Roof load question [RE-wrenches]

[Barbra Kerr]

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Allan Sindelar writes: 


> Wrenches,
> We are proposing a non-penetrating roof-mount system for a commerical
> installation.  We plan to use a UniRac SolarMount platform for this system.
> Working through the calculations provided by UniRac, we determined the
> ballast required for this system is 11 lbs. per sq. ft. excluding the
> modules and rack. 
> 
> My simple question is: what are flat roof building structures designed to
> carry?  Are there any rules of thumb for this? I will require the customer
> to get an engineers approval for this, but I would like to know up front if
> this is ridiculous or in the ball park?
> Thanks,
> Randy 
> 
> Randy Sadewic
> randy at positiveenergysolar.com
> Positive Energy
> 3225A Richards Lane
> Santa Fe, NM  87507
> Phone 505 424-1112
> Fax 505 424-1113 
> 
> Enjoy the Sun! 
> 
> 
Randy - 

My experience with commercial flat roofs (and the structural enginnering 
firms)lead me to suspect that the "total" anticipated load may exceed not 
only the intended design load, but also the margin for future possible loads 
that many engineers and building departments have built into both their 
designs and building codes.  You are wise to put the issue of engineering 
approval in the hands of the Owner. 

More directly to your question.  Each county has their own specific building 
codes.  What works in San Diego may not be acceptable for Sante Fe.  You can 
go to the building department having jurisdiction over that building and 
view the original building plans and the engineering/load calcs for the roof 
and the entire building.  You will be able to walk away with copies of the 
engineer's original calculations, take those to the engineer reviewing the 
PV design and load, and not only will you save the Owner money, but you will 
keep the project moving. 

Should the PV design call for "beefing up" the glu-lams/joists/etc...please 
be reassurred that this additional work doesn't have to result in excesive 
costs.  We have had to do modify existing glu-lam beams that resulted in 
only 2 extra days worth of work for the City of San Diego for another 
self-ballasting non-penetrating roof mounted PV system. 


Barbra K. Kerr
President
Kerr Enterprises, Inc
209-586-9011
209-586-8607 Fax
858-337-5097 Cell 

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