I am planning a new deck out our back door. The current deck is about five and a half feet off of the grade. What I’d like to do is to expand the footprint of the deck, and dig down to pour a concrete slab for a manspace room down there. I want room for my winemaking equipment, my piano, my books, and also we’ll make it something of a guest room with a half bath.

Anyway, I’d like to dig down as little as possible, which means that I’m looking for the best way to do the roof and decking so that it takes up as little vertical space as possible. My current plan is to cut a tapered pitch on the top of my ceiling joists/roof rafters for drainage. Then I’ll put CDX plywood, covered with maybe a hotmop roof. On top of that, I will glue tapered stringers that will serve as my joists for the decking material.

Is that a good plan? Are there any pitfalls to that approach? Are there better ideas?

What would be the minimum pitch for such a roof, realizing that there is a roof of sorts above the deck, and we’re in the Bay Area? I’m thinking 1/4″ per foot would be enough?

Would hotmop be necessary here, or can I use a bituthane membrane sort of product or other roofing material that I could apply myself?

And also, what would be the minimum ceiling height of the new room below, so that it will still feel reasonably comfortable for my tall-ish family visitors? (I know I don’t want to go for a full eight feet, that’s a lot of dirt to move and engineering issues that might crop up with our foundation).

Many thanks for any words of wisdom and/or experience.