Examples of Panels Weighing Too Much for House?

[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.

[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.

[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.

[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

[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.

[solarix]

Until recently, no one in the building industry envisioned designing roofs with the possibility of putting these solar panel arrays on them. Therefore it is scary to the building departments to allow something that was never intended by the designers. They are very risk adverse you know and subsequently you have to prove to them that the roof is strong enough. Which means some one has to dive into the ASME code SEC 7 on calculating wind loads either (which of course weren't done with solar panels in mind). They want a PE to ok the roof in order to absolve the building dept of liability. I know that there has never been a roof collapse due to a solar array, but that has nothing to do with it. I like the "can't be occupied by live loads" logic - I'm going to use that... thanks.

[ButchDeal]

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.

Sure the main problem is ballasted on flat roofs. These should ALWAYS require a stamp.

Then you have pitched roofs and these you have two different mounting techniques: railless and railed systems.
The railless systems reduce structural integrity and should be looked at closely
Railed systems when installed correctly increase structural integrity and should not need a stamp.

any tilted system has greatly added uplift forces and should include a stamp.

[emartin00]

Railed systems when installed correctly increase structural integrity and should not need a stamp.

If they use enough mounting points.

In many jurisdictions, as long as you have a mounting point every 4 feet along the rail, you don't need any sign off. If you go with a stronger rail, and longer spans, you increase the point loading on the rafter. This is where you could see a failure. But when you distribute the load evenly, by using 4ft spans, and alternating rafters, you really won't see a problem with an house that doesn't have any structural deficiencies.

[ButchDeal]

If they use enough mounting points.

In many jurisdictions, as long as you have a mounting point every 4 feet along the rail, you don't need any sign off. If you go with a stronger rail, and longer spans, you increase the point loading on the rafter. This is where you could see a failure. But when you distribute the load evenly, by using 4ft spans, and alternating rafters, you really won't see a problem with an house that doesn't have any structural deficiencies.

As I said, When installed correctly...

We like to use ironridge XR100 with 4 foot spans. It can handle longer spans but is more resistant to flexing
We do staggered in the middle and double up on the ends.

It is pretty much what you will get when you use the ironridge design tool online.

[J.P.M.]

Until recently, no one in the building industry envisioned designing roofs with the possibility of putting these solar panel arrays on them. Therefore it is scary to the building departments to allow something that was never intended by the designers. They are very risk adverse you know and subsequently you have to prove to them that the roof is strong enough. Which means some one has to dive into the ASME code SEC 7 on calculating wind loads either (which of course weren't done with solar panels in mind). They want a PE to ok the roof in order to absolve the building dept of liability. I know that there has never been a roof collapse due to a solar array, but that has nothing to do with it. I like the "can't be occupied by live loads" logic - I'm going to use that... thanks.

Not to sound like too much of an engineering prick about this, but I believe you are referring to ASCE sec 7, and probably SEC 7-16 that deals with wind loadings.

ASME Sec 7 covers recommended guidelines for power boilers. When working, I dealt with and designed to both.

As for building depts. being cavalier about such things, my limited experience with folks at the counters and others working inside of buildings is that they mostly try to do the best they can but tend to be a bit bureaucratic and once in a while more worried about covering their asses with paper than what's in front of them, but I won't paint everyone with the same brush. Besides, and as we all know, you won't win an argument with an inspector. And, I'll repeat, I've never won an argument with an inspector, except for one who worked for the Hartford Steam Boiler Inspection Service (the folks who are in effect the AHJ for the ASME Boiler and Pressure Vessel Code compliance) who showed up drunk. I had to physically remove him from the shop before he hurt himself or others.

I agree with you that a building dept. brushing off requirements is pretty scary but it does happen - and never acceptable.

But I do wonder just how a building gets to be occupied by a live load. And if it can't be an occupant, how does one remove it. Maybe a live load exorcism is required.

Because it is a standard that deals with minimum design loads for buildings and other structures, ASCE - 7 does not specifically reference PV panels, but for the most part it doesn't reference specific building features or components. Like any code the designer expected to be knowledgeable enough to use the code as a tool in/for a specific application. I used it a fair amount for the design of distillation columns, power boilers and other such things. I used ASCE 7 quite a bit even though that code does not specifically reference pressure vessels.

See SEIA (the Solar Energy Industries Association). They have a lot of information on how to deal with wind loads and interpretations of hjow to use/comply with ASCE 7 with respect to PV systems and wind loadings.

[J.P.M.]

Sure the main problem is ballasted on flat roofs. These should ALWAYS require a stamp.

Then you have pitched roofs and these you have two different mounting techniques: railless and railed systems.
The railless systems reduce structural integrity and should be looked at closely
Railed systems when installed correctly increase structural integrity and should not need a stamp.

any tilted system has greatly added uplift forces and should include a stamp.

1.) As a general precept, ballasted systems ought not to be used on horizontal roofs, or any roof for that matter, where the roof has not been specifically designed to carry such a load and associated external loads whatever such loads may be. Extra care is necessary in the checking of existing roofs or other structural elements of any orientation with respect to external loadings where such loadings may not have and probably weren't part of the original design considerations.

2.) While there is no way to ensure that any roof penetration, or any penetration for that matter will not leak, a little extra care in the design of penetrations to isolate/reduce the area of potential leaks such as the use of small, raised plinths - sometimes known rather indelicately as puss pockets when used for equipment supports - can go a long way, with one design consideration being a requirement for access and visual inspection and service of the penetration. An often unconsidered result of ballasted systems, particularly on flat roofs, is the induced strain (deformation) on the roof caused by the extra weight. That can be thought of as a ticking bomb. As the ballast strains the roof, the roof will sag. The low spot thus created will collect water. Enough said.

2.) I suppose it's possible to have railed and railless systems on a horizontal roof as well as a non horizontal (pitched) roof.

3.) I'm not sure that railed systems increase the structural integrity of a roof, or that railless systems decrease the structural integrity of a roof. I could make a case that the likely added number of attachments using a railless system might have some advantage of decreasing the point loadings at the roof penetrations. Also, and hard to say beyond speculation, depending on how it's done, if each panel is independent of its neighbors as might be the case with a railless system, a damaging wind event may have a slightly lower probability of taking/damaging the whole array, or at least maybe a smaller part of it.

4.) Depending on wind vector, systems that are non parallel to a (probably but not necessarily flat, and probably but not necessarily horizontal) roof will probably see larger tensile forces in the supports (uplift on the roof) than systems parallel to a roof. Systems that are non parallel to a roof will also see more compressive forces in the supporting members (that will exert downward forces on the roof) than systems parallel to a roof. There are also shear forces from wind as well, but those are usually small compared to the other loadings.

5.) Any system, should be properly designed, parallel to the mounting surface or not. Non parallel systems present different external loadings and so the mechanical design has different and probably additional considerations than systems parallel to the mounting surface. All of that doesn't necessarily require a P.E. to figure out. Follow ASCE - , etc. and plug and chug. A building code or dept. may require a P.E. to do or review the calcs. and drawings, but those can actually be done by anyone who's versed in the fundamentals. It might not be acceptable to authority having jurisdiction, but in a way that's only adding some assurance that the required calculations were done to the required standards.

[ButchDeal]

1.) As a general precept, ballasted systems ought not to be used on horizontal roofs, or any roof for that matter, where the roof has not been specifically designed to carry such a load and associated external loads whatever such loads may be. Extra care is necessary in the checking of existing roofs or other structural elements of any orientation with respect to external loadings where such loadings may not have and probably weren't part of the original design considerations.

exactly. Ballasted systems always need a stamp (and someone to review the structure to see that it can handle the load). This is why it is difficult to put ballasted on residential. Often the structure can not be certified as it is too old of uncertain integrity. Commercial often has specs on the large buildings to easily calculate to.

2.) While there is no way to ensure that any roof penetration, or any penetration for that matter will not leak, a little extra care in the design of penetrations to isolate/reduce the area of potential leaks such as the use of small, raised plinths - sometimes known rather indelicately as puss pockets when used for equipment supports - can go a long way, with one design consideration being a requirement for access and visual inspection and service of the penetration. An often unconsidered result of ballasted systems, particularly on flat roofs, is the induced strain (deformation) on the roof caused by the extra weight. That can be thought of as a ticking bomb. As the ballast strains the roof, the roof will sag. The low spot thus created will collect water. Enough said.

2.) I suppose it's possible to have railed and railless systems on a horizontal roof as well as a non horizontal (pitched) roof.

It is possible to do railed systems on flat (Horizontal) as well as both railed and railless on pitched roofs.

Railless on a flat roof though possible would result in a flat array which is not desirable.

3.) I'm not sure that railed systems increase the structural integrity of a roof, or that railless systems decrease the structural integrity of a roof. I could make a case that the likely added number of attachments using a railless system might have some advantage of decreasing the point loadings at the roof penetrations. Also, and hard to say beyond speculation, depending on how it's done, if each panel is independent of its neighbors as might be the case with a railless system, a damaging wind event may have a slightly lower probability of taking/damaging the whole array, or at least maybe a smaller part of it.

Railes have structural integrity of their own. They tie the rafters together and are considered to increase integrity more than the lag bolts would decrease the integrity, Because they tie the rafters together and spread the load and uplift force out, especially with staggered penetrations. Drilling holes into the rafters to put lag bolts in decreases integrity.
Railless on the other hand do not have any integrity of their own, they have more lag bolts and add weight but no integrity.

4.) Depending on wind vector, systems that are non parallel to a (probably but not necessarily flat, and probably but not necessarily horizontal) roof will probably see larger tensile forces in the supports (uplift on the roof) than systems parallel to a roof. Systems that are non parallel to a roof will also see more compressive forces in the supporting members (that will exert downward forces on the roof) than systems parallel to a roof. There are also shear forces from wind as well, but those are usually small compared to the other loadings.

exactly, and why I stated tilted systems should have a stamp.

5.) Any system, should be properly designed, parallel to the mounting surface or not. Non parallel systems present different external loadings and so the mechanical design has different and probably additional considerations than systems parallel to the mounting surface. All of that doesn't necessarily require a P.E. to figure out. Follow ASCE - , etc. and plug and chug. A building code or dept. may require a P.E. to do or review the calcs. and drawings, but those can actually be done by anyone who's versed in the fundamentals. It might not be acceptable to authority having jurisdiction, but in a way that's only adding some assurance that the required calculations were done to the required standards.

many jurisdictions do not require stamps in any of the conditions I suggested should have them but most of the larger well established do. We stopped doing ANY residential ballast system as it is too difficult to get anyone to review and stamp for the older ones and in jurisdictions that don't require it, we didn't feel comfortable either.

Ironridge has standards for revers tilt but that is for their equipment not for the structure it is bolted to.

[Roger57]

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.

There is a need to change codes and processes if they are unnecessarily preventing solar installations. I've worked with AHJs a number of times on unnecessary requirements and got them changed in the past.

The uplift requirement is not the issue here. The AHJ is worried about the actual weight on the house to the point that, in many cases, they are requiring a full engineering analysis clear down to the footings and foundation of the home. They want to know that a system isn't going to compromise the structural integrity of a home, even in an earthquake. But I'm pretty sure that a solar system isn't going to determine whether a house will collapse in an earthquake. We have engineers stamp our system designs all the time but this is a whole other level of analysis and costs.

[J.P.M.]

There is a need to change codes and processes if they are unnecessarily preventing solar installations. I've worked with AHJs a number of times on unnecessary requirements and got them changed in the past.

The uplift requirement is not the issue here. The AHJ is worried about the actual weight on the house to the point that, in many cases, they are requiring a full engineering analysis clear down to the footings and foundation of the home. They want to know that a system isn't going to compromise the structural integrity of a home, even in an earthquake. But I'm pretty sure that a solar system isn't going to determine whether a house will collapse in an earthquake. We have engineers stamp our system designs all the time but this is a whole other level of analysis and costs.

If dead weight is the governing issue rather than uplift from wind, I'm led to think that this is a ballasted system ?

Reason I ask: While both wind and seismic loadings need to be considered (but not acting simultaneously) as well as all other external and occasional loads, for most solar arrays, wind governs over seismic. If seismic is the concern, and from what you describe of their concerns, that's what it reads like, it's not the weight of the panels that's the problem.

From what you're writing, it almost sounds like you're planning a ballasted system. If so, the AJH's requirements are probably and entirely reasonable.

[solarix]

By the way, there is now a convenient service call greenlancer.com that is providing solar engineering services in just about all states. In most cases they can provide 24hr turnaround on standard stuff. I've pretty much quit using my local PE friend that my jobs were small potatoes to...