I am putting on a flat roof on a building that is 24 x 44 block. The roof consists of : top and bottom sill plate, face plate, 9 - 4x4 treated posts for support, 2x10x12 joist every 16" on center with a 2x4x12 wedged to allow for slope of 1 3/4" per 12 foot side, 5/8" osb, roof felt, tar, and rolled roofing. My question is when I start the osb should I leave a 5/8" gap on the edge between the osb and the face plate? I know there is a 1/8" gap between the osb but what about the first piece so it will line up to the 16" on center joists for nailing?
I would say that it depends on the drip-edge and how the fascia are laid out on the edge. Galvanized drip-edge should easily span a 5/8" gap, and will certainly be less waste than flushing up the sheathing and cutting off 15" for starter courses. It mainly depends on having the drip-edge supported on both sides and having adequate room for nailing it down.
This will work:
I'm not entirely sure I'm grasping all the details here but in general I would say its better to go flush to the edge. Here's how you could lay it out mathematically speaking, but I'm not sure this is the most efficient way to go about it. Also I've never heard of doing an 1/8" space on sheathing, that just seems like an invitation for leaks, not to mention that if your osb is 96" and you add an 1/8 between every sheet your layout will creep systemically and you'll be off your 16" centers in short order. Full disclosure: I'm not a roofer so you might want to find someone with firmer answers than I've got. Hope this helps, cheers.