Determining Maximum Load for Flat Roof for Solar Panel Installation
Determining Maximum Load for Flat Roof for Solar Panel Installation
(OP)
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!
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.
. 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!






RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Thank you for the reply. These trusses were installed in 1977....so yes whatever code was then is likely what they were designed for. I am near Tampa, so no snow load.
Is 20 psf the "norm" as far as live load designs?
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
You'd obviously need a local structural guy to review and sign off, I assume at this point you're just investigating feasibility.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Yes...right now I am just seeing what may be possible. Before I get too deep into this Id prefer to know if I am in the ballpark or not!
Here is a photo of a "legit" (i.e. permitted) installation of a ballasted system in my town. In looking at the ballast blocks, it doesn't look like a huge amount was required and the spacing may be enough to fall below the ~20 psf (which I know Id still need to confirm). I'm guessing that the ballast in each of the trays is ~50-60 lb. In looking at the area of each panel and the total load in a given area, it seems as though (if this photo is a reasonable guide) that the total load would be less than 20 psf?
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
I will def get a local structural for the permit drawings, but before going there (again as a feasibility study), Ive found the below truss load table online.
My trusses are ~30 ft long between bearing points and as mentioned above appear to be parallel chord type and are comprised of 2x4 members. The depth of the truss (measured between the outside of both the top and bottom chords) is ~68"
In looking at this table, and again just ballparking things to see how feasible this is, this truss might be capable of fairly significant load?
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Your roof framing system is a crazy looking parallel chord truss framing system, if that’s what it is or what you call it. It isn’t like the parallel chord trusses you show. The truss spans about 30', the trusses are spaced 2' o/c, but the top chord appears to be a 2x4 or 2x6 spanning horiz. 15', from one truss bearing line to the center of the truss (the ridge) and then another 15' on to the other truss bearing line. There are no diag. members supporting the upper chord in the plane of the truss. The real load carrying truss seems to be a 30' long std. gable truss with about a 4/12 pitch (5.67/15). Then there is a 30' top chord fixed to the top of the gable truss, to carry the ½" roof sheathing and roof loads. That won’t support a ballasted solar panel system and it won’t take much of the uplift in your neck-of-the-woods either. 15' is a very long span for a 2x4 or 2x6 with ½" sheathing.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Great stuff!
I was actually just looking online for other truss tables that matched more closely with mine. Your note that it really is just a standard/common gable truss with another 2x4 running horizontally 15 ft from the center of the truss out on each side immediately made sense to me. Thank you for this insight.
So as far as the load carry capability of the truss itself (ignoring the funky 15 ft 2x4 spans running horizontally), the below might be a reasonable approximation?
And, do you have any estimations on what the 15 ft 2x4 span can support? I am guessing its going to be less than 20 psf! If so, to correct that can vertical 2x4s be used to transfer that load downward at mid span (i.e. make the span ~7.5 ft rather than ~15 ft)?
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
You need to have someone come out, review and measure the framing, and analyze the structure as built to determine the capacity.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
This house was built (and untold thousands like it around here) by a giant builder of the day (Reutenberg/US Homes). They even provided a big heavy brass plaque to the new homeowners upon completion! So, it must have been good. LOL
In seriousness, we can be sure that this met the code requirements of the day considering whom it was built by (and empirically, this house has seen 100+ mph winds more than once without any uplift issues at all), and I also have 12 pool heating solar panels (each is about 50 sq ft and full of water) on another section of the roof and there are no issues with that (they have been through hurricanes as well) so funky as all this is, it was someones good idea at one time and seems to work
Unfortunately, the fact that it is non something I can look up in an online table or otherwise reasonably estimate, means it may have overly complicated my solar pursuits
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Dave
Thaidavid
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Some further estimations using the above installation photo of the ballasts. As I know this is an acceptable installation that passed all permitting requirements it may be a decent basis for speculation-----
I believe those ballast blocks are ~32 lb each and there are 3 on each ballast tray (for ~96 lb total ballast in each tray). The tray itself is ~4 lb, so we are at ~100 lb total per tray.
In looking at the above solar panel layout, for a given 8 panel section, there are 15 ballasted mounts. So, this means over that section there is ~1500 lb added, plus the panels themselves.
These panels will typically be ~40 lb each and measure ~65"x39" (or ~17 sq ft each).
The panel rows are spaced ~18" apart.
So, the total weight of the 8 panel array----
8 panels x 40 lb each = 320 lb
15 ballast trays x 100 lb each = 1500 lb
1820 lb total
The total area covered-----
2 panels x 39" (3.25') = 6.5 ft length
4 panels x 65" (5.4') = 21.6 ft width
Total panel area = 6.5' x 21.6'= 140.4 ft^2
Adding in the spacing between panels rows where panels do not reside-----
18" (1.5' spacing between rows) x 21.6 ft width = 32.3 ft^2
Total area of the 8 panel array = 140.4ft^2+32.3 ft^2 = 172.7 ft ^2
So, the total added load would appear to be---
1820 lb / 172.7 ft^2 = 10.5 psf.
Does this rationale make sense to you guys? I believe the ballast requirement is surprisingly low due to the relatively flat mounting angle of the panels. They appear to be <10 degrees.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Yes that building is in a high wind area less than a mile from the coast. It did see a hurricane 2 months ago. It wasnt a cat5 hurricane (maybe cat 2) but if a cat 5 comes through then the last thing well be worried about are solar panels
But, my area does require near cat 5 design conditions for anything new and I have confirmed with the building department that 145 mph is the design they want. So, these panels (which are a fairly new installation) must have had to have met this ~145 mph requirement at least on paper!?!?!?
I really think the low panel angle is what makes this permittable. If these panels were at a 30-40 degree slope, one would suspect the uplift would be much much higher. But, I am not sure on all of this (which is why I am here on this board).
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
It's tough to tell from the photo, but I expect that the ballast blocks may also weigh more than 32lb/ea.
----
The name is a long story -- just call me Lo.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
1820 lb total load added to roof over 15 locations = ~121 lbf uplift resistance per mount?
So, if I calculated the uplift imposed on the panel at ~145 mph and it was less than 121 lbf at each mounting location, the panels would be assumed to remain in place?
Part of this seeming to work may also be from the deflector panels which are used. It appears these are used on the high end of each panel mount and dont allow much wind to get under the panels (and therefore limit uplift).
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
I verified the ~32 lb/block for the ballasts. I too thought this seemed too low, but they are only 4x8x16.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Saying that you wouldn't be worried about solar panels in a Cat 5 hurricane is not good enough, in my opinion.
I think you need a Florida structural engineer to comment on this. So far, none of us are.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Thanks for the info.....I think the firms name might be "AG". Ill drop them a line.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Im working with a local truss engineer to sturdy up my trusses a bit.
But, it looks like the average load for the ballasted solar mount arrangement is only going to come in at ~6 psf including the panels. What do you guys think of this? Ill of course be digging into this more (apparently Unirac engineers will sign off on this), but at 6 psf its so much less than I originally thought.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RM10 U-BUILDER PRODUCT ASSUMPTIONS
RM10 – Ballasted Flat Roof Systems
Building and System Occupancy/Risk Category II
Building Height ≤ 60 ft.
Roof Slope ≥ 2.4° (1/2:12) and < 5.6° (1-3/16:12)
Friction Tested Roofing Types: EDPM, PVC, TPO, and Mineral Cap
Required Setback from Module Edge to Building Edge for Wind Tunnel: 3 ft
Surrounding Building Grade: Level
Wind Design: Basic Wind Speed Range is 85-150 mph (ASCE 7-05) and 110-170 mph (ASCE 7-10). Wind Exposure: B or C (ASCE 7-05/ASCE 7-10). Occupancy/Risk Category II buildings and systems with a 25 year design life. Insertion of the project at-grade elevation can result in a reduction of wind pressure. If your project is in a special case study region or in an area where wind studies have been performed, please verify with your jurisdiction to ensure that elevation effects have not already been factored into the wind speed. If elevation effects have been included in your wind speed, please select 0 ft.
Ground Snow Load (ASCE 7-05/ASCE 7-10): 0-60 psf. Results are based on uniform snow loading and do not consider unbalanced, drifting, and sliding conditions. Roof snow load reduction calculated per Section 7.3 of ASCE 7 with the following assumptions: Exposure factor = 0.9, Thermal factor = 1.2.
RM10 Ballast Bay Weight: ~3.5 lbs
Module Gaps (E/W) = 0.25 in
Seismic: Installations must be in seismic site class A, B, C, or D as defined in ASCE 7-05/ASCE 7-10
Ballast calculations are based on ASCE 7-05/ASCE 7-10 load combinations and product specific wind tunnel testing.
Ballast Blocks: The installer is responsible for procuring the ballast blocks (Concrete Masonry Units – CMU) and verifying the required minimum weight needed for this design. CMU to comply with ASTM standard specification for concrete roof pavers designation C1491 or C90 with an integral water repellant suitable for the climate it is placed. It is recommended that the blocks are inspected periodically for any signs of degradation. If degradation of the block is observed, the block should immediately be replaced. The CMU ballast block should have nominal dimensions of 4”x8”x16”. The actual block dimensions are 3/8” less than nominal dimensions. Ballast blocks should have weight as specified for the project in the “Inspection” section of this report.
Limitations of Responsibility: It is the user’s responsibility to ensure that inputs are correct for your specific project. Unirac is not the solar, electrical, or building engineer of record and is not responsible for the solar, electrical, or building design for this project.
The system is certified to UL2703 when properly installed. See the installation guide for more detail.
SEAOC PV1-2012: Structural Seismic Requirements and Commentary for Rooftop Soar Photovoltaic Arrays
Assumptions for unattached photovoltaic arrays:
Importance Factor: Array (Ip) = 1.0, Building (Ie) = 1.0
Site Class = D
S = design seismic displacement of the array relative to the roof
Minimum Separation: Array to Array = 0.5*S, Array to Roof Edge or Obstruction with Qualified Parapet = 1.0*S, Array to Roof Edge with Unqualified Parapet = 1.5*S
SS = mapped MCER, 5 percent damped, spectral response acceleration parameter at short periods per ASCE 7-05/ASCE 7-10
SDS = design, 5 percent damped spectral acceleration parameter at short periods per ASCE 7-05/ASCE 7-10
A minimum module return flange of 0.9in (when using 1-3/4 in. clip bolts) is required for all RM10 installations
A minimum module return flange of 0.65in (when using 2 in. clip bolts) is required for all RM10 installations
Coefficient of friction used for calculations per Unirac, Inc. Roof Mount Ballast Support Coefficient of Friction Testing report dated October 4, 2013. Roof pads are required for PVC, TPO, and EDPM roofs to attain a minimum coefficient of friction of 0.4. Mineral cap roofs do not require roof pads to attain a coefficient of 0.4.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
Im not posting here too much about the fact that I am also looking at the mechanically attached approach as well. That method is rather self explanatory and the uplift/downforce info is readily available (so I haven't needed any additional help with it).
As such, Im certainly not locked into the ballasted method, but its the more interesting one to chat about
It is looking like it will be less expensive (perhaps by a good margin) to take this route but well see.......
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
As I look into this stuff more, Im finding that type of letter is available from all of the manufacturers of these types of mounts (and mechanical ones as well), state by state. I suppose the efforts they've placed onto these systems is not surprising (as the companies who manufacture this stuff are not small entities). Hilti for instance owns Unirac (the manufacturer of the system I posted the calcs on above). They are certainly no newbies when it comes to structural elements.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
1507.17.3 Wind resistance.
Building integrated photovoltaic roofing modules/shingles shall be tested in accordance with procedures and acceptance criteria in ASTM D 3161 or TAS 107. Building integrated photovoltaic roofing modules/shingles shall comply with the classification requirements of Table 1507.2.7.1 for the appropriate maximum basic wind speed. Building integrated photovoltaic roofing modules/shingle packaging shall bear a label to indicate compliance with the procedures in ASTM D 3161 or TAS 107 and the required classification from Table 1507.2.7.1.
Were the panels tested in according to these tests? If not, then as a good building official/plan reviewer I would want testing data, site specific calculations, ect. from a Florida PE that would justify the design is within standard engineering practices and other code requirements. It looks like you have that already from the manufacture and that might be enough for an approval. However, the manufacture is in the business of selling their products and the building official might request a letter, plans, calculations, ect. from an engineer (3rd party, EOR) to further validate the design of the system.
From the EOR's perspective (my opinion), these panels were tested in a wind tunnel based on dead load and friction more than likely in perfect conditions. I would want to see at what pressures these panels/ballasts failed at, the mode of failure, and how they ended up with their allowable wind speed range and other criteria. After this I would need to look at the site specific conditions, apply a hefty safety factor and see if it works or not. There are a lot of variables to think about if you dont have this testing data which is why the code requires specific testing and product approvals.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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.
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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!
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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?
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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).
RE: Determining Maximum Load for Flat Roof for Solar Panel Installation
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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RE: Determining Maximum Load for Flat Roof for Solar Panel Installation