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I am putting some 3/4 plywood on top of 2x6 framing joists on my flat garage roof. The framing is spaced 16oc. I figured that I place 4x8 plywood sheets in such way that their short edge will be resting on joists. The entire roof is 17 1/2' X 10 1/2' so it requires about 7 sheets total.

That brings up 2 questions:

  1. Do I need to space sheets from each other and from concrete walls of the garage? I heard that 8d nails used as spacers between sheets work well for it.

  2. How do I nail those edges? I mean - there would be only 3/4" (not even counting spacers) under the edge. Do I just nail so close to the edge? Will it work? Should I put nails at the edge toward the center of the joist or just straight down?

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First concideration, I would encourage you to use a T&G roof sheathing such as AdvanTec instead of standard CDX plywood. This will give you a better fit and not have to worry about spacing gaps in the plywood. The price is comparable, so there is no cost advantage to using regular plywood.

The sheathing should be secured with 6d galv ring shank nails at apx 8" spacing. 1 5/8" coated drywall screws are great, but more time consuming to install. Don't use non galv common nails, as they tend to back out after a few years.

If your sheathing fits properly on the joists with 3/4" overlay, nailing straight down is fine as long as you bite into solid wood. A slight angle towards the joist is OK as long as the heads of the nails or screws don't protrude above the sheathing. Protruding nails can damage your roofing material, assuming you are not planning on using a layer of Grace Ice and Water shield under your final roofing materials.

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  • One add: I would use zinc-coated or galvanized construction screws (e.g. SPAX) instead of coated drywall screws. I'm not a fan of drywall screws, coated or not, for anything besides drywall.
    – Karl Katzke
    Oct 1, 2011 at 14:04
  • @karl: My bad, the ceramic coated screws I was referring to are actually decking type screws. Guess I call all coarse threaded screws drywall screws. Galv decking types are fine also. I like the ones with square head inserts and my DeWalt impact driver. Go in slick, rarely strip and sink well.
    – shirlock homes
    Oct 1, 2011 at 14:27
  • I should say however, my first choice is my trusty Bostitch 88 framer, with Bostitch 2 1/2 inch galv threaded ring nails set at 120 psi.
    – shirlock homes
    Oct 1, 2011 at 14:33
  • Thanks for the answer. I already got PT plywood but not t&g unfortunatelly. I planned to nail it with 8d hot dip galvanized nails. I am happy to accept your answer but feel free to add more comments. I'd appreciate as much info as possible.
    – Uncle Meat
    Oct 1, 2011 at 15:13
  • and since I don't have t&g, how do I space it? Should I?
    – Uncle Meat
    Oct 1, 2011 at 15:26

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