Determining Maximum Load for Flat Roof for Solar Panel Installation 2

[BufordTJustice]

Hello-

I have a flat roof onto which I am interested in installing some solar panel arrays. As I am in a high wind area (design is ~145 mph), I either need to penetrate the roof 50+ times to anchor the panel mounting rails, or use a ballasted system and avoid the penetrations altogether.

I am leaning towards the ballasted approach as it not only eliminates the cost of large number of roof penetrations but also the opportunities for leaks which will certainly become more likely with so many penetrations. The ballasted approach also removes uplift loads from being applied to the roof structure itself (as the ballast is sized accordingly to handle this).

In any case, the specifics of the roof are as follows----

The roof is flat (covered with a single ply TPO membrane over 1/2" iso board, which is over 1/2" plywood sheathing). The trusses are parallel chord type and are spaced 24" OC across the roof and are comprised of 2x4 members. The span between vertical supports is ~12 ft and the depth of the truss (measured between the outside of both the top and bottom chords) is ~68" A photo is below.

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Is there an easy way to determine the maximum load in psf this roof could be expected to support? I'm going for permit for the solar system this is going to be question 1 that I have to answer. Thanks for the help!

 

[BufordTJustice]

From what I have found so far (in many places of FL at least), an expedited permitting process is being used for PV installations which, if the installation meets certain simple criteria, additional engineering is not required.

Basically, it appears that if the total roof load to be added is <5 psf and the load per attachment point is <45 lb, and the mounting means are an engineered product, then no additional structural analysis is required.

 

[dcarr82775]

Any manufacturer of this type of system must get State approval in Florida, part of the process is the letter you posted along with submittal of calcs and tests results. YOu will need another engineer to stamp something saying the roof itself can handle the added loads.

Min roof LL in Florida has traditionally been 30 psf, but recently got reduced to 20 psf. So if things were adequately designed and built originally, you might have 10psf for free. All that said, those roof trusses look horrific. The 2x4 (imho) top chords in the first pic can't be good for 20psf LL much less 30. It has already sagged pretty good with virtually no dead load. Almost looks like somebody took the original roof truss and modified it (poorly) to make a flat roof, or maybe it was a piggyback truss truss that got installed backwards. The 2nd pic is more disturbing if that is possible. I wouldn't put anything additional on those truss because they can't be right. But if you can get an engineer to stamp something saying those trusses are good for the loads, good on you

 

[BufordTJustice]

dcarr---

You've done this before

The process as you've just described it is how I now know it will flow (in my area anyway). The building department will take all the manufacturers info for the PV equipment/mounts, etc and all I need to do is gain structural verification of the roof load that will be added. If the load was <5 psf, I wouldn't need to do this even, but if I go ballasted mounts I will be over 6 psf.

Ive found a truss engineer in the area who has already figured out how to modify the trusses. He also said they were non-standard, but there are very easy ways to make them much stronger (which Ill be implementing regardless of whether I go solar or not to harden the house). He said h sees this all the time in the area with houses that are a little older, and its usually a solar installation that triggers someone to address it!

 

[BufordTJustice]

All-

Another quick question (which I think is simpler)---

I do have another flat roof area which could be used for the solar panels.

Initially, I didnt think I could fit the number of panels desired in the area available, but upon more detailed measurement, I can get very close to my 11.7 kW target with the footprint available. The roof is a little lower than the other one (which is slightly less ideal for solar production), but still faces dead south.

This roof is also a flat TPO roof using 1/2" isoboard over 1/2" sheathing, but is more conventionally framed. It uses 2x8 roof joists as follows----

2x8 @ 16" oc ~8 ft span
2x8 @ 12" oc ~14 ft span
2x8 @ 16" oc ~14 ft span

What might be the ballpark load that could be added to this roof without modifications?

 

[jayrod12]

2x8 @ 16 over an 8ft span have lots of capacity. Enough that I wouldn't worry about the joists but rather the wall supporting the joist.

2x8 @ 12 over a 14ft span can take a LL=45psf with a dead load of 20psf.

2x8 @ 16 over a 14ft span can take a LL=30psf with a dead load of 20psf.

Keeping in mind I'm just a random stranger on the internet. And I'm accounting for gravity loads, not concerning myself with uplift (i.e. unbraced length of the bottom chord when in compression).

 

[JAE]

What might be the ballpark load that could be added to this roof without modifications?

Go hire a structural engineer to get you an exact load capacity - a "ballpark" load capacity by what jayrod12 correctly refers to as "random strangers on the internet" won't do you any good in reality.

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[BufordTJustice]

Thank you all for the help.