Travis is giving excellent advice. That roofhelp website is really good, and
reminds me that I used all the wrong terminology in my earlier comment.
Ok, to repeat,
On my 1.8 inch pitch, I had a built-up roof applied.
Fiberglass basesheet, then three ply hot tarred roll roofing (each
successive layer covers 2/3 of underlying layer, so that you get 3 plies,
net), followed by a tarred-on mineral surfaced capsheet. Worked like a
dream, and is very sturdy and walkable.