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RE: Unblaanced snow on 12 in 12 roof (UNCLASSIFIED)

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Classification:  UNCLASSIFIED 
Caveats: NONE

My reading on the code change was that before W was meant on the same side as
the unbalanced load and now W is on the unbalanced load.

The old way when W was less than 20' the unbalanced load was based on your
sloped roof snow load no problem - now it based on I*Pg.  This is an extreme
change for where you have a 12:12 pitch and a metal roof even if a cold roof.


Try it for Yakutat Alaska where the ground snow load is 150 psf for a 26'
wide building with a 12:12 pitch metal roof. I come up with the old code an
unbalanced load of 78 psf or something and with the new code 150 psf. Your
2x12 rafters @ 24" oc designed under the old code doesn't work anymore with
the new code. Your ridge beam does not work anymore either.

-----Original Message-----
From: AWC Info [mailto:AWCInfo(--nospam--at)afandpa.org] 
Sent: Monday, February 05, 2007 6:29 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Unblaanced snow on 12 in 12 roof

Jordan, 

I didn't think there was that much difference between 7-02 and 7-05. We did a
comparison of 7-05 to 7-98 and didn't even see the magnitude of differences
that you note below. Here's a link to our paper:

http://www.awc.org/pdf/PaperSnowProvisionsASCE7-05web.pdf 

Take a look at that and see if it helps. If not, give me a few more specifics
about your particular situation (e.g. span, ground snow load, etc.) and I'll
see if I can help.

Buddy 

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
http://www.awc.org 

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: "Jordan Truesdell, PE" <seaint1(--nospam--at)truesdellengineering.com>
To:  seaint(--nospam--at)seaint.org
Subject: Unblaanced snow on 12 in 12 roof 

I was just looking at an existing roof the the owner would like to add some
new load to.  The slope is 12 in 12, in an uninsulated attic space, 25psf
ground snow load. By my reading in ASCE 7-05, the entire roof effectively has
to be designed for Ixpg=25psf, whereas in the past
(ASCE7-02) the load would be 1.5ps = 1.5*(0.7*I...*pg)*Cs or
1.5*(0.7*1*...*25)*0.6 = 15.75 psf. 

Is there some errata I've missed? This is nearly a 60% load increase from the
previous code. I realize that it only affects steeper roofs (8 in 12 to 32 in
12), but it seems like there's a gap in the code application. 


By the way - does anyone actually check to see if ridge board roofs work for
unbalanced snow loads, or do we all just close our eyes and hope that a
couple .131 toenails and friction keep the roof from collapsing? 
Experience dictates the latter - both on paper and in the field. 

-- 

Jordan 


Classification:  UNCLASSIFIED 
Caveats: NONE


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