How to calculate wind loads for ballast rack requirements

[SFMike]

Hi Everyone,

My name is Mike and I'm in the process of putting twelve 230W REC panels on my roof in San Francisco. They would be in a 4x3 array. I have a flat roof with a reflective-coated membrane, so we decided to go with a ballast rack system. This lets us re-coat the roof when we need to and doesn't require any penetrations.

I'm at the point where I have all the panels, I have the microinverters and my layout, and I'm ready to build a rack system out of 2" galvanized angle iron. I've put together the rack for one panel just to work out the supports and bracing and it looks pretty good. Before I go any further, I have to pull the permit with the city and show my design to the city structural engineer since it's not a pre-manufactured system.

Can anyone help me with how to figure out what ballast weight is required for my system?

And does anyone have recommendations on what to put under the racking material to protect my roof? I was planning on putting down 6" wide strips of rubber roofing membrane, then 1" thick redwood, then screwing the 2" angle iron down to the redwood with 3/4" screws. The roofing membrane has adhesive on both sides, so that provides some connection to the roof. But I definitely want to put cement blocks in the racking trays too, I just don't know how many I need to put in there. Obviously, the least I could use to be safe would be preferable.

So if anyone could give me a few pointers, I'd really appreciate it!

Cheers,
Mike

[russ]

Hi Mike - Welcome to Solar Panel Talk!

I expect someone who knows that type of thing will be along shortly.

Russ

[So solar]

The best I've seen without cutting out roof and mounting to rafters. Is to simply adhere the same roofing material to the current material. Be careful and use the appropriate "glue" or caulking. Some of these adherents will eat the membrane. I believe all tar based ones eat the membrane. If you have the cash Id recommend cutting holes and racking to.the raftors with nipples or fast jacks. Have a roofed seal penetrations after. Its not that difficult of a process. Stay away from wood and galvanized material. The salty air will rust it fast. It may be difficult to get approval from the planning office with a DIY racking system. Because it hasn't been tested and the earthquake issues you will be faced with.

[So solar]

I've also seen where the membrane roofing material was wrapped around the wood. Stapled to it. Then glued down. With another extra layer of membrane glued to the roof. But again. I honestly don't know the earthquake issues you would be faced with. They could require a mounting "spring" to absorb any tremors. Commercially, it would be required.

[So solar]

I checked a couple of my sources. Without knowing, building height, exposure, wind conditions. Its hard to say and will greatly depend on the layout and how many ballast pans. They're usually in front and back. It may be best to purchase a ballast system, that's aluminum and already tested and certified. Otherwise maybe find a civil or stuctural engineer. Or attatch to raftors. The roof can still be replaced in the future. All a roofer will do is heat and glue a membrane roof jack around it and bandclamp it to the nipple.

[SFMike]

Hi Mike,
Thanks for all the advice. I originally wanted to cut into the roof and attach to the roof rafters, but the other owners were not okay with that. So our compromise was that I would install a ballast system without damaging the roof membrane.

It looks like it might be easier to get through the city permitting with a pre-manufactured rack system, so I'll have to check on that. The only problem is that all the suppliers I've talked to tell me there's a 1 to 2 month lead time on ballast rack systems since they're all custom made for the panels I have. If anyone knows of a good source for ballast racks that could deliver quickly, I'd love to hear about it!

And any other advice is much appreciated too.

Mike

[Pitter98]

Scrap the galvanized steel and go for aluminum - as soon as you cut galvanized steel you lose all your rust resistance. It's only a protective coating - it's not through the material like stainless steel, so when you cut or drill the steel you will have bare iron exposed to the elements which will rust. Also, with something as heavy as a ballast system, you may want to go with a protection board (not as in a wood board, but an actual roofing product) and a thick roofing membrane to help protect the current roof. I'm assuming this is a commercial project (with flat roof and other owners), so you may even want to talk with a commercial roofing contractor to see what they would recommend. Don't forget to make sure water can go around/under it - you don't want ponding on your roof. Hope it helps.

[SFMike]

Which ballast rack system do you recommend?

Hi Pitter,
Thanks for the advice. I'm seeing that more and more people recommend just going with aluminum or stainless steel. So my next question is which ballast rack do people recommend? The only ones I know of are the G10 Rapidrac by Unirac and the Rayport Stainless Rack by AET. Both of those seem like there is a big lead time to get them and most people don't even want to deal with me when I tell them I only want racks for 12 panels. Does anyone have any advice for me on where I can get a good ballast rack without a huge delay?

Or does anyone have a good source for either aluminum or SS angle iron so that I could build my own rack system?

Also, to clarify the situation, it's just a two unit residence, but I'm not the only owner.

Thanks again for all the help.
Cheers,
Mike

[NABCEP Russ]

Hi Pitter,
Thanks for the advice. I'm seeing that more and more people recommend just going with aluminum or stainless steel. So my next question is which ballast rack do people recommend? The only ones I know of are the G10 Rapidrac by Unirac and the Rayport Stainless Rack by AET. Both of those seem like there is a big lead time to get them and most people don't even want to deal with me when I tell them I only want racks for 12 panels. Does anyone have any advice for me on where I can get a good ballast rack without a huge delay?

Or does anyone have a good source for either aluminum or SS angle iron so that I could build my own rack system?

Also, to clarify the situation, it's just a two unit residence, but I'm not the only owner.

Thanks again for all the help.
Cheers,
Mike

RapidRac is simple and easy to do. UniRac will specify ballast requirements for you that will be acceptable to most municipalities. It can be a bit pricey though. I've always bought mine from DC Power Systems. They may or may not want to deal with a DIY guy though.

Without going with a manufactured item, you'll have a very difficult time getting it approved by the AHJ. Unless you are able to prove the design with calcs and a PE seal they'll make it very difficult for you. MOst companies like Unirac have done extensive wind tunnel testing and structural design to qualify their products for use.

[Naptown]

I believe DPW has a ballasted system also
Chack also with solar dock they are out of delaware
The REC mdule you are using is fairly common. You might also check with Wholesale solar who sells both that module and racking.
Go with the engineered system designed for the location the implied loads are greater than you would think.

[SFMike]

Thanks for all the replies and advice. I'm just trying to figure out what the real differences are between a manufactured ballast rack and one that I can build. If I use aluminum angle or SS, I'm pretty sure I can replicate the rack and ballast pans of the manufactured systems. The ballast pan is super simple. It's just a holder for the solid cinder blocks. There's no complicated engineering there. The panel support is just a piece of metal that the panel bolts to and then there is a long vertical support and a shorter vertical support so that you can get some tilt to the panel. I don't think anyone would argue that my supports built out of 12 gauge 2" angle and bolted together with 5/16" SS bolts are not going to be strong enough to hold up the panel. As far as I can tell, the engineering that the manufactured systems do is just to determine how much ballast is required to keep the system on the roof.

So if I can calculate the wind force on my panels and determine the ballast required, shouldn't that be enough to present to the building department when applying for a permit?

Has anyone gone through this process on their own or does everyone buy manufactured ballast systems? I'd love to hear from someone who has gone through the permitting process to see how much trouble it is.

I might end up buying a manufactured racking system in the end, but since I already have the material to build one, I'd really like to be sure that my rack won't work before buying another one.

Thanks again for all the help.

Mike

[Mike90250]

.....

So if I can calculate the wind force on my panels and determine the ballast required, shouldn't that be enough to present to the building department when applying for a permit?.......

The building department, will require a Certified Engineer to wet stamp the calculations / plans for the rack, before they will issue a permit. In earthquake country, I doubt ballast racks will get approval.

[Elkhornsun]

In California per the building code you can use a ballasted solar rack if the manufacturer has had it tested in a wind tunnel and certifies it. You get the piece of paper with the statement of certification by a structural engineer at no cost when you buy the racks. The problem in California is with seismic requirements that state that the system cannot be moved vertically or horizontally during an earthquake with the concern being that the equipment would land on someone outside. That requirement applies to the electrical as well which needs to be able to withstand flexing so is in some ways at odds with the structural requirements that the panel array not flex.

With a commercial roof there is usually a parapet or perimeter wall that serves to keep the equipment mounted to the roof on the roof during an earthquake. If you building has such a wall you can install an array without fastening it to the roof. One advantage of even 4 fasteners to the roof is that the weight requirement for the ballast (per the rack manufacturer's) diminishes substantially. For example with Unirac one RapidFoot for each 15 panels reduces the ballast requirement from installed from 8.70 lbs. per square foot down to 3.84 lbs. per square foot. Where this can help is in minimizing the flexing downward of the flat roof which can result in ponding of rainwater and this ponding is not allowed per the building code.

Have attachments at the windward size of the panels, i.e. to the north, allows for having a greater amount of tilt with what is in effect a hybrid ballasted mounting, and can significantly increase the output from the panels at your latitude. Most ballasted rack systems limit the tilt to a maximum of 20 degrees and some limit the tilt to 10 degrees (80 degrees from vertical).

[kellymas]

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