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RE: 1 hour exterior wall

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If the plywood is part of the structural system, then exposing the combustible plywood to the exterior without the gyp sheathing protection would not provide the required one hour protection to the exterior wall envelope.  704.5 tells you that exposure can come from both sides    Note also 1406.2.1.1




From: DA [mailto:dnae(--nospam--at)]
Sent: Friday, May 02, 2008 5:07 PM
To: seaint(--nospam--at)
Subject: Re: 1 hour exterior wall


Thanks Bob

It is planned to have wood siding

I know it is better to nail the siding to plywood 

is ok to nail the wood siding to dry wall?

Thanks for input

Dave A.


----- Original Message -----

From: Bob Freeman

Sent: Friday, May 02, 2008 4:13 PM

Subject: 1 hour exterior wall


Hi Dave:


You can do it either way structurally.  Plywood over drywall has reduced shear values in the code.


You may want to consider adding the plywood first, then applying drywall (or plaster).  On the exterior face I use 1 inch thk. cement plaster, but you can use 5/8” thk. Type X Gypsum Sheathing.  This is black coated paper drywall intended for exterior applications.  Check out the Gypsum Association web site for U.L. listed fire ratings.  Use an approved assembly and call out the approval number on your drawings.



IDS Group, Inc.

Bob Freeman, AIA, EIT

(949) 387-8500