I've got a home with a large rooftop deck that I'm trying to turn into a comfortable outdoor living space, and I'm at a bit of a loss as to how I can build on it. I'd like to tile it, build a grill island, and a few other structures, but I don't want to damage what seems like the main waterproofing layer for most of the roof. My neighbor's construction is similar, and he's owned his for much longer and has tentatively identified the surface as Parex Mer-Ko Shur Deck elastomeric coating. I'm not sure what the substrate is below it, but my best guess is plywood, possibly with a few other emulsion layers and some engineered fabrics in there. My neighbor says I should be able to tile over the top of it as long as I lay down a backer board first, but I can't think of how I'd attach a backer board (or secure any other structures I'd want to build) without breaking the surface of the coating. Does anybody have any experience working with this, and can you advise on how to go about doing what I want with it without making my roof leak?

I am within a couple years (at most) of needing to address my flat roof and I have old swamp coolers up there that I never use (central air was added many years ago and I doubt the previous owners used the evaporative coolers much once those were in). My question is to have those removed, do I need an hvac guy to seal off the duct work prior to a new roof or would you trust the roofers to do this. Haven't seen a clear answer on this with internet searches.