Uplift on Roof Mounted Solar Panels

[WSUgrad]

I'm getting ready to install a solar panel system on my roof. The supplier won't get in to the uplift calculations, probably because they wan't zero liability. Therefore, I will need to provide this information to my local building official. I've been searching the internet all morning for sample calculations so that I can side the lag bolts and spacing of them.

My two problems: How do I compute the uplift on the solar panels?

Where do I find the max uplift resistance of lag bolts?

Given the wind uplift it should be easy to specify a number of bolts & spacing to resist the uplift forces.

Background info:
Panels are 2 rows of 11 (1 panel = 39" wide x 59" tall).
Mounted on rails that are laid perp to roof trusses.
Roof trusses spaced 24" oc.
1 panel weighs 39.7 lbs.
Distance from eave mounted: 1'-2"

I'm buying my kit from the website listed below:

 

[JedClampett]

I don't know how much you're spending on the solar panels, but investing a few hundred dollars to hire a structural engineer would be money well spent.
For instance, the uplift needs to be calculated from ASCE 7-05. And the wind section of that is not an easy document to fight through. Then you need to go to the Wood Desgn Code (NDS) to get the lag bolt allowables. And that is dependent on the type of lumber you're anchoring to and the thickness.

 

[RacingAZ]

Jed is right. I don't see the logic in trying to save a few hundred dollars when your system cost is in the tens of thousands.

I do this for solar companies and when homeowners balk at paying a little extra to design the connections, I tell them to think of the expense as an insurance policy. In your case, just make sure the structural engineer is covered with E&O insurance.

Also as a side note, you're doing the installation as a DIY type project? You may want to check with your local utility as you'll probably losing out on their rebate, in addition to state and federal. Not sure where you at but here in AZ, solar installation would need to be done by registered contractor to qualify for that 3rd rebate from the local utility. That can be a substantial saving if that is being offered in your area.

 

[MiketheEngineer]

Jed has it right.

Unless you are in some weird wind zone or have a very strange roof - about the most uplift you might expect is in the 15 psf range - BUT it can vary widely about that. 3/8'' lags into good wood will probably give you soemthing in the 500# per lag range. Again it depends on wood type, loading type and length of bolt, etc - just to mention a few items.

Then how do you transfer that load from the roof to the building?? Thats where an engineer can really help out as Jed suggested.

I just toss those numbers out there if you want to get some idea of what it will take. Please know they may make no sense for your project.....

 

[Ron]

Agree with JC and RAZ...pay a few bucks and sleep well.
You're not in a particularly high wind area (Michigan), but even the code-mandated 90mph can produce some significant loads. You also are crossing over several uplift factor zones on your roof, so there will be variable loads across your panels.

 

[WSUgrad]

I can install the panels on my own and still receive the rebate offered by my utility company and by the federal government. I have a certified electrician that is going to assist me in the wiring (it's actually quite simple).

I plan on putting 3/16" x 3" lag bolts at 4' o.c. into my trusses to anchor the rail system. The uplift resistance should be more than adequate.