Rooftop Solar Panel question

[TJacobs]

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.

 

[RJJ]

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!

 

[Yankee]

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.

 

[Yankee]

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 : )

 

[Big Mac]

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.

 

[GHRoberts]

"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.)

 

[RJJ]

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

 

[Yankee]

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.

 

[FredK]

.... 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.

Good idea.