[RE-wrenches] truss loading

[Bill Brooks]

Rip,

Going down the road of engineering on every roof is a very slippery slope
(particularly with ice and snow). The idea that a roof designed for 30psf
would get in trouble with an additional 3psf is ridiculous. The Inspector
Guidelines that I wrote, which are based many jurisdictions across the U.S.
encourage the use of standard, engineered, stand-off mounting systems (e.g.
UniRac, ProSolar, DPW, etc...) with buildings that have previously been
inspected. If the building has never been inspected, then the rafter sizes
and spacings should be provided, or truss calcs for trusses.

I understand that snow loads can be considerable (the Sierra's have
locations requiring 300psf), but we also have hurricane locations and other
loading issues. The biggest concern that I see with heavy snow loads is the
point loading of the truss or rafter. In high snow load areas, the array
should be located near the ridge (3') so that snow cannot accumulate above
it, should be parallel to the roof, so that snow will not collect behind it,
and have the mounting feet every 4-feet with alternating rafters used for
the upper and lower rail connections. Rails should be run East-West on a
South facing roof so that attachments can be adequately staggered. Snow has
a greater tendency to shed from a glass surface making it more likely to be
removed earlier than the rest of the roof.

Encourage your local jurisdiction to use the Inspector Guidelines at
www.irecusa.org. 

Bill.

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Solar Plexus
Sent: Tuesday, October 28, 2008 1:25 PM
To: RE-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] truss loading

Hi All,

We were recently notified by our local building department that all  
future building permits will require a letter from a state licensed  
engineer stating the existing roof trusses are adequate for the  
additional load a solar system will add.  All roofs in our area have  
a 30 pound per square foot snow load requirement.  The building  
departments concern is truss manufactures design the trusses to very  
tight tolerances and there is no room for additional load.  Has  
anyone had to address similar requirements?  Any ideas on how to  
approach this issue would be appreciated.

Thanks in advance, Rip

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