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Rebecca,<br>
<br>
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 <a
href="http://www.unirac.com/pdf/ii227.pdf">manual</a> 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.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar<br>
<br>
<br>
<br>
Rebecca Lundberg wrote:
<blockquote
cite="mid:c64d24ab0912050548o1eec765q880b55a3f377c2b4@mail.gmail.com"
type="cite">Hi William,<br>
<br>
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.<br>
<br>
Rebecca<br>
<div class="gmail_quote">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><br>
Message: 3<br>
Date: Fri, 04 Dec 2009 22:21:43 -0800<br>
From: William Miller <<a moz-do-not-send="true"
href="mailto:william@millersolar.com">william@millersolar.com</a>><br>
To: RE-wrenches <<a moz-do-not-send="true"
href="mailto:re-wrenches@lists.re-wrenches.org">re-wrenches@lists.re-wrenches.org</a>><br>
Subject: Re: [RE-wrenches] PV wind load<br>
Message-ID: <<a moz-do-not-send="true"
href="mailto:6.0.1.1.2.20091204220550.061cd348@millersolar.com">6.0.1.1.2.20091204220550.061cd348@millersolar.com</a>><br>
Content-Type: text/plain; charset="us-ascii"; Format="flowed"<br>
<br>
Rebecca:<br>
<br>
It sounds like you have made an air-tight case (pun intended) and you
have<br>
still not been able to convince this thick-headed inspector. I have
had a<br>
similar experience last summer that might be relevant: After being
pushed<br>
around beyond my patience, I fought back. I worked up the chain of<br>
command, stayed professional and put it all in writing.<br>
<br>
I won the battle and set a precedent. I have forbidden the building<br>
department from assigning the problem inspector to my projects and, to
my<br>
amazement, the department obeys my request!<br>
<br>
Good luck, and let us know how it turns out<br>
<br>
William Miller<br>
<br>
<br>
<br>
At 05:55 PM 12/4/2009, you wrote:<br>
>Solar is somewhat 'new technology' where I live. :-) I have a
building<br>
>inspector that feels that (32) Sanyo modules (6.88 kW array)
mounted to a<br>
>house roof is going to cause undue stress to the roof truss (it is a<br>
>regular house in a Minnesota suburb, building height is 26', 30
degree<br>
>roof angle, in a suburban neighborhood. 2" x 4" manufactured
truss). We<br>
>are leaving 12-24" on each edge with no PV, and are careful to
attach to<br>
>the center of each rafter.<br>
><br>
>The inspector is not that concerned with dead load, but is
especially<br>
>concerned with wind load. We have provided an engineering letter
that<br>
>ascertains that a solar array mounted parallel to the roof surface
is well<br>
>within the parameters of what a typical roof truss can handle. We
were<br>
>required to have an engineer look at this address specifically, so
we then<br>
>provided an additional engineer's letter that affirmed that yes,
solar<br>
>mounted in the manner we proposed (with Quick-Mount attachment and
Unirac<br>
>SolarMount rails, all installed according to manufacturers'
instructions)<br>
>would be fine on this home. The building inspector insists that the
wind<br>
>load is excessive, and wants more analysis. Do any of you have
ideas,<br>
>experience, or data that might help me?<br>
><br>
>Rebecca Lundberg<br>
</blockquote>
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