A tile deck has been brought up over here before, I my own self have been trying to figure out how it would work and stay working down the road. In the past we All were thinking plywood/cement board/ tile one guy tried it and posted pictures of the failure,none of my little test decks doing it that way worked real well either. Building it like a shower and using the modified as a shower pan I have thought about but never tried. I have several projects out with modified/stringers/ decking 9 years or so out with no problems. And I know a torchdown will last done corectley on a comerical roof granulated or smooth surface for many years so all I can give you are ideas Dont make the fall center on a main drain that would be installing a leak, dont buy any thing serious at the box stores, if your going to do a tear out anyway frame the fall with cant strips fastened to your rafters and re deck,if you can dont have the posts going thru the roof if they have to be sure all needed flashing is instaled corectley and the posts have solid bearing. I still am coming back to the workers far as the leaks go, the worst thing for a roof is people walking on it x 100 the basic concret guy placing the creet base (dont use stones)the chances of a sharp heal,droped trowel,talking on the cell and droping a pencel then steping on it things like that are just to great to expect something not to happen to even a perfect roof/flashing install. I would come closer to trusting 3'' of light weight creet placed over plywood with flashing all around then using very close tolerance com tile,thin com grout line,instaled with construstion adhesive with several coats of high grade uv sealer. J

RedGard can go over interior plywood, but I don't believe they recommend it over plywood for exterior installations. Plus, composite decking over the membrane? How are you going to secure it? Are you going to float sleepers over the RedGard and fasten the composite to the sleepers? You don't want holes in your membrane. I recommend you contact Custom's Tech and see what they have to say. If the deck boards will have a gap between them and if sunlight can see the RedGard, I don't know what (if any) long-term UV resistance RedGard has. Plus, if the deck boards will be spaced bit, will you see red between the gaps? If using sleepers, I'd want a buffer between the sleeper and the RedGard to help protect the membrane.

When we had the shower/tub one piece thing removed in order for a cast iron tub and then tiled shower surround walls put in, the guy who did the demo of the old stuff was the one who had the qualifications to also correctly install the cast iron tub - the difficult part came when the guy doing the tile got involved in the project. If the cat hadn't managed to find her way inside the walls around/underneath the lip of the tub while he was out at lunch, and was too scared to move when he came back and got started up again making all the noise and whatnot - I probably would have never known that the water-resistant drywall he was putting up wasn't actually attached to any of the wall studs (he used random pieces to attach it to instead, but none of that was secured to wall studs either...yeah, I know...makes no sense but that's how it was!) ...so in a way I'm thankful to that cat for getting into a situation that required me to carefully take down each and every piece of drywall that he'd finished by the end of that first day, he'd only left a space around 10" wide by 18" tall with only some kind of paper stuff there which is how I'd figured out where the cat had gotten to in the first place (the paper wasn't as muffling as the drywall was so while I knew it sounded like she was somewhere in between the lower and upper floors, I had to go in and listen by the papered section to really figure it out). If the cat hadn't gotten trapped I'd have gone in to take a shower one day not long after the job had been completed and had an avalanche of tile fall down on me because the drywall wasn't secured and was installed with all that tile weight in a 'wet area'. Just because someone does installation of a given thing for the house for a living doesn't mean that they're necessarily going to know all the things there are to know about a particular thing that happens to be the thing you want them to install for you. It's also possible for someone who has been in the business for decades to get into certain habits when they do a job and become extreeeeeeeeeeeeeemely resistant to any newfangled ideas about how things should be done - ideally this is where the instructions from a manufacturer would help out, but considering how...unclear...those things can be, and how strongly manufacturers feel about having to shell out to customers when something goes wrong (why else would they provide hazy-at-best details on how to install things, along with a bunch of fine print that trust me, IS ALL THE EXCLUSIONS that will mean they don't have to pay anything for the failure of their product...and homeowners must sift through an assortment of professionals, put those professionals through an obstacle course designed to weed out all but the best for their particular job, pray that they can afford that best option, etc etc)... :P

@User, Thanks for your comments. There are concrete tile options appropriate for rural Mediterranean-style roofs that are very close to clay's look—particularly when mud-boosted—that are acceptable/recommended by the design review committee. If this was a home where I planned on moving, then clay all the way. My sole interest is in whether or not there is roman pan concrete roof tile. It's less about affordability and more about whether it makes sense for this spec house. Also, it's more than material, of course, there's also labor spread across a single-level home with 7500 sq ft under roof. The two builders who I interviewed indicated that a clay installation total cost is significantly higher than concrete and the ROI isn't there.