Roof Structural Analysis for Solar Panels

[chinmayhegde]

Hi all,

I'm installing solar panels on my home and the structural engineer I'm working with determined that my current roof will not support the weight of the panels. My roof (built in 1967) is comprised of a field fabricated truss with a 2x8 ridge plate running along the top of the roof, 2x6 rafters, and two diagonal members per side of the roof, a 2x4 and a 1x5. See attached for a layout of the rafters and a layout of the solar panel system.

The total weight for the solar panels, plus racking, etc. would be in the ballpark of 900 lbs. There are currently 2 layers of asphalt shingles on the roof but I'm having a new roof put on so there will be only one layer when the panels are put on.

Based on the engineer's analysis, he recommended sistering a 2x8 to every rafter and adding a 2x6 collar tie every other rafter. However, because of the layout of the diagonal members, it is not possible to add the sister members in the same plane and thus they could not be connected by the collar ties. Hopefully this statement makes sense. To clarify, if you look at the rafter diagram and consider the z axis as coming out of the page - the diagonal members on either side of the roof would be at say +2" in the Z direction on one side, and at -2" in the Z direction on the other side, and thus the sister members could not be put in the same XY plane.

So, the engineer is coming back out next week to see what alternatives there might be to sistering in new rafters.

My questions are the following:

1. Is there a good alternative to sistering? (I came upon this site because of an interesting discussion on using a coil strap, but I think the poster ended up sistering anyway).

2. Is is necessary to sister all the rafters, when only a select number will be exposed to a point load from the solar panels?

3. From an intuitive standpoint, if I have 2 layers of shingles currently on the roof, when I remove them and add just one back plus the solar panels, I will likely have less weight than what is currently on the roof (though the load distribution will be different, hence Question 2). If this is the case, doesn't that mean my current structure is unsound, or at least not up to code?

Thanks for reading! I appreciate any insight the forum can provide.

 

[msquared48]

In answer to your questions...

1. Sistering could be a problem, with what you describe, so a continouos steel strap, or two, or three on each of the bottom chords of the affected trusses could bde a viable option here. However, if required, sistering would be the only way to strengthen the top chord. He would have to figure out the details on how to do this.

2. If they see load from the array, each truss bottom chord should be strapped within the limits of the solar panel array. Same with sistering the top chord if it sees load.

3. It appears that the uniform load of the array is 3.5 to 4 psf, or about the weight of one layer of composite roofing material - three tab - depending on the years of life and style used. Removing the existing two layers and installing new three tab could help with the capacity, but if you currently have two layers of shakes over skip sheathing, the addition of a required OSB or plywood overlay on top of the skip sheathing would decrease the benefit. Your engineer would have to figure that out. Could be a trade off. He should also be concerned itf the top chord of the trusses are seeing point loads and make sure they can take it.

I can see his concerns.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

chin,

I have no idea what the snow load is in your area, but I believe there is a problem with your layout. By supporting the panels on every second rafter, snow load will be carried by the panels and supported on every second rafter. It would be better to support the panels on every rafter.

The total additional load is 900# and is spread over an area of about 13'x26' or 338SF. That is only 2.7 psf on one half of the roof. Are the trusses designed that precisely?

I would consider adding a short 2x4 vertical member between top and bottom chords 107" from the exterior support on the side with the panels. That would reduce the span and bending moment on the top chord/rafter.

If the bottom chord is not adequate for the extra load, you would have to beef it up as required.

BA

 

[BAretired]

Attached is my proposal.

BA

 

[msquared48]

Personally, I can understand why the company mounts on every other truss - to cut the numbefr of roof penetrations, possible leaks, and associated liability in half.

That being said though, I would recommend that the panel rail supports, since there are two rails to each panel line, and two panel lines, the rails alternate their supports so that no more than two support points bear on any one truss. It would give a more uniform deflection pattern to the roof. Not perfect, but better.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Mike,

Maybe they could use a block attached to the rail but not to the roof at alternate locations. It really is preferable to carry snow load on every truss.

BA

 

[chinmayhegde]

Thanks all for the responses.

BA,

We based the rafter spacing off the rack manufacturer's code code compliant installation manual (

http://www.unirac.com/pdf/ii227.pdf),

which takes into account snow loads (I'm in MD, I believe it is 30psf), and live wind loads. That specified a minimum spacing of 5.5 ft. However, I believe the manual generally takes the stance that the live loads created by the solar panels are more critical than the dead loads, so it is up to the engineer to determine whether or not the structure can handle the snow load.

I like your suggestion of adding the 2x4 to the top chord, I'll talk it over with the structural engineer and see what he thinks.

Mike,

I see your point about alternating the attachment points for the rafters. Although I don't think I have seen a layout like that, it would make sense to help distribute the snow loads. If I were to alternate, do you think I should put the mounts for the top row of panels on the "even numbered" rafters, and then put the mounts for the bottom row of panels on the "odd numbered" rafters, or should I alternate the mounts within the row of panels? I would think the latter option might be better, since then each rafter could have one load on the top chord and one load on the bottom chord.

BA,

I'm not really sure what you mean by a block attached to the rails but not the roof. Could you elaborate?

Thanks again.

 

[hokie66]

This is not really a truss roof, but rather a stick built rafter and ceiling joist arrangement with some diagonal members fixed to the sides. Maybe you could add some diagonals on the opposite side of the existing ones, then remove the ones in the way of the sistering and collar tie installation.

 

[BAretired]

chinmayhegde,
I was responding to Mike's post where he expressed concern about too many roof penetrations. At present, you have a Quickmount PV installed on alternate rafters. Between those, you could install an aluminum block fastened to the rail and bearing on the roof, but not attached to the roof.

In that way, the vertical reaction is applied to every truss but penetration occurs only on alternate trusses.

BA

 

[msquared48]

I agree with BA here. Good thoughts.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask