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

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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