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

Specs:

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).

Asked by Mai Tai
Posted Jun 10, 2018 2:24 PM ET
Edited Jun 11, 2018 9:26 AM ET

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3 Answers

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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.

Answered by Malcolm Taylor
Posted Jun 10, 2018 2:53 PM ET
Edited Jun 10, 2018 2:54 PM ET.

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.

Answered by KEVIN ZORSKI
Posted Jun 10, 2018 10:24 PM ET

3.

Kevin,
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.

Answered by Malcolm Taylor
Posted Jun 10, 2018 11:53 PM ET
Edited Jun 10, 2018 11:54 PM ET.

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