[RE-wrenches] Structural Review for Residential PV Systems

Greg McPheeters gmcpheeters at yahoo.com
Sat Jul 6 17:42:22 PDT 2013


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 <bruce at gridalternatives.org>
To: RE-wrenches at lists.re-wrenches.org 
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: RE-wrenches at lists.re-wrenches.org

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20130706/c8450337/attachment-0004.html>


More information about the RE-wrenches mailing list