Solar panels and roof load question

brn3aApril 28, 2010

Hi,

We live in Virginia and have a 4 panel solar thermal setup that contributes to our winter heating needs and provides ample hot water in the summer. We are able to use the panels for heat as we have hydronic radiant floor heat. Anyway, Virginia now has a solar thermal rebate and because of that we can add 3 more panels for essentially free since all the plumbing, etc has been done. Out current system uses 4 AET flat panel 4'x10' modules that weighing ~155lbs each. The are mounted on a 2 rail/track system with room to add more. I am worried about the point and dead loads of adding 3 more panels.

Our house is brand new. The rafters are hand cut (ie. not trusses) 2x12's. I assume the roof meets Virginia code. The panel rails are installed perpendicular to the rafter(each rafter carries 2 points, 1 from the lower rail and one from the upper rail). This past winter we had an (unusually) large amount of snow in Virginia.

I am not asking for engineering advice, but if a setup of 7 4x10 panels mounted on the roof exceeds the experience of this group.

BM

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solargary

Hi,
If I understand you correctly, the new panels would apply their loads to new rafters that are not currently loaded by the existing panels. If that is the case, then it seems like if the current panels are OK, the new ones should be OK as well since they are not adding load to the same rafters the current panels are on.

You could noodle around these pages and maybe get some help from the info they provide:
http://www.awc.org/technical/spantables/tutorial.htm

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

On the span calculator at the link above, if you use 2X12s at 24 inch spacing, deflection = L/180, with the 20 psf snow load plus 10 psf dead load (ie the roof) you get a max span of 21.25 feet.
If you increase the dead load to 15 psf to cover your panels (actually more than cover them by quite a bit), the allowed span goes down to 19.75 ft -- not exactly a killer change. You could compare the numbers the calculator gives to your actual span and see how (roughly) how much margin you have.

Gary

    Bookmark   April 28, 2010 at 9:09PM
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brn3a

Thanks for the links. They were helpful.

    Bookmark   April 29, 2010 at 10:19AM
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