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RE: Plywood sheathing on wood trusses

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Thanks for the advice!  I haven't designed a plywood diaphgram before and wanted to make sure there weren't any particular no-no's to look out for.  I'll keep your thoughts in mind and if the design loading doesn't end up too high, I won't worry about it.
If it does get too high, I'll add in the molasses!
Jim Wilson, PE
Stroudsburg, PA

Keith De Lapp <keith(--nospam--at)> wrote:
Shimming a 3/16" thickness doesn't seem practical.  Splitting would be my big concern.  I would take a look at the demand/capacity ratio.  If it's high I would require a shaped nailer along side the top chord.  If it's not high I would inspect for shiners and let it go.  The situation you describe is common on hip roofs.
Keith De Lapp, P.E.
-----Original Message-----
From: Dennis Wish [mailto:dennis.wish(--nospam--at)]
Sent: Tuesday, October 12, 2004 12:58 PM
To: seaint(--nospam--at)
Subject: RE: Plywood sheathing on wood trusses

This is a tough call. I think the issue here is that the shear transfer through the diaphragm creates an opportunity for bending in the exposed nail shaft and/or greater chance of pull-out through bending of the nails. In a tight connection, the bending issue is almost non-existent as the shear is tightly transferred from wood to wood.

So the question then comes it shimming the wood would provide adequate shear transfer. I would prefer to use a nailer to the side of the truss ripped at the angle causing the 3/16? gap and then boundary nail the sheathing to the nailer ? like molasses flowing through the load path from wood to nail to wood to nail. This might confuse some, but I?ve always considered the Molasses metaphor as ideal in understanding load path transfer from roof to foundations.


Sorry about the simplicity of this ? but I try to understand how the materials will work together in light-framing rather than to try and analyze any one component.




Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant



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-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)]
Sent: Tuesday, October 12, 2004 12:16 PM
To: seaint(--nospam--at)
Subject: Plywood sheathing on wood trusses


Does plywood sheathing need to be flat against the top chord of supporting wood framing to be acceptable/effective?  I have a case where wood trusses are not parallel to the pitch of a roof, and therefore, the plywood will rest on one edge of the top chord, but be approximately 3/16" off of the other edge. 


Jim Wilson

Stroudsburg, PA


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