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Advice for residential flat roof installation in New England

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  • Advice for residential flat roof installation in New England

    I am looking to have a 10KW system installed on a 1950's mid-century flat roof house in Rhode Island. The rubber roof is four years old and has only a very small amount of pitch. The rafters are 16" on center 2X12s.

    We do get some pooling on the roof after a rain, but multiple roofers and an engineer have all advised me that this is totally normal and not to worry about it. We've owned the house for three years and have not had any leaks or problems.

    I have had four or five estimates and settled on a company to do the installation. Currently, I'm waiting for their final design determination and not sure if they will go with a ballasted system (they use Ecofoot2+) or a penetrating rack (they use OMGRoofing).

    I've been unsuccessful in my search for homeowners with solar installations on a flat roof. So far, I'm relying on the good word of the solar company salespeople who all assure me that there's nothing to worry about. However, I'm nervous about both options and am wondering if there are any experts out there who can provide some advice on things to look out for or insist upon.

    Many thanks for any help!

  • #2
    1.) Most roofs are flat. Many are oriented at an angle with respect to the horizontal. You have one that is mostly horizontal with the usual slight pitch of something like a degree or two or so to help with runoff/drainage. Some puddling is common and expected, mostly unavoidable and mostly harmless.
    2.) For several reasons, I hope your panels will be at some angle to the horizontal, probably greater than 30 deg. or so.
    3.) Ballasted systems without some fixation can still move/creep, even on mostly (but not completely) horizontal roofs.
    Also, ballasted systems on (mostly) horizontal roofs need careful examination of the roof to ensure the roof can handle the added weight of the ballast, which can be quite significant, and also the other external loadings such as uplift from wind forces as well as consideration of the cyclic nature of those wind loadings, all of which were probably was not envisioned or designed for when the building was originally designed and constructed.
    4.) Check with the building dept. for requirements for PV systems with respect to design and permitting requirements for PV systems, including roof loadings.

    Comment


    • #3
      Thanks J.P.M! Clarifications:

      1) The initial proposal is for the panels to be a at 9 deg. tilt, though the design has not been finalized.
      2) The system will be permitted by the building department, so those load calculations will be required to pull the permit. The preliminary design shows 129 ballast blocks (32 lbs each), with a total array pressure of 6.7 lbs/square foot.
      3) The website for this ballasted system is at: https://ecolibriumsolar.com/ecofoot2plus/ and it shows video of how the system works.
      4) The roof did originally carry an air conditioning system (compressor, external ducts, etc.), that was completely removed before the new rubber went down four years ago

      Given your comments, it doesn't sound like this is a terrible idea overall... so that's encouraging.

      Comment


      • #4
        Originally posted by austinb View Post
        Thanks J.P.M! Clarifications:

        1) The initial proposal is for the panels to be a at 9 deg. tilt, though the design has not been finalized.
        2) The system will be permitted by the building department, so those load calculations will be required to pull the permit. The preliminary design shows 129 ballast blocks (32 lbs each), with a total array pressure of 6.7 lbs/square foot.
        3) The website for this ballasted system is at: https://ecolibriumsolar.com/ecofoot2plus/ and it shows video of how the system works.
        4) The roof did originally carry an air conditioning system (compressor, external ducts, etc.), that was completely removed before the new rubber went down four years ago

        Given your comments, it doesn't sound like this is a terrible idea overall... so that's encouraging.
        Thank you for the reply. Understood.

        While not a fan of ballasted systems, and certainly not a fan of low panel tilts, especially at higher latitudes such as RI, not my design, system, $$'s, etc.

        I would suggest keeping an eye on roof sagging as f(time) under the ballast and on the roof in general, by looking/observing at what happens to the roof profile and changes in puddling tendencies going forward.

        One perhaps (saving ?) consideration of low slopes for panels on (mostly) horizontal roofs is that on a flat roof that does not have significant shading, the max. output (size) of an array is approx. limited to the max. output of a horizontal array that covers the entire roof. That's because of self shading of the more northerly rows by the more southerly rows. The greater the tilt, the greater the annual output potential per panel, but that's approx. offset by the greater spacing between rows that's required to avoid the shading. For example, if that ballasted system's panel slope was, say, 30 deg. instead of 9 deg., annual output per m^2 of panel would be greater but the north- south space between rows would need to be greater, resulting in less panel area available per square ft. or square meter of roof area. To a first approx., those two sort of offset one another.

        A third comment I'd have is that the skirt at the north side of each row will most likely impair and reduce air circulation behind the panels. That lower(ed) air circulation will increase the temp. of the panels and so reduce panel efficiency and array annual output. My speculation is that the skirt was added for reasons of less ballast mass required to meet code wind loading requirements for uplift, but that skirt will probably cost a couple/few % in annual array output.

        Q: What are the roof dimensions and how big (STC kW) is the array ? (129 * 32)/6.7 = 616 ft.^2. Roof area or array footprint ?

        Last comment: Most AC systems don't weigh 129*32 = 4,128 lbm. Just sayin'.
        Last edited by J.P.M.; 05-17-2019, 12:26 PM.

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        • #5
          Thanks! This is great info.

          Roof dimensions are in a two connected sections (but all one plane):
          Total: 1312 sq ft
          Section A: 16X24 (384 sq ft)
          Section B: 29X32 (928 sq ft)

          The (preliminary) design calls for:
          Panels: 32
          Array Sq Ft: 867 sq ft
          Array weight: 5,800 lbs
          Inter row spacing: 18.9"

          Comment


          • #6
            Originally posted by austinb View Post
            Thanks! This is great info.

            Roof dimensions are in a two connected sections (but all one plane):
            Total: 1312 sq ft
            Section A: 16X24 (384 sq ft)
            Section B: 29X32 (928 sq ft)

            The (preliminary) design calls for:
            Panels: 32
            Array Sq Ft: 867 sq ft
            Array weight: 5,800 lbs
            Inter row spacing: 18.9"
            You're most welcome.

            What are the panels (mfg., STC output/panel) ?

            Any shading by surrounding structures ?

            Row spacing ? (not clear space between panels but top of panel to top of panel for adjacent rows.
            Last edited by J.P.M.; 05-17-2019, 12:58 PM.

            Comment


            • #7
              Panels: LG 335
              Shading: Yes, there is some shading from trees, though not too much
              Row spacing: not clear

              Comment


              • #8
                Originally posted by austinb View Post
                Panels: LG 335
                Shading: Yes, there is some shading from trees, though not too much
                Row spacing: not clear
                I should have looked a bit more at the product blurbs you included. W/fixed spacing between rows pretty much set by the ballast/support system, interrow spacing is pretty much fixed by the panel dimensions and panel orientation either portrait or landscape.

                Comment


                • #9
                  Originally posted by J.P.M. View Post
                  1.) Most roofs are flat. Many are oriented at an angle with respect to the horizontal.
                  Most flat roofs have a pitch of less than ten degrees. At least that is according to most Architects. Contractors and other building professionals. A quick search of Wikipedia, Google or Webster's most likely would confirm that terminology. However since this is a forum in which @J.P.M has posted over 10,000 relies we should change our conventional thinking when we enter the door. The logic is that when we put solar on a roof, or contemplate putting solar on a roof it somehow magically must change from a flat roof to a horizontal roof for the purposes of clarity. I am not trying to be the "thought police* here because I used to think I owned a townhome which had a flat roof. At least that is what the realtor describe it as when I purchased it. I dont know if we also need to drop our use of the term "pitched roof". That is a question that only J.P.M. can answer.

                  Comment


                  • #10
                    When people refer to their roof as flat I think it is generally assumed to be something near a, level, plain, level being the operative word. Now, when somebody says "I need a new hot water heater" that really bugs me. Hot water doesn't need to be heated, does it? Maybe they should be called a "Hot Water Maintainer".
                    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                    Comment


                    • #11
                      Originally posted by littleharbor View Post
                      When people refer to their roof as flat I think it is generally assumed to be something near a, level, plain, level being the operative word. Now, when somebody says "I need a new hot water heater" that really bugs me. Hot water doesn't need to be heated, does it? Maybe they should be called a "Hot Water Maintainer".
                      I've thought about bringing that redundancy up more than a few times. I believe the better term might simply be "water heater".

                      Comment


                      • #12
                        Originally posted by J.P.M. View Post

                        I've thought about bringing that redundancy up more than a few times. I believe the better term might simply be "water heater".
                        Exactly! Hot Water Maintainer is for the people who cant remove the "Hot" part of the term.
                        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                        Comment


                        • #13
                          Originally posted by J.P.M. View Post

                          I've thought about bringing that redundancy up more than a few times. I believe the better term might simply be "water heater".
                          I am good with that and apparently most architects agree. Often on plans just a circle with the initials WH inside is all that is needed for the plumber to figure out where the pipes go during rough in.

                          Comment


                          • #14
                            Originally posted by Ampster
                            Quibble? I think it was was quibbling to write three sentances about your preference for the word horizontal. The obvious inference of the words 'flat" and 'roof" when used in the phrase "flat roof" is what most people agree on, except you. How many posts have you tried to correct the posters use of the phrase, "flat roof"? What is your point in doing so? To prove you know more than them? Is it for some kind of clarity? Almost everyone already understands that except you?
                            Just in case you were wondering, my post was hardly a confirmation but I did try to make it less about you and more about the generally accepted meaning of the phrase, "flat roof". What is your opinion about the phrase pitched roof? What is the difference between a flat roof and a pitched roof?
                            Last response to you.

                            Re: To what I consider your imprecise and last question. Since you're asking what seems to be how I define a flat roof and a pitched roof, and the difference(s) I may draw between them:

                            I consider a flat roof to be a surface without any consequential or substantial surface relief (as in dips, pockets, abrupt height changes or visible surface irregularities) beyond what is normally and usually observed and considered acceptable and fit for purpose for the application and service requirements of a flat surface. Definitionally that implies that the variation in the direction of a "local" surface normal from some immediately adjacent surface area surface normals will be f(size of the "local area" considered) and, from a practical standpoint, with some "acceptably small" variation in the direction of that vector over the entire surface.

                            I consider a pitched roof to be a surface that is not horizontal, that is, from a definitional standpoint, one whose local surface normal is not vertical but can be in any other direction, but with that direction being mostly invariant over the entire surface within some acceptable directional variation as described above, with that acceptability criteria being f(application requirements/tolerance).

                            Not that you're asking, but definitionally, a horizontal surface (at least on the earth) is one whose surface normal is parallel to gravity. For example, the surface of still water is horizontal. I suppose if the horizontal surface were to be large enough, it would, under that definition, have the curvature of the earth, or have some irregularity as the gravity vector shifted slightly with location, but that's probably not much to worry about for what you're asking. A horizontal flat surface might also be considered a special case of a flat surface whose surface normal is parallel to gravity.

                            I bet even people in the building trades know that what they call flat roofs aren't really flat. I also bet many homeowners and readers of this forum do not. I consider raising public awareness might be a form of public service. I still don't see the harm in making the distinction. Similar to my opinions about the term "sun hours", just trying to be precise and avoid confusion to those less familiar with insider terms.

                            Any further comment is yours.

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                            • #15
                              Ampster and JPM. Are you both done pissing on each other? If not please look into another hobby or I might get pissed myself at both of you.

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