GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Installing Solar Panels on a Roof With Exterior Rigid Foam

kaster | Posted in PassivHaus on

I have a recent retrofitted passivHaus built in the Bronx, NY with new architectural shingles over a quarter inch thick plywood and 4 inch ridgid foam boards on top with another quarter inch plywood sitting on top of the old existing roof.

I’m now seeking to install solar panels but with all my engagements with local PV installers they don’t seem to have the ability or confidence to find a way to install/anchor the panels to my roof.  1. The PV installers seem to all agree they need to anchor the brackets to the rafters but how can they find it on one go without making my roof into Swiss cheese is the concern.

Do anyone know 1) best way to install solar panels to passivHaus post construction with at least 4 inches of exterior ridge foam boarding?  2) PV installers in NYC who can help?  3) install method?

Note:  I am not the architect but the owner of the passivHaus.

I’ve also saw the Q&A below but the info seem to be stale since 2016 entry.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Mark_Nagel | | #1

    Do some very exact measuring (to/with reference points that are accessible from the top) on the underside (to transfer to top)?

    Wonder if you could identify the rafters via some sort of thermal imaging (Flir?)?

    Might come down to finding out from an engineer what kind of fasteners might work (installers will have to provide specs). I'd think that some strapping (perhaps metal) could be used on the underside such that connectors from the topside would connect to it and spread any load out across the underside plywood (and maybe that strap could be anchored to rafters?).

  2. Expert Member
    Akos | | #2

    I'm assuming the roof deck is actually 1/2", can't install shingles over 1/4" plywood.

    If that is the case, there are many deck mount system, these use more screws per mount into the plywood instead of the single lag into the rafter like the ones from

    There are many out there, your installer should be able to point you to a brand they prefer.

  3. GBA Editor
    Kiley Jacques | | #3

    I think you would find the comments to a related question of value: Mounting solar panels above roof with exterior foam?

  4. kaster | | #4

    Thanks for the input. I had met with over 3 PV Installers that service the Bronx and they each turned the job down. Not sure if it is a NYC thing but they all only want to install via rafters only.

    The fast track rafter talons seems like the best option. If I can find a PV installer who has and can do it.

    Measuring the underside of the roof for the rafters was a good approach too but the PV installers didn’t want to put the effort into it.

    Another, I though of was finding strong magnets to connect under the roof and outside the roof to find the rafters or sonar...but that seems not likely either.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      I don't understand the installer's reluctance, and desire to anchor right back to the rafters. If the 2"x furring is properly fastened to the rafters, why aren't they satisfied with anchoring to it? That gives them 2" of meat to fasten to. Are you sure they understand that is there and available to them?

      1. kaster | | #10

        I had provided them the architectural drawings like in my post so each vendor’s installer understood what they were working with. They just never provided cause other than they won’t be able to take on the project post review. The only thing I can think of is my room is flatter/less slopped than most roofs and maybe worried if strong winds pulling the panels off?

        I tried the following PV installers venture, momentum, and sunrun

        1. Expert Member
          MALCOLM TAYLOR | | #13


          Glancing quickly at your section, and reading your description of the assembly, I initially assumed you had nothing but the sheathing to attach to as well. If I was approaching an installer I'd tell them I have a roof with sheathing fastened to 2"x4" strapping, and not worry about is under that as it doesn't concern them.

    2. user-3258290 | | #7

      My roof was 1/2" OSB over 2.5" polyiso over 1" skip sheathing, so a bit less than your situation at only 4" to the rafters. I knew the approximate rafter spacing, but only had access to the underside for one of 15 mounts. I bought a used Bosch D-TECT 150 scanner on eBay, used it for 20 minutes to hone in on the rafter locations and install the mounts, then resold it for a net rental cost of ~$25. Also, I had a flat surface to scan across, felt paper on OSB, as we pulled the steel roofing from the area where each mount was going ahead of time.

      1. kaster | | #9

        Ill have to ask vendors why they can’t is something like the Bosch D-TECT 150. This is what I was thinking as a “sonar” tool in my earlier post.

  5. Expert Member
    NICK KEENAN | | #5

    See if you can get a roofer to put the mounts in and then have the solar installer install the panels.

  6. Jon_Lawrence | | #8

    Have they acknowledged that deck only mounts exist and they are just are not confident it will work in your situation?

    My panels use Sunpower's Invisimount system that is approved for deck only attachment. I have 5/8" plywood on top of 4" of polyiso. This sandwich is attached via 7 1/2" HeakLOK screws (there is a 1" vent cavity between the plywood and the polyiso). We hit every 3rd rafter per the manufacturer's installation requirements. The Invisimount brackets are attached only to the plywood with lag screws. It is a solid connection that does not move at all even if I give it hard tug. This is a really strong system. If there were ever a failure I think it would be the panel detaching from the rail not the bracket pulling out.

    1. joenorm | | #11

      Your mounts that attach to the roof are likely not Sunpower brand, that look like these:

      they do have a "decking" option

      Everything after the L-foot is Invisimount racking.

      1. Jon_Lawrence | | #12


        You are correct about the mounts. Looking through the brochure, I now remember the one they used. There is a shoe underneath the flashing that is held in with (4) 1/4"x 3" deck screws.


        The link below has the brochure for the mount Joe identified. Page 12 shows the deck version. Maybe you can show that to the installers.

  7. kaster | | #14

    So I got an answer back from one vendor by their install design engineering and this seems to be the issue the past two vendors I checked with as well. Really odd...

    “ We don’t have a way to install on this type of roof. There’s not any install means I’m aware of that can both meet the minimum requirements for attachment strength and maintain the insulative properties of the foam.”

  8. Jon_Lawrence | | #15


    You could try getting my installer to give you a quote -

    1. kaster | | #18

      I reach out thanks

  9. andy_ | | #16

    Remember Hurricane Sandy?
    It seems to me that the installers are covering their butts. Yes, you can install over foam as that's how most commercial roofs are made, but it's not nearly as straight forward as bolting to rafters. They're just nervous about anything outside the norm on residential work and really, I don't blame them.
    If I was approached with the information you just gave here, I'd pass too. From the drawings it seems that there's only a 2x board attached to the sheathing above the foam, and no solid structure between it and the rafters below. I don't know if that's really the case, but it would look like it to anyone going off the drawing. Saying that it's just thin 1/4" plywood for the roof deck would also mean no surface anchor could be used. I'm guessing that plywood (or OSB) is more likely 3/8"-5/8", but adding that uncertainty to an unconventional (to most) assembly will just help scare the installer off the job.
    Can you contact your roof designer and contractor to get more comprehensive info about how it was put together? That would help ease the fears of your installer.

  10. user-1140531 | | #17

    I see the 2 x 4s lying flat on top of the Polyiso. I see they are attached to a short piece of framing in the soffits. How are the 2 x 4s attached as they extend up the roof over the Polyiso to the ridge?

    I see the existing rafter below the original roof deck. Is there another rafter standing on top of the original roof deck, and separating the layers of Polyiso insulation? And if there is, are the 2 x 4s shown flat on top of the insulation nailed to that second rafter? I don't see any of that detail in your drawing, so I assume that the 2 x 4s lying flat are just resting on the insulation board, and not connected to the original roof deck and rafters.

    If that is the case, I can see why solar installers would not want to connect to the structure, which consists of 1/4" plywood roof deck nailed to the 2 x 4 furring, and all of that just floating on the foam board except for attachment at the eaves, and probably the ridge.

  11. Jon_R | | #19

    Maybe ask the installers if they would accept an engineer stamped approval.

  12. user-1140531 | | #20


    You mentioned getting the following comment from a solar installer:

    “We don’t have a way to install on this type of roof. There’s not any install means I’m aware of that can both meet the minimum requirements for attachment strength and maintain the insulative properties of the foam.”

    I think what they are saying is that they need a structural connection to the roof rafters in order to meet the minimum attachment strength requirements. They have concluded that there are no rafters within the 6” layer of insulation, and that lack of new rafters was intended to eliminate thermal bridging through the insulation, which would result from rafters in the insulation.

    So they are saying that their necessary structural connection passing though the insulation would be a thermal bridge. They say that this thermal bridge would compromise the insulative properties of the foam insulation. So that leaves them with no solution for mounting the panels on the roof.

    I don’t see why a plan to mount the panels connected to the original rafters could not be devised, but it would be complicated and more costly than a straight forward mounting on a topmost roof deck as is usually the case. It would also add thermal bridging as the installer mentions.

    You could place mounting plates on top of the new deck, over the 2x4 battens, and screws them into the original rafters, assuming that the battens are currently placed directly above the original rafters. But you would need 10-12” long screws to reach the rafters. It would take a lot of measuring and layout to hit the rafters with the screws. It would require very accurate positioning of the drill, assuming you had determined exactly where the original rafter is located.

    Then the screws will draw down on the mounting plate in order to make the mounting plate resistant to rocking on the roof deck. However, when you tighten the long screw, the compressibility of the Polyiso may make it difficult to prevent the mounting plate from rocking by compressing the Polyiso.

    Unless there is more to the roof than what is shown on the drawing, I can understand what the installers are objecting to.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |