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  • mounting panels to treated-wood frame

    I am proceeding with a 4+kW ground-mount installation built on a base of treated wood. The base will be four concrete piers, supporting two beams (two-ply 2x12) with 2x8 purlins supported between the beams. I'm planning to use MCA-treated wood, which supposedly less toxic to metals (and people), particularly the aluminum in the panel frames; also, around here at least, it's easier to find the 20ft lengths I need in MCA (and #1 grade). The purlins will be perpendicular to the long edge of the panels, so each purlin will intersect the panel frame at two of the four mandated clamping zones (roughly 4" long and centered about 12" from the panel corner). It's a 4x4 array of Talesun TP660P panels, in landscape orientation.

    So, I'm trying to work out the details of attaching the panels to the purlins. I plan to use 5/16" stainless-steel lag bolts with washers. Things I can't quite decide:

    1. Should I use 18-8 or 316 grade stainless ? I'm sure both are strong enough and resistant enough to general corrosion. But I wonder if the more expensive Type 316 is more resistant to possible galvanic corrosion (at the interface with the aluminum panel frame). Talesun doesn't seem too worried, in fact, they specify SS (without mentioning a grade) if bolting the panels using the thru holes, and I believe all the makers of metal racking systems use SS for the clamps. IronRidge says "the high quality stainless steel used in the IronRidge UFO [clamping bolt] does not cause significant galvanic corrosion, even in extreme environments"; I wonder if they're talking about 316 instead of 18-8 and if indeed the former is more resistant to galvanic corrosion ? Anyhow, my environment is not marine, just probably not "extreme".

    2. Should I use spacers around the part of the lag bolt that is tight between two neighboring panels ? Spacers are inexplicably expensive, but I scored some SS tubing (not sure what grade) at the local scrap-metal yard (a consignment store for guys that is exactly 5/16" I.D. and 1/2" O.D., so I figure I can make spacers myself with an abrasive wheel on a miter saw. Might be a bit of a bore to clean 'em up after I cut 'em though, so I wonder if they're really needed ? I worry that without them, the rotating bolt (as I tighten them) could bugger the edges of the frames; just seems cleaner too.

    3. Is 4" a good length ? I figure that will bury about 2.5" of bolt (4" minus the 35mm panel thickness and the washer). I imagine less would be theoretically strong enough, but 4" isn't that much trouble or expense.

    4. Are washers actually suitable as clamps ? The installation manual says clamps should be 2" long (along the edge of the frame). But the IronRidge UFO (clamping bolt) just clamps the panels down with a washer, and one a bit smaller than I'd likely use (probably 1.25", the 1/2" spacer if I use it, plus 2x the 11mm width of the edge of the panel frame).

    5. Should I attempt to ground the panels with the washers ? If I could make contact between the washers and the top edges of the frame, I'd only need to run ground wire along the top or bottom edge, connecting to 4 panels total. Not sure where to get the washers I'd need, with teeth on the underside to gouge into the panel frame (like on the IronRidge UFO). Otherwise, I have to buy 16 grounding clamps (like the $4 Tyco 1954381-2 that Talesun recommends, and a lot more wire).

    6. The PE who stamped drawings of my frame was at pains to tell me to paint the wood, so it doesn't warp and then potentially damage the panels that are bolted to it. Wise words I guess. Except generally treated wood is pretty wet when you get it, so you need to let it dry out before painting, and it warps when it dries out. Maybe build the frame, THEN let it dry out, then paint it and mount the panels ?

    TIA !!

  • #2
    Wood warps, painted or not. Treated wood warps worse. The rail mount systems work well, the grounding is inherently integrated, but I guess you have already made those decisions by now, and I cannot bless it . You paid your engineer to bless it. go in peace
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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    • #3
      Originally posted by Mike90250 View Post
      Wood warps, painted or not. Treated wood warps worse. The rail mount systems work well, the grounding is inherently integrated, but I guess you have already made those decisions by now, and I cannot bless it . You paid your engineer to bless it. go in peace
      I thought warpage tended to occur as moisture content changes, and paint is meant to stabilize the content.

      Seems like a couple of other people here have done treated-wood mounts; I wonder if they've had 2nd thoughts ? One I believe is @bcroe , seems highly-regarded at other subforums.

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      • #4
        I built a patio cover two years ago using treated 2 x 4s.I didn't paint them and they are doing fine. They span 9 feet and are on 24" centers. There are two rows of panels so the structure is actually stiffened by the 4 Iron Ridge rails bolted to them. I selected the 2 x 4s for small tight knots to minimize warpage. The treated wood I see today is much less susceptable to warpage than the utility grade mudsill that used to be sold. The incisor marks and the pressure treating process tends to dry it out more evenly in my opinion.
        I bought inexpensive SS lag bolts and embedded them according to the Iron Ridge engineering. I used 1 1/8 SS fender washers and 1/4" bolts which fit in the top slot of the rails.

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        • #5
          Originally posted by RShackleford View Post

          I thought warpage tended to occur as moisture content changes, and paint is meant to stabilize the content.

          Seems like a couple of other people here have done treated-wood mounts; I wonder if they've had 2nd thoughts ? One I believe is @bcroe , seems highly-regarded at other subforums.
          Wood has more permanent dimensional change as f(time) than metal. Metal is also generally more durable and has more uniform properties.

          When considering material for support for flat solar panels, based on its properties, metal is probably a better overall choice than wood for such duty.
          Last edited by J.P.M.; 02-20-2020, 12:19 PM. Reason: Corrected 2d sentence with apologies for the error.l

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          • #6
            most of the movement of the wood is going to happen as it initially dries out. So you will want to let your lumber sit, stacked with space around each board for at least a few days.

            304 vs 316: 304 is perfectly suitable, and makes up the vast majority of SS parts used in solar. 316 is usually only used in Hawaii or other beach front locations.

            I would caution against using standard washers as clamps. They likely do not have the strength required. The IronRidge UFO has been extensively designed tested to withstand the required uplift forces. Washers have not. I would either use the mounting holes, or find a clamp that is designed for solar panels.

            And don't try to ground using washers. Again, solar panel grounding devices are extensively designed and tested for this purpose. You don't want to compromise on safety.

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            • #7
              Originally posted by Ampster View Post
              I built a patio cover two years ago using treated 2 x 4s.I didn't paint them and they are doing fine. They span 9 feet and are on 24" centers. There are two rows of panels so the structure is actually stiffened by the 4 Iron Ridge rails bolted to them.
              So the IronRidge rails run perpendicular to the 2x4s ? I suppose I could do that, but my wood frame is constrained to what the PE gave me. So I'd have to clamp the panels on the short sides (10" zone from corners). Or are the rails parallel to the 2x4s and just bolted to the top edge of them ? Anyhow, it adds considerable expense, especially if I can't buy them locally and avoid freight shipping.
              I selected the 2 x 4s for small tight knots to minimize warpage. The treated wood I see today is much less susceptable to warpage than the utility grade mudsill that used to be sold. The incisor marks and the pressure treating process tends to dry it out more evenly in my opinion.
              This is MCA treated and #1 grade, so I am optimistic.
              I bought inexpensive SS lag bolts and embedded them according to the Iron Ridge engineering. I used 1 1/8 SS fender washers and 1/4" bolts which fit in the top slot of the rails.
              Now I'm confused. If you used IronRidge rails, why didn't you use their UFO clamp bolts ? Or did you use the lag bolts to attach the rails to the 2x4s ? Would you point me to the IronRidge page where they talk about SS lag bolts being used ? I thought their system was all metal down to the concrete piers.

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              • #8
                Originally posted by J.P.M. View Post
                When considering material for support for flat solar panels, based on its properties, metal is probably a better overall choice than wood for such duty.
                No doubt. I choose wood for cost. Hope it doesn't end up being peenywise-poundfoolish, but I'm committed to it now. Hopefully I can make it work with some of the advice being given here.

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                • #9
                  Originally posted by RShackleford View Post
                  No doubt. I choose wood for cost. Hope it doesn't end up being peenywise-poundfoolish, but I'm committed to it now. Hopefully I can make it work with some of the advice being given here.
                  I would too if first cost and no concern for tradeoffs about future maint. or any concerns about how such things as effects on property resale value had no value to me.

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                  • #10
                    Originally posted by emartin00 View Post
                    most of the movement of the wood is going to happen as it initially dries out. So you will want to let your lumber sit, stacked with space around each board for at least a few days.
                    I thought of doing the build in two stages. Let the 6x6s and 2x12s dry out as you say, then install the posts and the 2x12 beams. Then let that assembly and the loose 2x8s dry for a couple weeks or months, then paint everything, then install the 2x8s onto the beams.

                    304 vs 316: 304 is perfectly suitable, and makes up the vast majority of SS parts used in solar. 316 is usually only used in Hawaii or other beach front locations.
                    Ok, solved that at least.

                    I would caution against using standard washers as clamps. They likely do not have the strength required. The IronRidge UFO has been extensively designed tested to withstand the required uplift forces. Washers have not. I would either use the mounting holes, or find a clamp that is designed for solar panels.
                    Mounting holes seems like a bad idea, as they'd be less forgiving or movement of the wood (clamps allow some movement of the wood relative to the panel, in one dimension at least, holes allow none. They do make extra-thick fender washers, 1/8" thick. Maybe best would be to use a clamp like https://www.renvu.com/Solar/SolarMou...p-Dark-302030M or https://www.solarflexion.com/product-p/c225imc-24.htm and toss the bolt and replace it with my lag screw.
                    And don't try to ground using washers. Again, solar panel grounding devices are extensively designed and tested for this purpose. You don't want to compromise on safety.
                    Argh ok, guess I have to use those Tyco grounding lugs. They are specified to use 10-12 awg wire. I thought such grounding required 6awg though.


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                    • #11
                      Not being a PE, and already having an array up, I decided to get 4 dozen extra panels (not in
                      use)up quickly to increase output at day extremes and under clouds. Hopefully the experiment
                      would last at least 5 years, its at 7 now and looking decent. All the wood hardware is hot dipped
                      galvanized, all the aluminum is 18-8 SS. Once anything is in the wood a while, it will likely
                      break off before being removed. Almost all holes were done on a drill press for accuracy.

                      The buried pieces went in, then a laser was used so the holes for the horizontal cross pieces
                      would all be in line. Aluminum rails were attached to the A frame by long SS bolts. You can
                      just see a big linoleum square between the aluminum and wood.

                      So panels are BOLTED to aluminum, no clips. The aluminum runs ground the panel frames,
                      connected to a common ground and 10 ground rods. Bruce Roe

                      NSlow.jpgNSsEnd.jpgNSAjig.jpg
                      Last edited by bcroe; 02-21-2020, 12:07 PM.

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                      • #12
                        I'm liking the idea of replacing my 2x8 purlins with IronRidge XR1000 rails. I'd stick with my PE-approved design for the piers, 6x6 posts, and 2-ply 2x12 beams running E-W. A bit more expensive, but should mitigate wood-warpage concerns and make panel installation easier with the UFOs.

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                        • #13
                          Originally posted by RShackleford View Post
                          I'm liking the idea of replacing my 2x8 purlins with IronRidge XR1000 rails. I'd stick with my PE-approved design for the piers, 6x6 posts, and 2-ply 2x12 beams running E-W. A bit more expensive, but should mitigate wood-warpage concerns and make panel installation easier with the UFOs.
                          Wow. 6x6 posts are pretty big. What was the uplift wind design criteria to come up with those posts?

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                          • #14
                            Originally posted by SunEagle View Post
                            Wow. 6x6 posts are pretty big. What was the uplift wind design criteria to come up with those posts?
                            Not really, to support the beams, which are 20ft-long twin 2x12s. I just gave PE sketches of what seemed reasonable to me, and I used the 6x6s. Cost savings of using 4x4 would be minimal (three 8ft pieces, one each for high end, cut in half for low end) and that sure feels flimsy to me

                            PE didn't share his computations, but I believe he said uplift was somewhere around 1000lb per pier. Piers are spec'd as 24"-diameter by 30" high (20" required to be below grade), which comes out to a little less than 8 cu-ft each. In fact, I can't imagine anything close to the design winds occurring; the biggest wind event we've had here in over 50 years, a hurricane which was producing 50-70 mph gusts when it arrived here ~100 miles after landfall, did not produce winds anywhere close to that strength at ground level. What it did do is to knock down a lot of trees; so my wind risk is not that the solar array is too flimsy or poorly-anchored, but rather that it's smashed by falling trees. Better check if I need an insurance rider !



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                            • #15
                              Originally posted by RShackleford View Post
                              Not really, to support the beams, which are 20ft-long twin 2x12s. I just gave PE sketches of what seemed reasonable to me, and I used the 6x6s. Cost savings of using 4x4 would be minimal (three 8ft pieces, one each for high end, cut in half for low end) and that sure feels flimsy to me

                              PE didn't share his computations, but I believe he said uplift was somewhere around 1000lb per pier. Piers are spec'd as 24"-diameter by 30" high (20" required to be below grade), which comes out to a little less than 8 cu-ft each. In fact, I can't imagine anything close to the design winds occurring; the biggest wind event we've had here in over 50 years, a hurricane which was producing 50-70 mph gusts when it arrived here ~100 miles after landfall, did not produce winds anywhere close to that strength at ground level. What it did do is to knock down a lot of trees; so my wind risk is not that the solar array is too flimsy or poorly-anchored, but rather that it's smashed by falling trees. Better check if I need an insurance rider !


                              Uplift is necessary but not necessarily complete information for design calcs.
                              Did the engineer mention or identify a value for the induced moment in the 6 X 6 posts at the point where they meet the piers ? Just curious.

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