Minor loads on substandard buildings: residential solar
Minor loads on substandard buildings: residential solar
(OP)
I'm hoping to get some opinions.
Very short summary: Assume existing, older, small, flat residential roofs with framing that don't meet modern code by a significant margin. Should it be generally allowable to add solar to them (on the order of 4 psf) without engineering (which, as explained below, won't meaningfully happen)?
Gory details: Start with urban row home roofs, 80 y.o., flat (5°), typically 14-22' wide, with beams most commonly comprising 1 or 2 2x12's at 10' o.c. Rafters are something like 2x8's at 20-24" o.c. 25 psf ground snow load. If you do the math, the rafters are generally relatively close to compliant with ASCE 7-10 for gravity loads. The beams, with their 10' tributary loads, don't come close. In practice, some of these beams have mid-span support to a wall below, but also in practice during renovations those walls are often removed without consideration to structural implications. The price point of solar installs is such that extensive, competent engineering of these projects is relatively impossible -- when engineering is done it routinely amounts to a letter that says "looks good to me" without any indication they considered the beams. I've never seen a convincing argument (that includes calculations) that the beams can support the load (I've looked at hundreds, from dozens of different engineers).
On one hand, the added load is on the order of 10% of the code-required capacity of the roof. But these roofs don't meet code requirements (in the event rebuilding the roof is part of a bigger project, they are brought up to code, and it's an entirely different situation).
I would be interested in well-reasoned thinking on these points. Thanks for anything you have to contribute (that's constructive)!
Very short summary: Assume existing, older, small, flat residential roofs with framing that don't meet modern code by a significant margin. Should it be generally allowable to add solar to them (on the order of 4 psf) without engineering (which, as explained below, won't meaningfully happen)?
Gory details: Start with urban row home roofs, 80 y.o., flat (5°), typically 14-22' wide, with beams most commonly comprising 1 or 2 2x12's at 10' o.c. Rafters are something like 2x8's at 20-24" o.c. 25 psf ground snow load. If you do the math, the rafters are generally relatively close to compliant with ASCE 7-10 for gravity loads. The beams, with their 10' tributary loads, don't come close. In practice, some of these beams have mid-span support to a wall below, but also in practice during renovations those walls are often removed without consideration to structural implications. The price point of solar installs is such that extensive, competent engineering of these projects is relatively impossible -- when engineering is done it routinely amounts to a letter that says "looks good to me" without any indication they considered the beams. I've never seen a convincing argument (that includes calculations) that the beams can support the load (I've looked at hundreds, from dozens of different engineers).
On one hand, the added load is on the order of 10% of the code-required capacity of the roof. But these roofs don't meet code requirements (in the event rebuilding the roof is part of a bigger project, they are brought up to code, and it's an entirely different situation).
I would be interested in well-reasoned thinking on these points. Thanks for anything you have to contribute (that's constructive)!






RE: Minor loads on substandard buildings: residential solar
When asked to certify existing roofs to add solar, we perform an analysis on the roof and indicate what needs to be reinforced. Obviously there is some level of overstress that is acceptable (and most engineers have a different level of confidence for different situations) but in a case like this where the roof already doesn't work under basic code level loading, then in no way would they be allowed to increase the load without reinforcement.
RE: Minor loads on substandard buildings: residential solar
RE: Minor loads on substandard buildings: residential solar
RE: Minor loads on substandard buildings: residential solar
We have required that roof systems be reinforced for solar applications or use racking systems that can span wall to wall to avoid placing loads onto the existing non-conforming roof system. We actually had a commercial motel application where they ended up putting some steel beams on the roof to support the racking system spanning wall to wall to avoid putting any loads on the substandard roof system.