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Maximum WEIGHT on roof ????....SOLAR PANELS

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  • Maximum WEIGHT on roof ????....SOLAR PANELS

    Different states , different counties , different cities has different rulesand permits regarding weight calculation on the roof .
    Los Angeles Building And Safety Department says that maximum solar panel weight you can install on the roof is 2 pounds per sq. foot.
    And different inspectors do different calculation .
    Lets say you have roof 40 feet by 18 feet = 720 sq. f.
    1- They take solar panel weight and size - lets say Canadian Solar 225watt
    Weight is about 42 pounds
    Dimensions are 5.5feet by 3.25 feet= 17 sq feet
    42 / 17 = 2.42pounds per square feet
    And after this calc they told me you cannot install . Which is not TRUE
    2 - Lets say you want to install 20 panels
    30 times 42 = 1,260 pounds
    1260/740=1,7 pounds per sq feet
    Which is more logical ....

    Who can tell acctually calc of weight on the roof ???
    I know it depends on structure and roofing material but I would like to know the RIGHT WEIGHT CALCULATION ,,,,
    Thank you guys ....

  • #2

    Re: Maximum WEIGHT on roof ????....SOLAR PANELS

    Re: Maximum WEIGHT on roof ????....SOLAR PANELS

    Use this document as a starting point. If you do not understand basic engineering principals hire an engineer. I went to school for a lot of years and like to get paid to figure stuff like this out.

    http://www.unirac.com/pdf/ii227.pdf
    10.125KW Netmeter System, (3) 15 panel arrays
    (45) REC225PE-US + (45) M190-72-240-S12
    SolarMount I Racking

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    • #3

      Re: Maximum WEIGHT on roof ????....SOLAR PANELS

      Re: Maximum WEIGHT on roof ????....SOLAR PANELS

      There is no universal number, every structure is unique and its NOT something you can provide information on to get a permit unless you happen to be a state certified civil engineer.
      www.solar-guppy.com
      sigpic

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      • #4

        Re: Maximum WEIGHT on roof ????....SOLAR PANELS

        Re: Maximum WEIGHT on roof ????....SOLAR PANELS

        I think you are asking to figure out the pounds per square foot, do you use the area covered by the panels or the area of the roof.

        Iím not an engineer, but both the load calcs I had done for my system used the area covered by the panels and included the weight of the rails. For my KD215 panels it worked out to 2.48 PSF for the panels and .5 PSF for the rails for a total of 2.98 PSF.

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        • #5

          Re: Maximum WEIGHT on roof ????....SOLAR PANELS

          Re: Maximum WEIGHT on roof ????....SOLAR PANELS

          Source for the 2lbs/sf?

          I've been to some cities that have waived structural calcs for anything under 3lbs/sf.

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          • #6

            Re: Maximum WEIGHT on roof ????....SOLAR PANELS

            Re: Maximum WEIGHT on roof ????....SOLAR PANELS

            it boils down to the inspector is right and you are wrong when he says so. too often you are under the mercy of an inspector even if you can prove him wrong, which many of them would not like especially those that are egomaniacs.
            NIEL

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            • #7

              Re: Maximum WEIGHT on roof ????....SOLAR PANELS

              Re: Maximum WEIGHT on roof ????....SOLAR PANELS

              Originally posted by Batikikik View Post
              Los Angeles Building And Safety Department says that maximum solar panel weight you can install on the roof is 2 pounds per sq. foot.
              Responding to this purely from a "common sense" standpoint,
              i.e., not from rules and regulations...

              Putting aside wind load for a moment, 2 lbs per square foot is
              a rediculously low number. Think about this: if you stand on
              top of the roof, your weight is pressing on about a square foot
              (less, probably) and you may weigh 200 lbs. That means you
              have 200 lbs per square foot pressure on that portion of the
              roof, or 100 times their "maximum" solar panel weight.

              When I originally spoke with my contractor about this topic
              prior to installation, he told me that the weight of solar panels
              is a nit compared to what a roof can hold. I recall that he
              said that a roof is supposed to be capable of supporting
              something like 600 lbs per square foot.

              Now back to the wind load part. The manufacturer of my
              mounting hardware, Unirac, did run a wind load analysis to
              ensure that the proposed configuration was safe. In other
              words, static weight load is one factor, and stress load is a
              different factor and must be accounted for as well.

              John
              15 Sharp 235w NU-U235F3 PV modules and 15 Enphase M190-72-240 Micro-inverters. 8 Suniva 265w and 8 Enphase M215.

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              • #8

                Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                When I did my 22 panel install in LA county, I had to have an engineer look at and certify the existing roof layout. Since it was fairly new, I still had the plans, and it went really easy, with 2x12 roof joists and earthquake sheathing (nailed every 4 inches) that were there.

                And there is a ratio between weight load and how much of the actual surface will be loaded at any point in time. if you loaded the 2nd floor of a house with the max load psf, it will fall down. That's why your foot does not go thru the roof. It's something like 30 or 40% total space loaded, not 100%.
                http://tinyurl.com/LMR-Solar
                http://tinyurl.com/LMR-BigLug
                http://tinyurl.com/LMR-NiFe

                Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
                Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

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                • #9

                  Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                  Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                  Probably what the LA county bldg dept meant was that if you were <= 2lb/sqft you didn't need to get an engineer to sign off. It is certainly possible to construct a roof in such a way as to support more but that is not likely to be the case on the average existing residence and if you want to claim it is so you're going to need an engineer to sign off on it.

                  Its not just the localized psf load... you can stand on your roof forever taking up 2sqft and thus loading it at 100psf. But fill your roof with enough other 200lb guys to use up all the other 2sqft spaces and your roof will fall down... either you will break rafters/trusses or the supporting structure will fail. You have to consider not only the load bearing capacity of the trusses but the load bearing capacity of the walls that hold up those trusses.

                  Likely the more important is to consider how many linear feet of perimeter wall the roof's weight transfers to and then calculate the weight bearing capacity of those walls based on their construction (likely 2x4 stud around here). A center support beam/post/wall will likely help but there will need to be an appropriate footing or support for those too.

                  And you are lucky not to have to account for live snow loads

                  This is why the rules exist and why engineers get paid to do analysis.

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                  • #10

                    Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                    Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                    Excuse me but ... two pounds per square foot?
                    That can't be right, unless you build your roofs out of cardboard and tissue paper.

                    Try this: measure the square area it takes for you to stand in (approximately 1 sq. ft.). Now divide your weight by this number.

                    200 lbs. per square foot I'd believe.

                    Maybe you do things really differently in The States.

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                    • #11

                      Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                      Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                      Probably related to the old around 230 lb per square 3 tab roofing specifications... 1 square being 10'x10' or works out to 2.3 lbs per sq foot.

                      And, we are allowed to have something like one or two layers of existing roof under a new shingle roof before we have to strip down the roof to the original sub roof (also, in our area, we need to add structural plywood when re-roofing).

                      So, the ~2.3 lb per sq ft is probably the "don't care / don't need a structural engineer to sign off the roof".

                      -Bill
                      20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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                      • #12

                        Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                        Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                        We recently designed a building to serve as storage and hold a 22kw system. The specifications we were given by the installer was to have it built for an additional 5 pounds per square foot.

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                        • #13

                          Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                          Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                          Originally posted by SolarLurker View Post
                          We recently designed a building to serve as storage and hold a 22kw system. The specifications we were given by the installer was to have it built for an additional 5 pounds per square foot.
                          I don,t know whaere in Pa. You are but I am in west pa. . I had a couple ft. of snow and ice on my roof a couple times and was dam glad I didn,t install my panels on my roof. There were lots of buildings that had roof caveins lasr year in this area. Mostly pole barns. solarvic
                          16 KC 158G & 3 KD185GX-LPU panels on Fronius IG PLUS 3.0-1 inverter and 14 SHARP NDU3A & 1 KD185GXU panel on FRONIUS IG-3000 inverter. All mounted on pole top racks. Retired and enjoying it!!

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                          • #14

                            Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                            Re: Maximum WEIGHT on roof ????....SOLAR PANELS

                            We had the building engineered for an additional 10 pounds per square foot to accommodate the solar. A saltbox/carriage style truss was chosen to maximize the southern exposure. South will be almost 31 degrees pitch, north will be even steeper. I don't think snow will be a problem.

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