Solar Panels on flat roof
Solar Panels on flat roof
(OP)
Our client owns a industrial/commercial building. Masonry walls with steel framed roof (OWSJ and beams, alternate demising walls are bearing walls). Building is divided into 2500 sq. ft units , approx 25' x 100', OWSJ @ 6.5 ft cc and span 25'. One tenant is planning to install a number of solar panels with concrete pad base (floating on roof). Owner obviously has concerns.The tenants engineer states the roof is fine as the solar panels only add about 4 psf (each unit is about 330 lb and footprint is 27 sq ft.) However, and this is where I am at odds, they say that the units 'occuppy' 81 sq. ft, and this is how they arrive at 4 psf additional. I am of the opinion that there is a potential for some joists to be overloaded (concentrated load) and don't agree with 81 sq. ft. Ideas? What about snow piling and wind? Something not right!?






RE: Solar Panels on flat roof
Something is not right here - how thick is the pad?
Does it cover the full extent of the solar panel array?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Solar Panels on flat roof
RE: Solar Panels on flat roof
Since the solar arrays will be tilted at some angle, snow drift could be an issue, as well as wind. Assuming your project is in Ontario or northern US, wind could be less of an issue than snow.
RE: Solar Panels on flat roof
2. "floating"...i assume you mean ballasted and resting on the roof as opposed to connected to the roof with roof penetrating connections. As I understand it, this ballasted system is the solar panel industries answer to your wind problem. They in effect, by ballasting, assume the responsibility for the wind problem. The only real load wind load imposed upon your structure by the ballasted system is not uplift, but impact, as their undercalculated ballasted system slides across the roof and slams into your parapet.
3. 4 psf? agreeing with "square". It may be 4psf, but best ask the question 3 different ways (does this include the panel?, does this include the ballast, does this include everything?) to make sure they have given you all the information.
4. Drifting snow is an issue. The sun sloped panels will prevent snow from blowing. This may be your most significant load. I don't think the solar industry has any "snow piling studies/diagrams for panel". You will have to go to the basics and calculate these loads. They will add to the load on your roof.
5. The responsibility for roof reinforcement is not well established in the solar industry. The industry itself is young and barely understands its own issues. Whether the Owner can roll the roof reinforcement cost into the total solar package cost or not, I don't know. I know I would try. But our engineering responsibility is clear.
Best of luck!
RE: Solar Panels on flat roof
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Solar Panels on flat roof
Having worked an investigation on OWBJ collapse, where ponding contributed, I second the caution about that.
You mention that some framing is steel (wide flange?), can the load be distributed to demising walls and steel members using a rooftop frame?
And for snow loading, a quick look at ASCE 7 will clear that up. If the panels and supports create a place for drifts to form, you must consider that if snow load controls. Now you have extra dead load deflection, snow, maybe rain-on-snow adding live load deflection. As the snow melts, it can pond significantly, even if there is a typical 1/4" per foot pitch.
The joists where undoubtedly designed on the margin, but a visit to the SJI book, looking at joist tags or plans will define max DL/LL. Get a handle on the actual, as-built DL, rather than design DL, for your evaluation.
If the roof is ballasted, you can remove ballast where the concrete footings sit, to reduce loads, but don't just dig a hole in the gravel and leave it piled nearby.
RE: Solar Panels on flat roof