I have a house with an add on that has a roof with rolled roofing. The roof has a slight slope. There is a door from the second story leading out to the roof for the first story. I want to place a deck on the roof. The structure is more than sufficient. What I am worrried about is ruiing the non-leaking roof. I figure that if I just lay boards on the roof it will eventually wear through and leak and then I have to tear up the deck. Is there a better way??

Wish I had saved the This Old House show where they did exactly what you want. They hot mopped a new membrane to the existing roof and then glued sleeper 2x4 pressure treated lumber down running parallel with the slope to allow water to run off. Then then nailed their joists to the sleeper and built the rest of the deck in a traditional manner.

Instead of the hot mop ( which I don't like too much of a hassle to repair and out on) I use rubber roof. Tear off old roof. Its only a 4 to 7 year roof to begin with. Add you nailer's on the corners Now install the felt board. Glue new rubber ( here we use 060) and run the rubber over the nailer's. X cut that to tuck good around the nailer's. Now take a piece of rubber roof and cap over the nailer's Post deck from there and then another piece of rubber roof around deck post and SPM seal around that and glue the rubber to the rubber and SPM seal that. Be careful about roof load here.

A deck of size is not light and the rafters are for snow load. You nay have to double up on rafters for the deck attachment points. If that can't be reached from below pop some ply off roof and sister up roof rafters to carry the deck weight.