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Roofing question: Do I NEED 5/8″ plywood decking under a 50-year steel roof?

Mai Tai | Posted in GBA Pro Help on


House 6:12 pitch, garage 3.5:12 pitch

Trusses 24″ OC, 2X6 top cord (solar ready). Climate zone 6A, mid to low snow load.

I’ve already moved off the 7/16″ OSB decking that is on the plans, and have temporarily settled on 1/2″ plywood decking for the roof. Roofing contractor said this would be much better for the screw holding power (using click-lock standing seam panels, 26 gauge, 16″ wide). Now the question is would I see any benefit over time going to a 5/8″ plywood roof decking? The cost is incremental vs 1/2″ ply (let’s call it 1k), but it all adds up, and if 1/2″ is plenty i am content leaving it at that. Will 1/2″ ply sag over time on 24″ OC trusses, and if so will 5/8″ provide any benefit for this over a 50 year life span? Assume venting will be more than adequate (full length ridge vent gable roof, full vented soffits).

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  1. Expert Member
    Malcolm Taylor | | #1

    Mai Tai,
    No you don't need 5/8" plywood. It's better but not necessary. The most noticeable difference between 1/2" and 5/8" is when you walk the deck before installing the metal and feel the flex if you stand on a horizontal seam. Once the roofing is on this disappears. There is no reason to believe the plywood will sag over time.

    Having said that, I've suggested and used 5/8" on two houses, and liked the feel of both the deck and the way it held fasteners. But I think it falls in the category of upgrades which are nice, but over time makes no discernible difference.

  2. KEVIN ZORSKI | | #2

    Mai Tai - use plywood clips between the 1/2" sheets in between the trusses. Good stiffener for edges, though may not matter once metal roof is on.

  3. Expert Member
    Malcolm Taylor | | #3

    I wouldn't use H-clips under metal. Too great a chance they get slightly misaligned and telegraph through. The metal roofing fasteners end up doing the same stiffening of the plywood edge.

    Using H-clips really slows down the sheathing. Do you use them where you build? I haven't seen them here on a house for years.

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