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solar water panel weight on roof
i want to install solar water heating panels on a roof. how can i calculate the dissipated live load ? example: if a panel weighs 135 lbs and holds 3.5 gal {28 lbs} 163 lbs each. are there standard live load allowances for existing roof structures ? example: 2 by 4 or 2 by 6 rafters spaced 16" or 24" oc. the pitch of the roof probally factors in. is there a rule of thumb ? i don't want to beef it up if it dosen't need it and at the same time i don't want to drive by a clients home a few months later and notice a sag
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depends on what part of the country you are in and the design that was used. here in the mid-west 20-30 psf live loads are common. in south texas - 12 psf may be used.
you really need to analyze the roof to verify the capacity - it may be already be at maximum or there may be quite a bit of "fat" left.
check w/ a local structural engineer.
good luck.
where i am, the snow load is 20 to 25 psf below 1000 feet for the roof, however, for a solar panel, you are talking live load in addition to the snow load since snow can build up on top of the panel as it can the roof.
sometimes, for simplicity of application, the truss manufacturer will design to a minimum live load which may exceed your local code requirements. you could check with the manufacturer, if possible. aside from that, it would be a good idea to engage the services of a licensed structural engineer in your area.
mike mccann
mccann engineering

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