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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?

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  • It's a bit unclear from your description what would be on the other side of the 5/8" gap. Will your drip-edge span the gap and still be supported on either side with enough room for nailing it down?
    – Comintern
    Jul 9, 2014 at 23:25
  • Yes it will, Sorry to take so long to get back to you been really busy. I liked your answer. Thanks for all your guidance.
    – Tina
    Jul 13, 2014 at 21:54

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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:

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This won't:

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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. enter image description here

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  • The 1/8" space on sheathing is to allow for expansion and prevent buckling.
    – Comintern
    Jul 10, 2014 at 23:01
  • osb doesn't expand or contract that i'm aware of. I'll talk to a friend of mine that's does structural roof work and get back to you with a hard answer.
    – user23534
    Jul 11, 2014 at 1:01
  • It expands like mad, and can get ugly: google.com/…
    – Comintern
    Jul 11, 2014 at 1:46
  • Okay so my roofer guy said that he would 1: start flush to the edge and cut your first seam back as described and 2: you use the tongue and groove stuff which is made specifically to account for the expansion and contraction of the building (the osb itself does not) The t&g absorbs movement on the long seems and the short ends are and 1/8" short of 96" to stay on layout.
    – user23534
    Jul 14, 2014 at 2:23
  • Also I found this pdf from an osb manufacturer on panel installation that promotes just using 1/8" so...? it will tell you more about installing sheeting and roofing than you'll ever want to know.file:///C:/Users/Dell%20User/Downloads/RoofInstallation_APAE30.pdf hope this helps sorry it took so long.
    – user23534
    Jul 14, 2014 at 2:37

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