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Structural requirements for PV

mattj | Posted in General Questions on

I’d like to super-insulate the roof and install a PV array on my ca. 1880 house in Massachusetts. The roof is approximately 25 ft from eaves to ridge, framed with ~5″ deep rafters roughly 19″ on center, and insulated with cellulose in the rafter plane (unvented). The top floor is finished, with knee wall attics.

I gather that the current roof framing isn’t adequate for a conventional racked PV installation and I’m wondering if there is any way I can avoid reinforcing the rafters. That work would need to be done from the outside, would require re-sheathing the roof and reinstalling the cellulose, and leave the roof open longer than I’d prefer.

If my plan is to add 6 inches of polyiso on top of the roof decking, would that affect an engineer’s considerations for the PV? What if I used metal roofing and mounted the panels to the standing seams? Would solar shingles do the trick? Is there anything else I should consider?

Thanks,
Matt

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matthew,
    There are lots of factors here. I think you need to consult an engineer.

  2. Expert Member
    Dana Dorsett | | #2

    Solar shingles might do the trick, but they are expensive and proprietary solutions with minimal follow-on support after the company goes bust, or ceases production. Panelized systems are somewhat standardized.

    A structural insulated panel (SIP) roof over the existing structure can be specified to meet the load requirements.

  3. user-6184358 | | #3

    When adding to a house you need to be concerned with the added weight on the structure. The California Building Code "CA 2016 Existing Building Code [BS] 402.4 & 402.4.1 Existing Structural Elements carrying lateral load -Any existing lateral load structural element whose demand capacity ratio is not to be increased by more than 10% . Comparison should account for the cumulative effects of alteration since original construction." This would apply from the top of the structure down.
    If adding 6" poly above the roof 2x6 or 4x6 could be added to add structure. Seem like you need an engineer

  4. seabornman | | #4

    Most of the structural reinforcing I've seen for PV is done from the inside. The engineers who specialize in that type of design are very creative. You need to keep in mind that adding lots of insulation to your existing roof has the possible effect of adding a lot of snow load to the structure.

  5. ranson | | #5

    Consult a structural engineer. You may be surprised how little it costs for them to come up with roof reinforcement. I was considering adding external foam and a second deck to my old house in MA, with almost identical framing. The site visit, roof analysis & plans, including unrelated basement work, was about $500. You probably have some constraints we didn't that will take a few more hours work. It's worth it to not compromise the structural integrity of your house.

  6. ranson | | #6

    Another thing to consider is that your house was probably built with rough lumber which is ~25% wider than standard 2x lumber. Standard load tables don't apply. What I didn't expect in my place, was that the attic joists were a bigger issue, especially around our staircase, and the rafters were okay. An engineer will sort this all out.

  7. mattj | | #7

    Thanks everyone. I will definitely be working with an engineer. I had a casual conversation about my question with our regular structural engineer who has done several conventional systems for us (I work for a remodeling company that does DER's). I got the impression that the roof would probably be ok with the increased weight if it was evenly distributed. He said the problem is that racking typically isn't attached to all of the rafters, just some, and the solar installers won't tell you which ones, so we end up reinforcing all of them. I asked about distributing the load more evenly and he just said he'd never done it, and the conversation ended there.

    I've heard of installations where panels are attached to metal roofing, and it seems unlikely that they'd be getting a direct load path to the rafters with that approach (right?). But I've never worked on one and so far haven't found anyone who has. Same with solar shingles. My solar installer has only done racked installs.

    So, I'm still in the feasibility/planning stage and wondering if there is any precedent for this kind of thing. I could justify a premium for fancy panels if they get me out of the structural work.

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