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Plywood over 2x Decking for Diaphragm

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Title: Plywood over 2x Decking for Diaphragm

AF&PA's Special Design Provisions for Wind and Seismic, 2005 Edition contains the following exception to plywood directly applied:

Exception: Wood structural panel sheathing in a
diaphragm is permitted to be fastened over solid
lumber planking or laminated decking provided
the following requirements are met:

1. Panel edges do not coincide with joints in
the lumber planking or laminated decking.
2. Adjacent panel edges parallel to the planks
or decking are fastened to a common member.
3. The planking or decking shall be of sufficient
thickness to satisfy minimum fastener
penetration in framing requirements as given
in Table 4.2A.
4. Diaphragm aspect ratio (L/W) does not exceed
that for a blocked wood structural
panel diaphragm (4:1).
5. Diaphragm forces are transferred from
wood structural panel sheathing to diaphragm
boundary elements through planking
or decking or by other methods.

You can download the full standard at:



John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791

The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.


From: "Joe Grill" <jgrill(--nospam--at)>
To: "seaint" <seaint(--nospam--at)>
Subject: Plywood over 2x Decking for Diaphragm

I have a project where the client has (so far) provided a floor system consisting of 3/4" plywood over 2x6 T&G decking. The decking spans 3 ft to 6x10 purlins. It seems to me that I can use the plywood diaphragm values for a blocked diaphragm assuming nail size (short enough to not protrude clear through the decking) and penetration is taken into account in the diaphragm values. I think this is the case as all the panel edges are backed up with the decking so the diaphragm can be considered "blocked" and the 2x6 decking will span the 3 ft.

Any thoughts?
Joe Grill
Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
P.O. Box 3924
Sedona, AZ 86340
PHONE (928) 282-1061
FAX (928) 282-2058