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

BatikikikBatikikik Posts: 141Solar Expert ✭✭✭✭✭
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 ....

Comments

  • mr.radonmr.radon Posts: 158Solar Expert ✭✭✭✭✭
    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
  • Solar GuppySolar Guppy Posts: 1,959Solar Expert
    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.
  • JburgessJburgess Posts: 130Solar Expert
    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.

    I’m 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.
  • newenergynewenergy Posts: 291Solar Expert ✭✭✭✭✭✭
    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.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    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.
  • jcgee88jcgee88 Posts: 154Solar Expert ✭✭✭✭✭
    Re: Maximum WEIGHT on roof ????....SOLAR PANELS
    Batikikik wrote: »
    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
  • mike95490mike95490 Posts: 6,446Solar Expert ✭✭✭✭
    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%.
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  • SlimDieselSlimDiesel Posts: 17Registered Users
    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 :D

    This is why the rules exist and why engineers get paid to do analysis.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    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. :roll:
  • BB.BB. Posts: 25,152Super Moderators admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarLurkerSolarLurker Posts: 118Solar Expert ✭✭✭✭
    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.
  • solarvicsolarvic Posts: 954Solar Expert
    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.
    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. :Dsolarvic:D
  • SolarLurkerSolarLurker Posts: 118Solar Expert ✭✭✭✭
    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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