Examples of Panels Weighing Too Much for House?

[sdold]

Roof loads are calculated for both Live and Dead loads in the worse conditions such as including someone on the roof (live) when there is a snow load (dead)

Funny you should mention that, at my AHJ meeting to turn in my plans, they asked me to go get a PE to sign off, and I told them that it "ought to" be fine since the panel area can't be occupied by live loads, and they said "Huh...OK, sounds good to us" and gave me the permit. I was really sweating it there for a minute.

[sdold]

There are some cities that require engineering only if the array is over something like 5 pounds per square foot. If you show a few of those examples, maybe it would help. I think the typical array is 3-4 pounds. The uplift loads are typically higher, that's where the screws really need to hold. There may be other requirements such as seismic, but I haven't seen those first-hand.

Edit: Here's an example of a city's requirements, note the top of page 2 that says "Provide structural calculations, prepared by a registered California design professional, if the total weight of the photovoltaic system is over five pounds per square foot." I've seen for at least a couple of cities.

http://www.ci.milpitas.ca.gov/_pdfs/bld_permit_sr_solar_panel.pdf

[SunEagle]

Roof loads are calculated for both Live and Dead loads in the worse conditions such as including someone on the roof (live) when there is a snow load (dead).

When it comes to equipment on the roof you also have to include the uplift forces caused by wind getting underneath or creating a vacuum over the top. This is especially important in high wind areas when you install a PV panel array.

[J.P.M.]

Has anyone come across a scenario in which a rooftop solar system actually weighed Too Much for a home? I'm working with a city to loosen up their permitting requirements and thought that we'd be more successful in drawing more appropriate lines for when an engineering analysis should be required if we actually had examples of when solar weighed too much.

The short answer is no. For all practical purposes, codes in force cover roof loadings quite well. You will not get an AHJ to change them. there is no need for that.

PV panels have a mass of ~ 2.5 lbm/ft.^2. Most roofs are OK with a dead load of probably 40 lbm/ft.^2. It's hard to come up with a scenario where the added dead load from panels would present a problem even with a design snow load added. As for uplift from wind, even in what's a completely absurd situation of a wind vector of 120 MPH applied normal to the face of a panel (blowing upward and away from the surface of the roof, the uplift would still only be ~ 37 lbf/ft.^2 less the panel weight, and that'll never happen. Same for a downward force but that's already accounted for as it'll be close to the same force on the roof without the panel plus the ~ 2.5 lbm/ft.^2 dead weight of the panel.

You need someone familiar with wind loadings on structures to help you understand why you're barking up the wrong tree.

[Roger57]

Has anyone come across a scenario in which a rooftop solar system actually weighed Too Much for a home? I'm working with a city to loosen up their permitting requirements and thought that we'd be more successful in drawing more appropriate lines for when an engineering analysis should be required if we actually had examples of when solar weighed too much.