[RE-wrenches] wind load

[Dave Click]

Something that may help with the inspector would be to alternate 
trusses. If your array spanned ten trusses, which I'll call A-J, and you 
have three rows of six rails:

Rail #1, 3, 5: attach to A, C, E, G, I, J
Rail #2, 4, 6: attach to A, B, D, F, H, J

It's not helping his concern on the end trusses but it would help with 
the interior trusses, anyway.

The SolarABCs.org expedited permitting report may help with things as 
well. Since these are engineered trusses that you're attaching to, I'm 
surprised that the inspector is giving you such grief!

We Florida guys love hearing about wind loading concerns in a 90mph 
zone! I work in a 130mph wind zone...

DKC

-------- Original Message --------
Subject: Re: [RE-wrenches] wind load
From: Kent Osterberg <kent at coveoregon.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Date: 2009/12/5 11:45

> Rebecca,
> 
> With three rows and six lags in every other rafter there is a tremendous 
> force on each of the rafters.  Unirac has excellent information in their 
> installation manual <http://www.unirac.com/pdf/ii227.pdf> that shows all 
> the engineering info needed for ASCE-7, which is meets the  requirements 
> for most building departments.  If you go through the calculations, you 
> can determine the maximum force or load on each lag bolt.  And you 
> should go through the calculations to make sure that the spacing and lag 
> bolt penetration is adequate.  The adequacy of the roof structure is a 
> question for the structural engineer.  Modern truss structures will 
> handle it, and presumably that's what your structural engineer looked 
> into.  At this point, I agree with William: it is time to go over the 
> inspector's head with the argument that he doesn't have the authority to 
> question the engineering.
> 
> Kent Osterberg
> Blue Mountain Solar
> 
> 
> 
> Rebecca Lundberg wrote:
>> Hi William,
>>
>> Thanks for responding. We have three long rows of modules, with rails 
>> attached every other rafter, every 4'. The inspector's argument is 
>> that two attachments for one row might be fine, but that the wind load 
>> for the entire array theoretically could be born by one rafter, and 
>> therefore it would pull the rafter right out of the house. This is 
>> only my year 3 as an installer, and while I'm not afraid to push the 
>> inspector if needed, I admit I want to be sure he isn't right, even 
>> though an engineer says so (because I'm not an engineer :-). So in 
>> your experience, is there a limit to an array size that would be 
>> 'safe' for a home if the array is within the edges of the roof and 
>> flush-mounted? I feel like that's the real argument, you might safely 
>> attach one module, but he is saying when you multiply the modules you 
>> multiply the wind load to the house by the same factor.
>>
>> Rebecca
>>
>>
>>     Message: 3
>>     Date: Fri, 04 Dec 2009 22:21:43 -0800
>>     From: William Miller <william at millersolar.com
>>     <mailto:william at millersolar.com>>
>>     To: RE-wrenches <re-wrenches at lists.re-wrenches.org
>>     <mailto:re-wrenches at lists.re-wrenches.org>>
>>     Subject: Re: [RE-wrenches] PV wind load
>>     Message-ID: <6.0.1.1.2.20091204220550.061cd348 at millersolar.com
>>     <mailto:6.0.1.1.2.20091204220550.061cd348 at millersolar.com>>
>>     Content-Type: text/plain; charset="us-ascii"; Format="flowed"
>>
>>     Rebecca:
>>
>>     It sounds like you have made an air-tight case (pun intended) and
>>     you have
>>     still not been able to convince this thick-headed inspector.  I
>>     have had a
>>     similar experience last summer that might be relevant:  After
>>     being pushed
>>     around beyond my patience, I fought back.  I worked up the chain of
>>     command, stayed professional and put it all in writing.
>>
>>     I won the battle and set a precedent.  I have forbidden the building
>>     department from assigning the problem inspector to my projects
>>     and, to my
>>     amazement,  the department obeys my request!
>>
>>     Good luck, and let us know how it turns out
>>
>>     William Miller
>>
>>
>>
>>     At 05:55 PM 12/4/2009, you wrote:
>>     >Solar is somewhat 'new technology' where I live. :-) I have a
>>     building
>>     >inspector that feels that (32) Sanyo modules (6.88 kW array)
>>     mounted to a
>>     >house roof is going to cause undue stress to the roof truss (it is a
>>     >regular house in a Minnesota suburb, building height is 26', 30
>>     degree
>>     >roof angle, in a suburban neighborhood. 2" x 4" manufactured
>>     truss). We
>>     >are leaving 12-24" on each edge with no PV, and are careful to
>>     attach to
>>     >the center of each rafter.
>>     >
>>     >The inspector is not that concerned with dead load, but is especially
>>     >concerned with wind load. We have provided an engineering letter that
>>     >ascertains that a solar array mounted parallel to the roof
>>     surface is well
>>     >within the parameters of what a typical roof truss can handle. We
>>     were
>>     >required to have an engineer look at this address specifically,
>>     so we then
>>     >provided an additional engineer's letter that affirmed that yes,
>>     solar
>>     >mounted in the manner we proposed (with Quick-Mount attachment
>>     and Unirac
>>     >SolarMount rails, all installed according to manufacturers'
>>     instructions)
>>     >would be fine on this home. The building inspector insists that
>>     the wind
>>     >load is excessive, and wants more analysis. Do any of you have ideas,
>>     >experience, or data that might help me?
>>     >
>>     >Rebecca Lundberg
>>
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