[RE-wrenches] truss loading

[David Brearley]

Rip,

Structural engineering is not required on every residential project in my
local jurisdiction (Austin, TX). But it is required for all PV systems on
flat residential roofs and for any residential racking system that is tilted
up off of the plane of the roof. In my experience this is a $400 - $500 line
item per project. 

You will probably find any number of engineers in your area willing to
provide this service. Look for small firms, even one person shops. If you
work with any local architects, they can recommend someone. If you find a
firm you like and keep working with them, the process will just get easier
for everyone. 

Just make sure you understand in advance what the structural engineer
requires from you in order to complete their review. In some cases, you may
have construction plans you can share. But you won't have plans for most
existing construction. So you'll probably need to create a new site analysis
form to make sure you get all of the construction details that the engineer
needs. You'll probably also wind up taking extra pictures during site
visits. 

What you basically want to avoid are extra trips to the site to verify if
the roof sheathing is 1/2" or 5/8", for example, plywood or OSB, 8" rafters
16" on center or 10" rafters 24" on center. Like every other step in the
process, return trips are profit killers. If most of your work is centrally
located, you may find an engineer who will drive to the sites to do the
review. That just makes it easier for them to write you the letter that you
need. If the travel time is insignificant, it may not add cost to the
engineering service.

In most cases, I suspect that a roof designed to carry 30 pounds per square
foot of snow can carry 3 additional pounds per square foot of PV. The
problem roofs will probably be the older ones that were built before the
structural codes were as clearly defined. I hope this is the case.

Best,

David Brearley, Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
david.brearley at solarprofessional.com
Direct: 541.261.6545
Fax:  541.512.0343

Visit our Web site at solarprofessional.com

On 10/28/08 3:25 PM, "Solar Plexus" <solplex at montana.com> wrote:

> 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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