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[SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [SEAOC] Number of plywood veneer layers.

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Thank you John and Michael. That was exactly the information I needed to know!

Jeff Smith

>You've got it almost right. 15/32" and 1/2" plywood sheathing are available
>in 3-, 4- and 5-ply layups, and 19/32" and 5/8" plywood sheathing are
>typically available in 4- and 5-ply layups. Five-ply plywood is available,
>especially in the CA market, but may be a special order item in some cases.
>As long as the face grain is across supports, the uniform load capacity is
>not affected by the number of plies.  When the face grain is parallel to
>supports, the cross-panel stiffness and strength are higher when there are
>more plies, and thus the panels can support more uniform load. There is a
>table in Sec. 23 of the '94 UBC that covers the uniform loads when panels
>are applied parallel to supports, as typically occurs for panelized roofs.
>For shear walls and diaphragms, the number of plies does not affect the
>shear values (except that the City of LA and County of LA have regulations
>which limit the shear values for 3-ply plywood to 200 plf - which APA will
>address when cyclic load shear wall tests are completed).  The shear values
>depend on the panel thickness and grade (e.g., Structural I or regular
>Sheathing), fastener schedule, panel layout (for diaphragms and shear
>walls), and lumber species.
>When re-roofing over spaced boards, APA has a Technical Note that provides
>recommendations on installing roof sheathing panels in this application,
>whether parallel to the spaced boards or perpendicular to them (e.g., up the
>slope of the roof).
>John D. Rose
>Tacoma, WA
>>I do not believe that table 23-I-J-1 of the UBC implies any requirement as to
>>the number of plys required for the given thickness of plywood.  This table
>>is only for the allowable shear for a total thickness of plywood.
>>As I understand it, 3/8" and 1/2" Exposure 1 plywood is only 3 plys.  5/8"
>>Exposure 1 plywood is typically 4 plys.  If you want a minimum of 4 plys,
>>than you need to call for 1/2" structural 1 plywood, or 5/8" Exposure I
>>plywood.  5 ply 1/2" structural 1 appears to be more of a special order which
>>most contractors do not want to do. Typically in our office  we are using
>>1/2" structural 1 plywood for all shear walls, since in Los Angeles all 3 ply
>>plywood is limited to 200 plf.
>>Michael Cochran  S.E.
>>Brian L. Cochran Assoc.
>>Los Angeles, CA