# Rooftop Solar Panel question



## Yankee (Apr 1, 2010)

What do you think about the theory that the dead load of a rooftop solar panel (***uming it is installed as a uniform load, not point load) displaces the calculated snow load that would otherwise act upon the area of the roof under the panel?

haha , , should I say "asksuming"?


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## Mac (Apr 1, 2010)

I don't buy it.


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## steveray (Apr 1, 2010)

We require engineering. What if the snow slides off of the panels onto a lower roof, or another part of that roof such as a valley? Either way it is point loading rafters (hopefully not sheathing) that weren't necessarily designed for it!


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## fatboy (Apr 1, 2010)

We also require engineering..........


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## mtlogcabin (Apr 1, 2010)

Depending on the angle of the solar panel you could have a significant increase in snow drift loading on certain portions of the roof.


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## bgingras (Apr 1, 2010)

We require and  evaluation of  the existing  roof frame, drawings indicating any modifications needed to carry the loads or a statement indicating that the structure is sufficient for the newly imposed loads.


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## Yankee (Apr 1, 2010)

bgingras said:
			
		

> _We require and  evaluation of  the existing  roof frame, drawings indicating any modifications needed to carry the loads or a statement indicating that the structure is sufficient for the newly imposed loads._


Are those evaluated in your office, or submitted by a stamp?


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## bgingras (Apr 1, 2010)

The evaluation would be submitted by an engineer or RDP


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## GHRoberts (Apr 2, 2010)

I don't know what the dead load ***ociated with solar panel is. I do know it is a rather small per square foot load. but ...

The fact that the loads are point loads is not important. The standard practice is to  treat the panels as uniformly distributed loads. By treating them this way one might find that the dead load is within the prescriptive limit.

---

We really need the filter to be fixed.


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## Yankee (Apr 2, 2010)

GHRoberts said:
			
		

> I don't know what the dead load ***ociated with solar panel is. I do know it is a rather small per square foot load. but ...The fact that the loads are point loads is not important. The standard practice is to treat the panels as uniformly distributed loads. By treating them this way one might find that the dead load is within the prescriptive limit.


That uniform/point load issue was discussed. I do feel that it is important if it is a point load or not, doubley important if the rafters are engineered I's or truss. I do think there is a way to install the panel that has a tendance to remove the point load and make the load more similar to a uniform load. I did recieve info from the panel manufacturer with calculations for the wind loading, and the wind loading is siginifant on the bearing points if askuming a point load installation.


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## FredK (Apr 2, 2010)

Doesn't snow here and if it did I'd move farther south.

I'm sure that I would take some look at engineering in the snow climates.

And I sure that you require the same for skylights?????


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## peach (Apr 2, 2010)

test.. thanks for your patience


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## Yankee (Apr 3, 2010)

> And I sure that you require the same for skylights?????


Good question, yes, I guess I'd want to see some accounting for the additional dead load if a skylight was installed on an existing roof. I guess that answers my question, doesn't it.


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## peach (Apr 3, 2010)

maybe.. maybe not.. there may be some drifting on the skylight/solar panels if they are slightly elevated..

Up to plan review to ask the question; once the permit is issued and inspections have started, it's inappropriate to put the responsibility on the inspector.  They will go with the approved plans.

trust me.. we don't have time to do plan review in the field.


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## Yankee (Apr 3, 2010)

You mean drifting of snow, as a previous poster suggested?

I thought about that. My thoughts were that whatever drifting might occur from a sloped/elevated skylight is no different re: the snow loading, than if an odd-ball roof slope of similar shape were to be built and as far as I know, that wouldn't require any additional "drifting" snow calculation other than a traditional uniform snow load factor. I don't see how that aspect can effect or be required re: engineering.


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## bgingras (Apr 3, 2010)

last time I checked, I didn't see any specs for the installation of  rooftop solar collectors, solar water heaters, etc in the code book. Given that fact alone, it's not prescriptive, thus an engineer should put his name/stamp/seal on it. This keeps us from having any liability for approving something that isn't in the code, right?


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## Uncle Bob (Apr 4, 2010)

Hello Yankee,

Welcome home. First time I've seen your posts on this new site.

The problems I've seen with solar panels on roofs; is they were installed long after the house was built.

Put money asside for replacement after the "every three to five year hail" we get.  

Where does it say a permit is required for installation of solar panels and them stupid TV dishes that cause leaks, on roofs?

For crying out loud! Put a-s-s back! If you don't like the words folks use; sanction them or kick them out; but were over reaching here! 

Oh, you did.  

I'll just quietly slither out the side door,

Uncle Bob


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## GHRoberts (Apr 4, 2010)

bgingras said:
			
		

> last time I checked, I didn't see any specs for the installation of  rooftop solar collectors, solar water heaters, etc in the code book. Given that fact alone, it's not prescriptive, thus an engineer should put his name/stamp/seal on it. This keeps us from having any liability for approving something that isn't in the code, right?


The prescription is there. One uses the tables with the appropriate dead load.

The prescription is there so that the AHJ can not force unneeded expenses on the designer/builder/owner.


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## Yankee (Apr 4, 2010)

GHRoberts said:
			
		

> The prescription is there. One uses the tables with the appropriate dead load.The prescription is there so that the AHJ can not force unneeded expenses on the designer/builder/owner.


How do you view the use of a uniform load for calculations (the tables) in the event that the panels are on legs attached to individual rafters (point loads/engineering)? Do you see that as a conflict that would need further attention, or a matter of insignificance?


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## JBI (Apr 4, 2010)

Most of the panels I've seen installed on roofs recently are installed parallel to the slope, and very low profile. I always ask what dead load they are adding, and for calculations to verify the rafters are adequate to carry the dead load. I don't see much more need for information than that with this type of panel. The additional loads have only been problematic on a small percentage of jobs, and were easily mitigated with additional support (usually a knee wall in the attic or doubling up (sistering) of rafters).

Other designs I have seen (less often and not newly installed) that have a slope much steeper than roof, or a high box-type profile, I could reasonably see as posing a different loading problem. Drifting snow and accumulating ice behind the panel or box may need to be considered.


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## GHRoberts (Apr 4, 2010)

Yankee said:
			
		

> How do you view the use of a uniform load for calculations (the tables) in the event that the panels are on legs attached to individual rafters (point loads/engineering)? Do you see that as a conflict that would need further attention, or a matter of insignificance?


a matter of insignificance


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## RJJ (Apr 5, 2010)

I have had a bunch of these over the last few weeks and at one point was going to kick them back for engineering. It seems that most of the panels I have reviewed have a 2.5 lbs per sq Ft. Seems to work within the guidelines. One of the last three was stamped by an engineer. The span and rafter size were very close to each other! They all seem to have the same slope for the panels, weight added and so on.

When reviewing these issues I consider this factor. If a roof had a 20 year shingle at 225 per sq. and they change to a 50 year shingle at 350 to 400lbs the numbers seem close!?????? Or just consider a second layer of shingles!  ?????? If the span and rafters work for those is this a bigger deal? I would like to hear George Roberts thoughts!


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## bgingras (Apr 5, 2010)

Bare in mind I have not been employed as an inspector for almost 1 year now, but when I was we got a bunch in a row, sent them for engineering, and they ALL came back with some type of framing modification required. We are in the northeast, snow loads are considerable.


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## GHRoberts (Apr 5, 2010)

RJJ said:
			
		

> I would like to hear George Roberts thoughts!


So a fellow buys a new couch and puts it in his bedroom - converted to a TV room for his 250 pound football buddies. Does he really need to be concerned about the point loads from the 4 legs of the couch? I don't think so.

The solar panels are a lot like the couch. There is enough of a safety factor in the prescriptive codes that there is usually no problem. For there to be a problem one would need to build at the extreme limits - max span for rafters, max span for the sheathing, and of course the max design live load.

Heavier shingles, solar panels. AHJs have a tough time.


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## TJacobs (Apr 5, 2010)

IF and WHEN we see solar panels we will do what bgingras said: engineering due to snow loads, and the fact that they are not uniformly supported across the roof.


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## RJJ (Apr 5, 2010)

George: interesting response! I would agree. I was thinking a similar situation.

Tjacobs: Why would you say they are not uniformly loaded? It is an interesting point you raise!


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## Yankee (Apr 6, 2010)

GHRoberts said:
			
		

> So a fellow buys a new couch and puts it in his bedroom - converted to a TV room for his 250 pound football buddies. Does he really need to be concerned about the point loads from the 4 legs of the couch? I don't think so.The solar panels are a lot like the couch. There is enough of a safety factor in the prescriptive codes that there is usually no problem. For there to be a problem one would need to build at the extreme limits - max span for rafters, max span for the sheathing, and of course the max design live load.
> 
> Heavier shingles, solar panels. AHJs have a tough time.


So by your comment, one would need an evaluation of the existing roof to determine if it were built to its extreme engineering limits. Does that not call in  an engineer right there (unless you are suggestion asking the homeowner to investigate and provide a drawing or statement as to what is existing?).

How about if the existing roof structure is not convevebtional lumber, but instead engineered rafters.


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## Yankee (Apr 6, 2010)

RJJ said:
			
		

> George: interesting response! I would agree. I was thinking a similar situation.Tjacobs: Why would you say they are not uniformly loaded? It is an interesting point you raise!


If they are installed on legs instead of on a runner of some sort it would be point loaded, wouldn't it?

Like couch legs : )


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## Big Mac (Apr 6, 2010)

Probably not much of an issue for Arizona or Southern Cal, but in those areas with potentially high snow laods it could be a significant issue.  I know that in our jurisdiction the snow load can vary between 25 and 450PSF.  Granted there is not a lot of buiklding in the 450PSF area, but what about those that do?  They might be exactly the kind of folks that want independance from power grids, gnerators, and the like.  Of course the secondary issue then is whether or not the solar panels themselves could support those loads.


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## GHRoberts (Apr 6, 2010)

"If they are installed on legs instead of on a runner of some sort it would be point loaded, wouldn't it?

Like couch legs : ) "

The engineering solution for a point load or a uniform load is not that much different.

"Granted there is not a lot of building in the 450PSF area, but what about those that do?"

The 2.5PSF is not significant relative to the 450PSF snow load.

I just typed some sample numbers into my rafter/joist span program to see what happens. 2x10 with 40 live/10 dead spans about 15-16'. Add 2.5PSF dead load for the solar panel - 5.2'. About 2.5-4.5" difference in the span. I can get that back by adding the sheathing strength to the computations.

I am sure there are some people with really slick software that can model the snow load as it bridges over the solar panel, compute the point loads, and determine the stresses. But to so is just a waste of resources. (Could you compute the loading on the nails also. I think 450PSF will cause the shingles or the sheathing to slide off the roof.)


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## RJJ (Apr 6, 2010)

George: you always seem to boil things down to the simplest response.


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## Yankee (Apr 7, 2010)

Thank you all for your thoughts and discussion. I have decided to require engineering verification considering the additional loads proposed. The decision is based on several factors.

1.) Ground snow load is 90lbs/per and therefore already out of the prescriptive

2.) The unit(s) either have a point loading effect which would require assesment of the structure, or a uniform effect which would add to the maximum prescriptive dead load of 15lbs/sf (R301.2.2.21)

3.) The existing roof framing is engineered trusses

4.) I am not able to assess the additional live loading for wind and snow given the factors involved.


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## FredK (Apr 7, 2010)

Yankee said:
			
		

> .... I have decided to require engineering verification considering the additional loads proposed. The decision is based on several factors. 1.) Ground snow load is 90lbs/per and therefore already out of the prescriptive
> 
> 2.) The unit(s) either have a point loading effect which would require assesment of the structure, or a uniform effect which would add to the maximum prescriptive dead load of 15lbs/sf (R301.2.2.21)
> 
> ...


Good idea.


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