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Adding second mounting type to S-5-N clamps with rails for more uplift resistance

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  • Adding second mounting type to S-5-N clamps with rails for more uplift resistance

    This past winter we got hit with wind gusts up to 80 mph, the wind was strong enough it broke a neighbor's glass patio table. I inspected my system and didn't note any changes but it got me thinking about my wind uplift design loads. So I took a closer look at everything and not that content with the numbers I'm seeing.
    My system as installed: https://imgur.com/gallery/aDSbsJC
    More details below on my system and roof after my questions if anything is confusing.

    I'm considering adding SnapNrack Metal Roof Bases to the ends of my arrays in addition to my S-5-N standing seam clamps. I'd likely have to get in the attic and block the trusses for every mount using a 4"x4" but it seems feasible.

    MY QUESTIONS
    1. Does anyone have any advise on products I should consider?
    2. Do you think mixing mounting types would cause any problems?
    3. Can I add these additional mounts down the same trusses or should I attempt to stagger them knowing the center truss will likely be blocked for both.
    4. On the metal roof panel is it better to be closer to each panels nail strip?
    5. Does anyone ever put additional boots over the mounts for waterproofing/water diversion?

    Thank you for any help and advise,

    DETAILS ON MY SYSTEM
    I thought I was doing everything right in my solar install. I DIY'd and used a local DIY company from which I bought a stamped engineer letter and a set of plans for my 38 module SolarEdge install. I have a concealed nailstrip 29 guage 16" standing seam metal roof which I used S-5-N clamps with SnapnRack rails. The 16" metal roof is secured every 17" with wood screws through 7/16" OSB. I have 2 roofs both are east and west facing: SOUTH - gabled roof with scissor truss built with 2"x6" with 16 modules on the east and 16 modules on the west and NORTH - gabled roof with common trusses built with 2"x4" with 6 modules on the east. Both roofs are about 27 degrees and modules are flush mounted about 5" high. All modules are 4' or more up from the eve. The modules on the West roof are all 3' from the roof's rake and ridge. The North Roof on the east has an array 18" from the ridge and 18" from the rake and the South roof on the east has an array 18" from the rack (4' drop to north roof). The local company and AHJ had said was fine and approved. My house is on a hillside and the West slope is wind exposure C with the East slope exposure B and a roof height of around 30-35 ft high. The plans I bought called for clamps on every 3 seam (48"); I upped the frequency to every other seam during installation trying to be conservative and because it just made sense. So as installed:
    6 clamps per 162" rail; 12 clamps per row; 5 clamps per 122" rail; 10 clamps per row; 3 clamps per 81" rail; 5 clamps per row

    My rough numbers are uplift looking into uplift forces are 20 lbs/sq ft seems like a reasonable design number. If so a row of 4 modules is about 1,400 lbs of uplift or an array of 2 rows of 4 modules is about 2,800 lbs. Mass of 8 panels 350 lbs so 2450 lbs uplift to resit. OSB has a pull out strength of 173 lbs/in with #10 wood screws so 75 lbs per screw. If you assume every clamp will distribute to at least 2 screws then the 8 modules has 48 screws or 75 lbs*48= 3,600 lbs of resistance. This should be about a 1.5 safety factor but it doesn't make me that happy and I'd like to reinforce the system.
    At the start of the project I was worried about putting holes in my metal roof worrying about leaks, I didn't see how I would manage to connect to the trusses, and S-5 clamps seemed so elegant and simple. Now I'm more worried about he West arrays popping off the roof. After reading posts I should have used a clamp on every seam. I have added some S-5 clamps to the seam of the top and bottom rails of my arrays as it wasn't hard to retrofit. However, the middle rails are a very difficult to access. This is why I'm considering adding the Metal Roof bases at the end of the arrarys. Using 4'x4' blocks horizontally connecting trusses with 5/16" lags has a pullout of 300 lbs/in so 300*3.5" = 1,050 lbs per lag. 8 lag bolts is then 8,400 lbs of uplift. Those are some over design safety factor numbers I like better.

    https://snapnrack.com/resources/draw...f-base-rafter/

    https://www.zillarac.com/Portals/0/D...20Pull-out.pdf

  • #2
    Did you get a set of wind calcs specific to your site with the stamp ?

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    • #3
      No all they said in the stamped letter:

      "Design wind speed: 115 mph (3-sec gust) per ASCE 7-10
      Wind exposure category: C

      The solar array will be flush-mounted (no more than 6" above the roof surface) and parallel to the roof surface. Thus, we conclude that any additional wind loading on the structure related to the addition of the proposed solar array is negligible."

      Overall, I feel like there is a lot of "it'll be fine" attitude, a bar of good enough to get AHJ approval, and a bunch of CYA language at the end of the letter. That all leaves me not trusting it much and thinking, I should just add on some mounts to be sure it is structurally should and worry about leaks if they're a problem.

      Comment


      • #4
        Originally posted by Sierrasclimber View Post
        No all they said in the stamped letter:

        "Design wind speed: 115 mph (3-sec gust) per ASCE 7-10
        Wind exposure category: C

        The solar array will be flush-mounted (no more than 6" above the roof surface) and parallel to the roof surface. Thus, we conclude that any additional wind loading on the structure related to the addition of the proposed solar array is negligible."

        Overall, I feel like there is a lot of "it'll be fine" attitude, a bar of good enough to get AHJ approval, and a bunch of CYA language at the end of the letter. That all leaves me not trusting it much and thinking, I should just add on some mounts to be sure it is structurally should and worry about leaks if they're a problem.
        FWIW, A few brief points of many possible things to think about with respect to wind loading and arrays on roofs:

        1.)That statement may apply to the racking and the panels, but without some analysis that deals with the roof and the attachment methods that are similar but perhaps/probably more involved than what your have provided, it's necessary but not sufficient information.
        2.) Furthermore, unless someone has calcs. to prove it, to say a maximum wind loading on a roof with an array on it that's parallel to the roof and 6" away from the roof is the same as the wind loading on the same roof with the same wind loading but without an array on it is simply not correct. It also may be dangerous to say so, particularly if something goes wrong.
        3.) The wind induced load will be different for a bunch of reasons and may be uplift as well as downward.
        4.) Any wind loading on a roof as a result of adding an array will also be more variable because of the array's presence and will probably have a cyclic or vibrational component to it as a result of the array's presence. Such loadings were probably not part of the original design of the roof.
        5.) Even if the wind loading transferred to the roof were to be of the same magnitude with or without an array, any wind load transferred to the roof will manifest at the roof as point loads through the array supports rather than as a more distributed load as would tend to happen if the array were not there. That will change the loading characteristics of the wind from those of a roof without an array.

        All that written however, I'd agree with you that the AHJ may well not understand or even care much for some of the finer points of the effects of wind on structures.

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