truss load calcs.

My local building department red-lined my permit application to expand my solar system wanting structural load calcs for the trusses and additional load of the solar panels. Is this common?


  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: truss load calcs.

    For our building permit (California) we needed the sign-off of a PE (Professional Engineer) to validate the roof loads and mounts met code.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,359 ✭✭✭✭✭
    Re: truss load calcs.
    Jburgess wrote: »
    My local building department red-lined my permit application to expand my solar system wanting structural load calcs for the trusses and additional load of the solar panels. Is this common?

    Very common. You will be adding a fairly heavy load, and possibly increasing the wind load of the roof.
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  • solar_davesolar_dave Solar Expert Posts: 2,382 ✭✭✭✭
    Re: truss load calcs.

    I have 72 panels on my roof, they surely wanted the load calcs but they were also concerned that the racks were attached into the truss directly I suspect for wind loading.

    This house is somewhat over trussed because of the concrete roof tile.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: truss load calcs.

    A properly built roof would be able to handle the panels because they are nothing compared to what it should be able to withstand. But there's that bug-a-bear: "properly built". That's what they want to make sure of. In some cases it's the building department's own fault for having too little demanding code in the first place. I couldn't believe what is allowed here in BC when I first found out. Things like 3/8" sheathing, which would be laughed at back East (insufficient for any sort of snow load unless the rafters are about two inches apart!)
  • JburgessJburgess Solar Expert Posts: 130 ✭✭✭
    Re: truss load calcs.

    Thanks for the feedback. A couple of days and a few hundred dollars will get me my permit.

    Has anyone ever had trusses fail to be sufficient?
  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: truss load calcs.

    In Florida, one can most certainly fail as building codes have become more strict over the years. Homes built in the 70's for example don't meet the current wind load required of new structures. Now they don't make a home owner have to meet the new codes UNLESS your doing structural changes, which adding a few tons of solar panels clearly is. Your mileage will vary ...
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: truss load calcs.

    Yes, engineered mountings are now the rule here in my area. I kind of think it is a make work program because the building dept here is twiddling a lot of thumbs lately and since solar projects are about half of what they are doing - they are giving us full scrutiny. Will admit however that this has meant the discovery of two roofs so far that were weak even with out the solar addition.

    It can be a real hassle though, as many roofs are difficult to quantify. The homeowner may not have the original plans (or the plans have poor or no roof loading specs on them), or the attic is sealed so you don't even know what is in there, or the steel building company farmed out their design to a guy that died 10 years ago (my case), or you have to go back to the truss company that made some complicated scissor truss. Nobody wants to be responsible for the liability involved with structural roof loading. We are adding solar systems to roofs that were not designed for it, and while the 3psf or so the solar array adds may seem pretty trivial, if for whatever reason there is a failure, it will get the blame. I've made friends with a local small time PE, willing to do minor calcs like this for $200-$400 (if I do the drawings for him), and it keeps everybody happy.
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