One thing we've had to do here is stagger the connections to the rafters from row to row thus each rafter is secured to, not every other one. This is easy enough to do, and just adds one extra standoff or Quick Mount in your case on the staggered row. (we are in a high snow load area and the idea of offsetting the connection points to the rafters on every other row is so the snow load doesn't stress the rafters too much, spreading out the weight)
This is a win-win solution that your inspector should have no problem with. Otherwise the truss to top plate connection might need to be addressed by your structural engineer for its uplift value. Hopefully the home had Simpson truss holdowns, like A-35 clips, installed during the framing, and if this is in the original engineering that should suffice. Sincerely, Chris Worcester Solar Wind Works NABCEP Certified PV Installer Phone: 530-582-4503 Fax: 530-582-4603 www.solarwindworks.com <http://www.solarwindworks.com/> [email protected] "Proven Energy Solutions" From: [email protected] [mailto:[email protected]] On Behalf Of Kent Osterberg Sent: Saturday, December 05, 2009 8:46 AM To: RE-wrenches Subject: Re: [RE-wrenches] wind load 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 <[email protected]> To: RE-wrenches <[email protected]> Subject: Re: [RE-wrenches] PV wind load Message-ID: <[email protected]> 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 _____ _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] Options & 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 _____ No virus found in this incoming message. 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