Minor loads on substandard buildings: residential solar

[CodeQuandary]

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)!

 

[jayrod12]

No, it should not be allowable. That's not to say it won't/doesn't happen.

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.

 

[XR250]

I would not do it. Is there any way to make the rack self-supporting so it bears over walls? FWIW, on proper roofs, make sure you alternate the feet on the upper and lower rack rails to better distribute the load to all of the rafters.

 

[CodeQuandary]

Thanks for the replies! In fact, edge to edge installs (these roofs have parapet walls, typically, between adjacent row homes, so "parapet to parapet" installs) are the most common way of avoiding structural tangles on these projects. And yes, in those cases where rafter-mounts are happening (flat or pitched roofs), distributing loading by alternating standoffs to all rafters is a common concern of mine also.

 

[capeengineerma]

I agree with all of the above. We run into residential solar applications on old roof systems a lot. The installers like to say the panels only add 2-3 psf of load to the roof, but if you do the analysis or use the design tools the racking system manufacturers provide you will find that there are some big uplifts and down forces due to wind loads. The racking systems typically want to have spacing of 24 to 48 inches for the standoff bases which put significant point loads onto the rafters. Even when the rafters are current code, we require that they alternate the standoff bases so all the rafters have loading, otherwise you end up putting 3 rafters worth of load onto one rafter.

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.