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Adding solar if a roof has two layers, of a metal roof over shingles

  • 1.  Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-09-2021 05:41 AM
    We have an opportunity to install solar on a shingle roof that was covered with a metal roof.  I think the default wisdom is if a roof already has 2 layers we are not to add solar.  Is that a hard and fast rule or if an engineer looks at it and calculates the loads are OK can one proceed?

    Thank you

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    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
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  • 2.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-09-2021 10:58 PM
    Edited by Duane Ediger 04-10-2021 09:06 AM

    My best guess would be that a primary criterion would be attachment strength.  PV racking is most commonly attached to a steel roof either via clamping hardware on the ribs of the metal material, or with brackets that screw through the metal roof into a structural member (or plywood deck).  With clamps, the design wind uplift force per clamp (which would determine the minimum number and maximum spacing of clamps) would be a function of the type and gauge of the metal roof material and the spacing and pullout force of the metal roof fasteners.  Penetrating brackets would eliminate the need to account for the likely unknown method of metal roof attachment, but the compressible shingle material between the metal roof and the roof deck may not be allowed by bracket manufacturer specifications -- with standoff instability and leaks being two potential failure modes.  If allowable, the installation crew would need to verify proper attachment to the deck or structural members.



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    Duane Ediger
    Owner
    RafterEye LLC
    duaneediger@gmail.com
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  • 3.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-10-2021 05:23 PM
    We planned to use a generous amount of S-5 Solar feet.  Each one accepts 4 stainless screws that goes into the decking which is 1by planking. The 1.5" screws will securely get into the planking which is in good shape.  So I am not worried about pulling out.  But my manager says he does not want the liability by going against the conventional wisdom of not installing solar if there is two layers.  I have not been able to confirm if that is a hard and fast rule, a typical solar convention or just a local convention.  It seems from most of the comments it is not a nation wide convention.

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    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
    ------------------------------



  • 4.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-10-2021 10:06 PM

    Sounds like the intended S-5! product might be the SolarFoot (https://s-5.com/products/solarfoot/).  Their installation instructions say that the brackets "must be mounted directly over and into the supporting structure of the roof, i.e. wood decking, wood or steel purlins, or trusses...."  There is no restriction mentioned regarding existing substrates.



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    Duane Ediger
    Owner
    RafterEye LLC
    duaneediger@gmail.com
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  • 5.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-11-2021 05:49 AM
    Correct I can not find any restrictions on adding solar for an older home with two roof layers.  Maybe a better way to ask the question have other installers put solar on a home with two roof layers?

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    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
    ------------------------------



  • 6.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    ASES Life Member
    Posted 04-11-2021 05:58 AM
    The question would have been easier to reply to if the sender had supplied more information.  Seems the real challenge is the manager.  The roof is shipslap over what must be REAL two by fours.  What might be considered is hot water under the panels, and painting the metal roof white, if the climate is in a hot climate.  This is a good challenge for which the diverse membership of ASES is made.

                               dnb





  • 7.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    ASES Life Member
    Posted 04-10-2021 07:13 AM
    Never heard of this.  Your thinking is the roof is not 'prefect' or what?   Your concern is what?  
    Thanks.
                                                               dnb

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    Dale Bickenbach
    Tacoma WA
    annadale.bickenbach@att.net
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  • 8.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-10-2021 04:35 PM
    I do not know if it is code or just a convention in this area, but I have been told solar acts as a second roofing layer.  Most homes are designed to support two layers, as sometimes people put on a second layer of shingles or put a metal roof over top of old shingles.  So if the roof already has two layers we are not supposed to put a third layer on.  I assume it is more for structural integrity as at some point one could have so much weigh on a roof it could collapse say with a BIG snow storm on top of 3 layers of roof.

    ------------------------------
    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
    ------------------------------



  • 9.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-10-2021 08:56 AM
    id say reroof! ur going to be installing Solar and do not want problems cropping up and it will be so dificult to do it latter and when u reroof u can do a very good job!

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    Rolland Riives
    President
    Guilt Free Hot Water
    cambellrives@comcast.net
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  • 10.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-10-2021 04:37 PM
    Yes that is an option but the metal roof is in good shape and there are no leaks or other reason to replace the roof.  So to remove two layers of roofing and then put a new single layer back on would come close to doubling the cost of solar.

    ------------------------------
    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
    ------------------------------



  • 11.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-11-2021 06:55 AM
    Typically you can add a 2nd layer of asphalt shingles, rather than stripping them off to reshingle a roof.  This has little to do with adding PVs to a metal roof.  As an engineer and an old solar project developer, my main concern would be whether the roof's structural design could handle the added load. If the rafters or trusses are undersized, it might be a concern.  The metal roofs, over asphalt shingles that I've run into have had strapping, either 1x4 or 2x4 firmly attached to the rafters sandwiching the existing shingles and often times enhancing the roof's structural capacity.  The metal roofs are then firmly attached to the strapping, thus using the appropriate metal roof clamps to attach the PVs should work just fine. A sloped metal roof with PVs typically sheds snow better than a metal roof without PVs.  My general rule of thumb on this installation might be not to sweat the petty stuff. (The other side of that might be to not pet the sweaty stuff.)  Peace!  





  • 12.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    ASES Life Member
    Posted 04-11-2021 07:22 AM
    Excellent reply, I would like to know the climate zone for the project, too.  A structural analysis would be less costly, if the engineer knows how to make the analysis, then, a new roof.
    Thanks,
                                                                                            dnb

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    Dale Bickenbach
    Tacoma WA
    annadale.bickenbach@att.net
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  • 13.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-11-2021 12:16 PM
    Edited by David Hrivnak 04-11-2021 12:19 PM
    The house in question in Appalachia VA climate zone 6a I believe.  They average 53 inches of rain and 21" of snow for an average year.  Design low temp is -5 and high temp of 85F.

    I guess I should have started with this question.  Are you a solar installer and if so would you add solar to a home that has two layers of roofing?  If the answer is no is it due to the fact you have never run across it or is it a company policy not to add solar if two roofing layers are present?

    Structurally I am not worried about the house, it is well built with maximum rafter spans of 6'.  But if the convention is to never put solar on a roof with two layers, then I understand going against convention. 

    Thank you

    ------------------------------
    David Hrivnak
    Sales/Engineering
    EcoLogical (part time)
    dhrivnak@chartertn.net
    ------------------------------



  • 14.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    Posted 04-11-2021 12:31 PM
      |   view attached
    That's not a rule in my book, in fact, I only have a metal roof over a single layer of asphalt roofing, but I used 2x4s for strapping and added 1-1/2" of foam in the cavity created, then put on the metal roofing, which is under my SHW collectors. If there's no structural issues and the metal roof is attached firmly to the strapping as it should be, it would not be a problem here on the stormy coast of Maine.   






  • 15.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    ASES Life Member
    Posted 04-12-2021 01:29 PM
    Edited by Mike Curran 04-12-2021 01:30 PM
    I think the "2 layer rule" would definitely apply if they were 2 layers of asphalt shingles.  But with the 2 layers consisting of a steel roof over shingles, the only thing you need to worry about is the roof structure's ability to support the load and to securely hold the array to the roof.   If those aren't an issue, then don't worry about your 2 layers.

    I'm not a roofer and only an amateur PV installer.  I put up 14 panels on an asphalt-shingled garage roof (8 in 2007, 6 more in 2017), and 58 panels (pictured in my profile) on a barn standing-seam metal roof (30 in 2012, 28 in 2019).   So far, so good.  Knock wood!

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    Mike Curran
    Retired from (dare I say it?) Fossil Plant Mgt.
    joacchim57@gmail.com
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  • 16.  RE: Adding solar if a roof has two layers, of a metal roof over shingles

    ASES Life Member
    Posted 04-14-2021 03:06 PM
    Better check the State Building Code for roof loading limits and details.
    David Stout.