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Solar panels - TJI Rafters - Uplift??

TimNY

TimNY

Platinum Member
Joined
Jan 13, 2010
Messages
1,133
Location
Charleston, SC
Finally got the connection details for a solar panel (photovoltaic) roof installation. Brackets are to be secured with a 5/16 lag with minimum 1.8" embedment into the rafter (I don't recall the uplift capacity).

The problem (or maybe it's not a problem) is the roof rafters are TJI Trus-Joists. Pitch about 3:12.

Any issues with using the top cord as an anchorage point? Seems to me these are not designed to work in tension, but I may just be overthinking.

We're in a 120mph wind zone.

Thanks!
 
Engineered product, engineered solution. While it's possible that the connection is ok, I wouldn't put my name on it by approving it without documentation, at least from the I joist manufacturer, indicating the connection would be allowed. We require stamped drawings for all engineered product that indicate the connection points and loads applied, this is no different. How does one meet the 1.8" fastener penetration requirement into a 1.5" thick top cord?

As far as I joists are concerned, the bottom cord works in tension. You are applying a different load to the top that created a point load of X that's uplift, my concern is that that point load isn't what the joists are designed to handle.
 
Ok, thanks. Top chord on these is more than 1.5" deep, not sure exactly what it is, but will seek more documentation.

I know the bottom chord is usually in tension, which is why i question using the top chord in that manner. All I can picture is the panels blowing away with the top chord bolted to them, leaving the web and bottom chord attached to the house. Of course the plywood sheathing would figure in there as well. That's for somebody else to figure out.
 
Would you allow a metal roofing panel to be attached to the top chord?

Or structural sheathing?

~Same uplift under wind load as the Solar Panel.

When the wind blows, there's uplift on the TJI.
 
"~Same uplift under wind load as the Solar Panel."

Uh, not an engineer, but I seriously doubt it. A metal panel, or sheathing directly attached to the top cord, vs. a collector panel fastened with airspace underneath?

I think it's apples and oranges, and would also require a specific engineered solution.

JMHO
 
You're loading the top chord of the TJI either way...

and the solar collector would fall under components and cladding just like everything else on the roof.

Unless of course you require wind tunnel testing for other roof mounted equipment...such as commercial HVAC units and satellite dishes.
 
Really? You don't think there is a difference between directly attached sheathing/roofing material, and an elevated component? RTU's typically sit on curbs, no airspace below. And you can't really equate an 18" diameter sat dish with several hundred square feet of elevated panels........can you?

And yes, I have required engineering on RTU's that were elevated above a rooftop surface. Had one a couple years ago, that after analysis, engineer required skirting around perimeter of elevated RTU, for uplift concerns.

Again, JMHO, just a dumb ol' buiding inspector, I'll still require you educated guys to tell me if it's OK.
 
While it's good practice to use curbs in lieu of posts for roof mounted equipment, it's not required by the code (nor does the IBC implement NRCA requirements for clearance under elevated equipment but that's another story).

Alas wind tunnel testing is rare and using C&C formulas for uplift is common for both equipment and sheathing.

With C&C, the larger the area of the element the lower the minimum psf.

Just to clarify, my original response was in regards to concern that placing the top chord of a TJI rafter in tension was a problem. If this is your concern, then you should not allow sheathing to be attached either.

The way it works is that the connection from the Solar panel to the roof needs to resist the C&C load for the tributary area of the solar panel. However, the TJI Rafter needs to resist the C&C load of the tributary area it supports regardless of what is attached.

C&C loads account for localized variation in pressure and are not additive...a 30' rafter supporting cedar shake roofing sees the same uplift load as one supporting a single continuous standing seam panel.

I'm not suggesting that the loads can be ignored, hell I once rejected a roof mounted AHU because of lateral wind loads.

On the other hand, (8) 10d in shear are going to hold down a 2' X 3' PV array in most cases.
 
TimNY said:
Brackets are to be secured with a 5/16 lag with minimum 1.8" embedment into the rafter (I don't recall the uplift capacity).The problem (or maybe it's not a problem) is the roof rafters are TJI Trus-Joists. Pitch about 3:12.

Any issues with using the top cord as an anchorage point? Seems to me these are not designed to work in tension, but I may just be overthinking.

We're in a 120mph wind zone.

Thanks!
Click to expand...
The manufacturer (maybe the engineer) has given one method of fastening. If it is doable, it is good enough. Otherwise some engineering needs to be done.

If the panels are close to the roof, the uplift is most likely not significantly greater than for a roof without the panels.
 
A few things to add! I kinda agree with George, but without knowing the separation of the panels from the roof the up lift is an unknown.

The other thing is the attachment. TJI's may require blocking for this type of installation. I would go back to the DP and discuss this installation. If he gives a complete plan with details I be ok.
 
I tend to agree with George on this one as well, but better safe than sorry. Most manufacturers of EWP are more than willing to answer these types of questions and provide details to insure their product performs as expected. Contact the TJI manufacturer if you have any doubts.
 
The applicant's calculations show the rails spanning 3.9' with a calculated uplift of 369.3 lbs at each point the rails are fastened to the top chord of the Trus-Joist.

The manufacturer of the solar panel mounting system has furnished one method of fastening the panels. I question whether this is applicable to engineered lumber.

From what I can gather from the Trus-Joist documentation, fastening to the chord is good for up to 200 plf. In excess of 200 plf it shows web stiffeners installed and the load transferred to the web (in the details I saw, it was a steel strap and the strap was nailed through the stiffener into the web).

How I can transform that information into something that applies to this scenario I have no idea. What it does do is cause me to require more documentation.

I apologize to brudgers if I misspoke. The question here is about fastening to the panels to the top chord of a TJI. Regardless of tension, compression or shear, I am soliciting responses to this particular mounting scenario.

I am going to go out to the site today.
 
Last edited by a moderator:
TimNY said:
The applicant's calculations show the rails spanning 3.9' with a calculated uplift of 369.3 lbs at each point the rails are fastened to the top chord of the Trus-Joist.The manufacturer of the solar panel mounting system has furnished one method of fastening the panels. I question whether this is applicable to engineered lumber.

From what I can gather from the Trus-Joist documentation, fastening to the chord is good for up to 200 plf. In excess of 200 plf it shows web stiffeners installed and the load transferred to the web (in the details I saw, it was a steel strap and the strap was nailed through the stiffener into the web).

How I can transform that information into something that applies to this scenario I have no idea. What it does do is cause me to require more documentation.

I apologize to brudgers if I misspoke. The question here is about fastening to the panels to the top chord of a TJI. Regardless of tension, compression or shear, I am soliciting responses to this particular mounting scenario.

I am going to go out to the site today.
Click to expand...
(pushed the wrong button)

The 400 pound point load needs some attention.

I would have added some structure below the deck and fastened to it. But ...

Perhaps the TJI rep can provide some suggestions. The builder should ask.
 
TimNY said:
The applicant's calculations show the rails spanning 3.9' with a calculated uplift of 369.3 lbs at each point the rails are fastened to the top chord of the Trus-Joist.The manufacturer of the solar panel mounting system has furnished one method of fastening the panels. I question whether this is applicable to engineered lumber.

From what I can gather from the Trus-Joist documentation, fastening to the chord is good for up to 200 plf. In excess of 200 plf it shows web stiffeners installed and the load transferred to the web (in the details I saw, it was a steel strap and the strap was nailed through the stiffener into the web).

How I can transform that information into something that applies to this scenario I have no idea. What it does do is cause me to require more documentation.

I apologize to brudgers if I misspoke. The question here is about fastening to the panels to the top chord of a TJI. Regardless of tension, compression or shear, I am soliciting responses to this particular mounting scenario.

I am going to go out to the site today.
Click to expand...
There's no need to apologize to me.

If I were reviewing the plans and because TJI's are alternative means and methods, I'd accept a letter/email from the TJI manufacturer that addresses the specific condition in lieu of engineering.

In my experience, squash blocks/web stiffeners will generally take care of specific point loading in TJI's. At 120mph, the supports of a TJI are going to see more than 400# uplift anyway.
 
Answered by tech staff

I called the technical department for Trus-Joist and explained the situation.

Rep stated the chord-to-web connection is rated for 500 lbs at any point.
 
We don't have any way to directly determine code compliance of a "point load" under the IRC, do we? EWP or conventional lumber. Engineering of some sort would be required.
 
steveray said:
We don't have any way to directly determine code compliance of a "point load" under the IRC, do we? EWP or conventional lumber. Engineering of some sort would be required.
Click to expand...
I would agree with that, unless the reviewer has enough background to asses if it falls under accepted engineering practice. If you can't asses it well within your knowledge comfort zone, the applicant needs to supply more info.
 
Adequate design is important. Engineering is silly for many situations.

In my opinion, if the plan reviewer can't read a load table, they shouldn't be in the business.
 
Brudgers,

It is very difficult for us here to get a "design" here with out a "design" professional. We get answers like: it's the same as another layer of shingles, it reduces the weight because the snow slides off, etc. Without the follow up questions of: Well, how many layers of shingles are already there or where does the snow slide to, off the panels or off the roof, onto a lower roof? The contractors are unwilling or unable to look at the big picture or at least provide us with the info we need to comfortably approve the plans, the engineer seems to be simpler.
 
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