Hey Bruce -
Engineer's Toolbox lists Asphalt Shingle Roofing at 2.7psf, which seems about
right. Further down they also have listings for plywood and OSB, which is
often added to structures during a re-roof.
http://www.engineeringtoolbox.com/roofing-materials-weight-d_1498.html
The main 'trick' used by Structural P.E.s to allow for "overloading" of roof
structural members is to argue that the roof no longer needs the code required
14 psf live load capacity because nobody will be able to walk in the area that
the solar panels are installed, so an 11psf live load capacity is more than
adequate for safety (14 minus 3 for the solar panels)
Other than that, it is certainly reasonable and appropriate for the Plan
Checker to want to check the load capacity of the roof when weight is being
added, and once they are doing that the most recent span tables are a pretty
reasonable place for them to look for guidance. Try the live load argument
above and see if that gets you anywhere when you run into this.
Bruce I had a great time at Solarthon in Castroville this past month - Hope
Grid is treating you well!
Greg McPheeters
________________________________
From: Bruce Leininger <[email protected]>
To: [email protected]
Sent: Saturday, July 6, 2013 3:04 PM
Subject: [RE-wrenches] Structural Review for Residential PV Systems
A local jurisdiction is starting to require that we document non-truss rafter
dimensions, span and attachment points. This seems to be a fad that other
jurisdictions have required at times, and other times not. When the existing
structure was deemed insufficient, we've either paid a structural engineer to
do some calcs and recommend more attachment points, or sistered the rafters to
match the spacing given in the 20 psf table in the UBC.
Would the dead load from the PV modules ever be more than that from
adding a second layer of comp roofing, which does not require a
structural analysis? Would the live point loads from the PV system
ever be greater than the point load of a person walking on the
roof? If the answers are "no" and "no", then it seems that a roof
that supports a person walking on it would not be adversely affected
by the installation of a non-ballasted residential PV system.
I looking for help with the following:
1. Has there been an occasion when a residential PV system (non-ballasted),
that was installed per the manufacturer's instructions, detached from a roof or
caused damage to the structure of the roof?
>2. Is there a good explanation for why structural review for
residential PV systems is necessary?
>3. Are there studies or resources that I can present to building
officials to explain why structural review for residential PV
systems is not necessary?
>
Thanks for your help.
Bruce
_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Change email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org_______________________________________________
List sponsored by Home Power magazine
List Address: [email protected]
Change email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
