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I'm trying to build structures to exclude some percentage of rainwater on experimental plots. I have three 20ft beam supports across the plot, spaced 3m apart and held in place by 4x4 posts that were stepped down in height by a total of 1ft across the three of them-creating a slope.

However I realized that the polycarbonate roofing can't span the 3m gap between supports without sagging. In the specs they suggest support every 3ft, so I am looking for a way to achieve this support with my existing materials- roofing panels (6" wide by 12ft) and pvc irrigation piping (depicted on the right below).

Here are the material specs: Suntuf specs

One thought I had was creating rafters with the pvc, or creating additional support beams of pvc between the wood posts to shorten the span to 1.5m. I'd like to find a solution that uses minimal additional materials and has the potential to last for several years at least. I'm really trying to avoid any pooling or spilling of water, as it would kill the point of the experiment.

Any clever engineering solutions on how I can help the polycarbonate sheet span this 3m gap?

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  • $\begingroup$ For any of you interested in the progress on this, we ended up needing to keep the holes augered where they were, so now we're considering running aluminum bars on the outside parallel to the slope (maybe 6063 or 6061, something like this cooperindustries.com/content/dam/public/bline/Resources/Library/… , and runnning a wire with tension between the two aluminum pieces to support the panels in the middle of the span.The question now is whether the tension will bend the aluminum... $\endgroup$
    – Julie
    Commented Oct 7, 2019 at 4:55

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If you want to avoid re-auguring holes, you could go out and get some 10' (~3m) long 2"X6" (or 2X8). You can run these in the direction of your corrugated plastic roofing. Probably want at least 3 per panel, edges and middle, though you may get away with just edges. Lay your Corrugated sheets on top of this. The 2X6s should take care of the bending. This would only be for rain loading. no one walking on top or snow accumulation. The deeper the section the more capacity you will get.

If you only support the edges of the panel, they may sage in the other direction (perpendicular to the corrugations)

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  • $\begingroup$ Thanks for your suggestion! Yes, I think we do want to avoid re-augering the holes after all, however there will be somewhere on the order of 22-25 strips that are only 7" wide per structure, so I think the most cost effective solution might be simply putting intermediate supports in between the existing support beams. Maybe this is what you are suggesting but I just misread it. I have recently been considering metal tubing for this, as it is less likely to sag, and comes at around $4 for 10ft strip. At 10 per structure that is under $50 x 12 structures, still not cheap but... $\endgroup$
    – Julie
    Commented Aug 17, 2019 at 3:16
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Cut strips from a sheet so that three strips glued together form a triangle - these may then be strong enough to reduce the sagging. Make the height of the triangle match the peak height of the sheet.

However, proper bracing is the only solution and that comes down to the design. Usually these panels have sufficient overlap, bracing and slope so they don’t have pooling issues when they sag slightly.

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  • $\begingroup$ Thanks for your response! I like the idea of using a triangular configuration. Just to be sure I understood your suggestion you are thinking form a triangle on top of the wooden support beams and then lay the roofing on top of that? Or to use the triangle as a bottom brace? $\endgroup$
    – Julie
    Commented Jul 26, 2019 at 12:53
  • $\begingroup$ Brace the roofing panels - the wooden beams are strong enough by themselves.. $\endgroup$
    – Solar Mike
    Commented Jul 26, 2019 at 13:48
  • $\begingroup$ Thanks, I'll try this out today and see how it goes! $\endgroup$
    – Julie
    Commented Jul 26, 2019 at 14:42
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Looking at the sag I'd say space the supports one sixth of the length from the end of each sheet and bond the sheets together that the moment forces combined with each other cancel sag sufficiently. This way you only need to space the supporting beams 2m apart with hogging and suing moment forces cancelled by bonded sheets acting as the counterbalance to their own self weight.

Edit: slope of entire arrangement should be such that mass of water does not remain on surface long enough to compromise anti-sag effects of bonding sheets together. Ultimately gravity will exceed structural strength due to material selection.

Slope should be about 1:24.

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  • $\begingroup$ Hmm, this is a good point I hadn't considered. The only challenge would be re-augering the holes. I'm curious how many sheet strips I would need to overlap or bond to counteract the sag, might be a trial and error approach. Thanks! $\endgroup$
    – Julie
    Commented Jul 29, 2019 at 1:42

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