your slab from becoming cracks in the tile. But if the slab is stable and crackfree, I don't think it would do anything but add about $1.50/ft to your materials cost. You DO need to use a tile approved for outdoor use-- tiles that aren't won't survive the thaw/freeze cycle. Personally, wouldn't add the heat system at this time unless you have a pretty clear intention of finishing the space later. I'm not sure an electric system can be used outdoors, and a hydronic system for a single room is rarely done due to the complexity and cost of other equipment, like the boiler, etc. If the rest of your home already has radiant hydronic and the slab is in a spot where it would be simple to tie the deck system into the whole house, it would probably be worthwhile. Personally, I'd save the money now, spend it later on a new floor if needed. BUT I know of people at Big Sky and other ski areas that have hydronic radiant heat under their decks, sidewalks and driveways. They melt the snow as it falls rather than paying someone to shovel it [they are in the income bracket that doesn't shovel its own walks]. Seems pretty environmentally irresponsible to me...

I have an exterior patio that is partly covered and partly uncovered. The covered part is protected from rain and other like elements, but all of the patio is subject to the outside temperatures and humidity levels. I have a contractor who is supposed to be installing a porcelain tile floor for me over the existing (and sealed) concrete floor. He is telling me that because the humidity outside is over 60% that he doesn't feel that he can commence the installation without a high risk of the tile not properly adhering. He said that the manufacturer of the thinset he uses recommends installation in conditions of 40% humidity or less. He said that based on personal experience he is comfortable installing in condition of 60% humidity or less. I inquired about other thinset options intended for exterior air conditions like we are experiencing in our area, and he said that what he uses is the best (of course he would say that). Does anyone have any experience or knowledge in this area who can help? I'm wondering if maybe he is using this "technical" excuse to cover for some other reason why he doesn't want to start right now (thinking that I won't know the difference and just take him at his word). Please advise. Thanks in advance!

If it wasn't so sad and ironic the situation would be laughable. If disabled people fell on tile all the time, there would be no tile in rehabs. There is non slip ceramic, porcelain, and vinyl tiles to select from which are more safe and sanitary than concrete stain. If you want an easy to clean surface with no grout lines, then there is an Armstrong tile called. Safety Zone. Sand the surface to remove whatever was installed and install this over top. Easy Peasy Damp Mop to clean. https://www.armstrongflooring.com/commercial/en-us/products/srf/safety-zone.html https://www.armstrongflooring.com/commercial/en-us/products/srf.html