Solar Panel Structural Calculations/Sealing Drawings 2

[mfstructural]

I was contacted by a solar company interested in hiring a structural engineer. They install solar panels on roofs and are looking for an engineer to work with to stamp the drawings.
The way they do things is have an inspector go out and take drone pictures of the roof, take pictures of the inside of attic (roof framing, including a tape measure showing measurements). They also have pictures of the roof framing layout and member sizes and any posts supporting ridge beams, etc. All this is for the engineer to review.
They also create a set of drawings showing how many panels are on each roof facet. There are a couple drawings they would want me to sign, including drawings showing how many panels on each face, roof slopes, and how to install them.
For example the one I reviewed showed the roof slope, and connection to roof of the panel, with was a 5/16" lag bolt (4" long) into rafter/truss. The drawings also show the clips that attach between the lag bolt and panel, presumably by the manufacturer.
Lastly, there is a letter drafted for the municipality talking about the loads and how the structural assessment was based on pictures/inspection performed by the solar company.
The solar panels are shown to be 3.5 psf on the drawings, which also show 115 wind load, 25 psf snow load, and 9 psf roof DL shingles, etc.
I don't think putting 3.5 psf of panels on a roof is going to overstress it. The other thing I'm wondering about is what the responsibility is to have performed a site inspection some of these are several hours from me and I can't get to them so most of them would be based on review of photos/inspection information.

Has anyone come across something like this?

 

[jayrod12]

The only thing I've ever come across in regards to solar panels is a slightly interesting situation.

New home build, fairly large and expensive home, entire thing had been constructed from a structural perspective. After the shingles were installed I get a shop drawing for some solar panels. Phone call to architect Me: Hey, I don't remember solar panels being part of the project?
Him: They weren't the owner added these to the scope about a week and a half ago. Is this going to be a problem?
Me: from a beam, wall, foundation perspective no, but someone should let the pre-eng roof truss guy know.

Pre-eng roof truss guy: No way in hell can our trusses support these. The connections are a 48" o/c and therefore load up every second truss as opposed to every truss so it's more like adding 7 PSF instead of the 3.5 psf they are claiming. Secondly, the lag screws are into the 1.5" dimension of the lumber therefore I also have to account for a reduction in capacity of approximately 20-30% for the top chord.

I thought it was interesting that they consider a reduction in strength of the top chord (compression only member) from the bolts. But I couldn't argue with the guy, it's his design and there's no way in hell I'm taking liability for the design of his trusses. So I told the architect that everything except the roof trusses was capable but without approval from the truss guy I couldn't allow the panels to be installed.

Fast forward a few weeks later, receive a letter from the AHJ asking for my certification on the newly installed solar panels. Sent a strongly worded letter regarding where this conversation had left off. Haven't heard anything since but I assume they did not remove the panels, but I'm not sure how they got the AHJ to buy in because I'm pretty sure the roof truss guy wasn't budging.

 

[NorthCivil]

I would not be keen to sign off on additional load to an existing structure without seeing it in person. I think the best way to proceed would be that you could sign off on the panel & its connection to existing structure, and the solar company should hire someone local to evaluate the existing structure and approve of its suitability to carry additional loads.

 

[TehMightyEngineer]

I'd say you could make this work and it's worth it but I'd be really careful.

First, the solar panel guys need to be honest with you and trustworthy. They'll want to put a solar panel on anything that will hold up long enough for them to get paid. Beyond that they likely don't care as it's your stamp and you would take the fall. You need to be clear that you will be brutal about ensuring each and every install is something you're comfortable with and if it's something you need to see in person then they have to understand that your fee may change to reflect the site visit. Ensure that you have a solid contract that defines your scope and protects you and your client.

Next, I'd suggest that the first few will require you to perform site visits with their personal so that you can both get a feel for the jobs and get them to understand what you're looking for and what information they need to get when they're on site (pictures, pictures, and more pictures). On the last one maybe even just observe as they do the site visit and ensure they don't miss anything and provide comments on their methods. This would also allow them to feel you out.

From there you should have a decent system going that you can review the jobs remotely and identify areas that need further review or that require personal attention. Have language in your reports that explain how you're relying on their site visit and thus you're not liable for conditions you weren't made aware of. My understanding is this practice is generally considered acceptable but might be worth an hour or two of a lawyers time to be sure.

Finally, I'd just be extra cautious. Imagine how each job would be defended if it went to court and make sure you have reasonable defenses and supporting data.

I would expect that this should cover you much of the time. Ideally most roofs never see their full design load and thus you could probably get away with it on blind luck. What would get you is someone with an already deficient roof (snow drift prone roof, rotted trusses, etc.) and the solar panels are the final straw.

Good luck.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WARose]

One thing you didn't mention was budget. I've had these jokers contact me before.....and for $300 they wanted me to assume the liability of them covering a whole roof with this stuff. I don't even return their calls anymore.

The field work they did would be pretty much worthless if something went wrong. They don't know what to look for. The good news is: typically where there is a panel.....there can't be any live load. That's how a lot of people justify these additions.

You just have to work it out if their budget for your services will cover site visits and so on. What I have discovered with them is: you wind up doing a lot of free estimates that they won't accept.

 

[JAE]

If I was hired by a homeowner to be their expert witness after their roof collapsed with solar panels on it, and I was asked by the attorney as to the validity of:

a licensed engineer performing an engineering sign-off on an existing wood framed structure based on photos and dimensions provided by someone that was not directly supervised by the engineer,

I would find myself stating that the engineer did not perform his services to the standard of care required to protect the safety and welfare of the public.

But hindsight is 20-20.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[kmart30]

I have a similar situation but not with solar panels. Basically I rely heavily on the data that is given to me and in all my reports I state this so it is known to all parties. Manufactures/contractors dont care....they are just looking for a seal so they can get paid. I did uplift calcs once for some roofing and the roofer was so dumb that just because he got a sealed doc from an engineer he thought that he didn't have to install a weather barrier. Really? and it clearly said in my report that it was limited only to the fastening. You need to be clear and bold in your report where your limitations for design are and that your calculations/data relies on the data you have been given. You should state that If anything is different in the field you should be notified immediately before construction begins or materials are paid for and delivered.

I had something happen to me recently where I wasnt given the "whole picture" of what the structure really was. I got an email from the EOR asking me all these questions and how my calcs were wrong. Basically the site information I was given (by the contractor and manufacture) was completely wrong and the materials were bought and already delivered on on the job. O well....read my report it says it in bold letters and is clear as day. This still wouldnt prevent a lawsuit but at least I have a leg to stand on.

You should be able to tell if the data you are given is a bit off or suspicious of at least asking to clarify but at the end of the day if you dont see it with your own two eyes(in the field) then you have to really trust the people giving you the data. Plan reviewers and inspectors are also a line of defense as well so be clear and bold.

 

[Ron]

kmart30...good ol' Florida has a pretty good delegated engineer clause in the law. One of the requirements is that the structural engineer of record provide you with the parameters of your delegated design. If he did not do so, it is his issue, not yours!

mfstructural....you did right in objecting with a strongly worded letter....don't cave.

 

[kmart30]

Yes you are right Ron....when it's newer construction with an EOR but what happens when it a building was built/designed 10-20 plus years ago and there is no way to determine who the original EOR is? You essentially become the EOR for what you are designing right? The existing building code would govern and you would have to rely only on field data....