How do I properly install underlayment and a modified bitumen roof on a flat garage roof with concrete walls?

Question

My project is to replace 18'x10' flat roof on my concrete garage. So far I installed 16" oc framing and 3/4" plywood over it. Now I am planning to install some fiberglass based underlayment (#30, presumably) and 2-ply self adhering modified bitumen (like SBS Liberty from Home Depot).

Plywood sits about 3" below the top edge of concrete walls (there is a drainage hole at the low point).

So I am figuring how to properly attach underlayment, MB roofing and flashing to the whole structure. I assume that underlayment will have to be nailed or stapled to the plywood. But what size nails do I choose? If I take anything longer than 3/4" it will protrude from beneath the sheathing. Is it ok? How do I properly space it? It surely will not be nice to have this roof covered in spikes from inside.

Do I need to fasten base sheet in addition to it self-adhering to underlayment? What is the best way to attach it to concrete walls? The only way I see is to use some kind of concrete screws. Do I need flashing along all sides of the roof?

I could not find any consistent information on internet so far, so some expert advice would be highly appreciated.

Answer 1

You will want to contact the manufacturer of the product to get their recommendations on the type of fasteners, the fastener spacings, and the necessary penetration of the fasteners.

Yes, you will need flashing around the entire perimeter if you wish the roof to be waterproof.