To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: drywall type shearwalls- l
From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
Date: Wed, 7 Mar 2001 07:52:02 -0900
<<RE: Code question: Plywood over gypsum wallboard (not gypsum shea thin g)
shear walls>> This is a copy of a message from Phil Line from APA and a memo
about the differences between C36 and C79.
Date: October 13, 2000
To: Plan Reviewers and Structural Inspectors
From: Scott Haan, Plan Review Engineer
Subject: Wood structural panel over gypsum board shearwalls [instead
Frequently the Structural Inspectors find interior walls that are supposed
to be wood structural panel shearwalls but that are only covered with
sheetrock. The proposed fix is often to apply a plywood panel over the
gypsum wallboard. 1997 UBC Table 23-II-I-1 has allowable unit shear values
for wood structural panels applied over gypsum sheathing, and not for wood
structural panels applied over gypsum wallboard.
ASTM C36 for gypsum wallboard has the same strength-durability requirements
as ASTM C79 for gypsum sheathing. The only difference is that gypsum
sheathing has a water resistant core and a water resistant covering. This
is because it is for an exterior application. APA Research Report 154 is
the basis for the UBC wood structural panel shearwall capacity tables. The
original APA Research Report 154 indicated gypsum wallboard, and this was
changed to gypsum sheathing. This is probably because gypsum sheathing is
required for exterior fire assembly applications, and only an idiot would
put wood structural panels over gypsum wallboard on an interior wall. See
the attached e-mail from Thomas Skaggs Ph.D. P.E., Senior Engineer for the
Based on this information the capacity of plywood sheathing over gypsum
wallboard is the same as plywood sheathing over gypsum sheathing.
Scott M Haan P.E.
Plan Review Engineer
Building Safety Division http://muni.org/building,
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183 fax: 907-249-7399
> -----Original Message-----
> From: JCohen [SMTP:jccpc(--nospam--at)email.msn.com]
> Sent: Wednesday, March 07, 2001 7:44 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: drywall type shearwalls- l
> Is there any documentation or testing which has been undertaken with
> to the use of ASTM C630 vs. ASTM C79 as sheathing material? A question has
> been raised with regard to either's use as exterior sheathing on a
> commercial building in the East Coast.
> Any information or references would be very appreciated.
> Thank you.
--- Begin Message ---
To: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>, "Phil Line (E-mail)" <philip_line(--nospam--at)afandpa.org>, "Alan Carr (E-mail)" <carr(--nospam--at)icbo.org>
I conducted a little research on your question. The testing to justify the
columns you cite in Table 23-II-I-1 in the 1997 UBC are summarized in APA
Research Report 154. The current version of Research Report 154 has a
slight contradiction. The text of this section references "Gypsum
Sheathing", and the title of the Table summarizing the data references
"Gypsum Wallboard". I then looked back at the original draft of Research
Report 154. The original memo, written in 1990, references Gypsum
Wallboard. Given the original memo referenced gypsum wallboard, I'd have to
conclude that the testing was most likely conducted with gypsum wallboard
Now the reference to gypsum wallboard in the original memo was struck out by
an ink pen (likely by a technical editor), and replaced with Gypsum
Sheathing. Given that Research Report 154 states that wood structural
panels applied over gypsum sheathing is for fire rated assemblies, I would
assume that the assembly in question was primarily intended for fire rated
constructions. Perhaps this is the reason gypsum wallboard was replaced by
In addition, I asked our helpdesk contacted the Gypsum Association and got
the following response from our helpdesk:
Gypsum Wallboard, IAW ASTM C36, is intended for interior applications.
However, it will perform suitably if protected by wall sheathing and siding.
Gypsum Sheathing, IAW ASTM C79, is intended for exterior applications. It
has water repellant face paper and may have treated gypsum core. Sheathing
is preferred for exterior applications since it may be exposed to moisture
during construction. If moisture exposure is not an issue wallboard is
So, in conclusion, I'd say that if you keep the gypsum wallboard dry, we
would not anticipate any structural degradation of the gypsum product, thus
the columns in Table 23-II-I-1 that reference gypsum sheathing could also be
used for gypsum wallboard.
If you have any additional questions, please feel free to contact me. Also,
your photos of the Cojo Salmon were quite impressive! Fishing must be good
Thomas D. Skaggs, Ph.D., P.E.
APA - The Engineered Wood Association
P.O. Box 11700
Tacoma, WA 98411-0700
> -----Original Message-----
> From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
> Sent: Friday, October 13, 2000 11:40
> To: Tom Skaggs; Phil Line (E-mail); Alan Carr (E-mail)
> Subject: Code question: Plywood over gypsum wallboard (not gypsum
> sheathin g) shear walls
> Please help me. I frequently encounter interior walls that
> were supposed to
> be wood structural panel shearwalls but that were covered
> with sheetrock
> without plywood underneath. The proposed fix is often to
> apply a plywood
> panel over the gypsum wallboard. 1997 UBC Table 23-II-I-1
> has allowable
> unit shear values for wood structural panels applied over
> gypsum sheathing
> and not for wood structural panels applied over gypsum wallboard.
> ASTM C36 for gypsum wallboard has the same
> strength-durability requirements
> as ASTM C79 for gypsum sheathing. The only difference I see
> is that gypsum
> sheathing has a water resistant core and gypsum sheathing
> board has a water
> resistant covering.
> I the only reason I see the code does not indicate wood
> structural panels
> over gypsum wallbord is that gypsum sheathing is intended for exterior
> applications and it is a waste of material to apply a wood
> structural panel
> over gypsum wallboard because another finish covering is then
> Can the same allowable unit shear values from UBC Table
> 23-II-I-1 be used
> for wood structural panels placed over gypsum wallboard in
> substitution for
> gypsum sheathing? If not why?
> Thank you!
> Scott M Haan P.E.
> Plan Review Engineer
> Building Safety Division,
> Department of Public Works,
> Municipality of Anchorage
> phone: 907-343-8183 fax: 907-249-7399