Plywood for new Roof

shianeAugust 18, 2008

We are having an extension put on our house. The GC is planning on using 1/2 inch plyood for the roof. Is this the correct size or should it be 3/4 inch?


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It would help to know the slope of the roof and the spacing of the trusses.
I see a lot of 1/2" but prefer 5/8" for 4/12 slope and trusses at 16".

    Bookmark   August 18, 2008 at 8:54PM
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I specify 5/8" to increase nail holding capacity. I have had contractors insist on using 3/4".

    Bookmark   August 18, 2008 at 9:44PM
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Thank you randy427 and mightyanvil for your responses. I will definitely insist on at least 5/8" thick. Thanks again!

    Bookmark   August 20, 2008 at 9:02PM
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In most cases anything thicker than 1/2" plywood is just a waste of money for most roof sheathing.

But the real answer depends upon your local laods and conditions.

Unless you have extremely high wind and or snow loads, or have rafter or truss spacing more than 24" on center....

Then 1/2" plywood or 7/16ths inch OSB is all any reputable builder would require despite cries to the contrary from designers and others who have never built anything....

    Bookmark   August 21, 2008 at 10:13PM
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And clips! Clips to bridge one sheet to another can add a lot of rigidity for a very small price.

    Bookmark   August 22, 2008 at 4:16PM
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It was made clear earlier that 5/8" sheathing is not necessary for structural bending capacity. It has been recommended only for its better nail holding capacity. If good nail holding capacity is not important to the owner, 1/2" sheathing can be used.

H-clips are not needed for 5/8" sheathing or 16" o.c. rafter spacing.

    Bookmark   August 22, 2008 at 10:41PM
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Thanks everyone for all your suggestions!! Certainly food for thought!

    Bookmark   August 24, 2008 at 12:13PM
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Has anyone mentioned the number of plys? It has been stressed to me on many occasions that 3-ply is weaker, no matter the thickness.

    Bookmark   August 25, 2008 at 3:00PM
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A 3-ply plywood structural panel is not weaker in bending strength in either direction for a given panel thickness and max. span rating, but it does have less resistance to shear across the thickness of the panel which is not an issue for residential sheathing. It may have other characteristics that builders don't like.

An APA plywood sheathing specification consists thickness, species, panel grade/use, bond, and span rating. Plywood mills are allowed to use different layups (plies) for the same panel thickness and span rating to make optimum use of their raw material resources.

Here is a link that might be useful: APA info

    Bookmark   August 25, 2008 at 7:14PM
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