Solar panels on flat roof — ballasted or penetrated?
| Posted in Green Building Techniques on
I have a proposal in front of me for a solar installation on a nearly flat roof — 1/4:12 slope. Roof membrane is TPO. The proposal says it can be either ballasted or mounted on standoffs that penetrate the roof.
I can think of pros and cons of each method but I’m curious in the community’s thoughts.
Thanks.
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I have worked mostly on commercial flat roof solar installs. All were ballasted.
I have worked on many commerical properties that specifically disallow ballasted mounts for antennas, and anything else. There are two reasons for this:
1- Ballasted mounts can blow off the roof under severe weather conditions, which can cause all kinds of damage (and liability issues)
2- Many installers don't put enough ballast on the ballasted mounts, which makes #1 MUCH worse. I've seen a lot of satellite dish mounts with only four concrete blocks, one in each corner, for example. Typically those mounts need at least double, and sometimes four times that many blocks to meet their specs.
Unless you're doing a temporary installation, I would strongly recommend real hard-point style mounting. Depending on the size of your installation, you might be able to use a relatively small number of roof penetrations (all of which are potential weak spots in your roof that could leak), then build a frame between them to support the panels. I've seen this done with unistrut many times, some solar panel installers have custom frames available, and for big systems, it's not terribly expensive to have a structural steel fabricator fabricate something for you.
I suppose the short answer is I prefer permanent mountings over ballasted mountings because it's safer under all conditions. That doesn't mean a ballasted mount can't be made pretty safe if installed correctly (most that I've seen aren't installed correctly), it's just that I personally prefer bolts to gravity for keeping stuff in place in a storm.
Bill
Thanks Bill, that's basically along the lines I was thinking: bolts hold better than gravity, but holes in roofs can leak.
I'm leaning toward the through-bolts and having the roofer put them in when the roof is installed. I think it would be just four and then a rack on top of that.
If the attachement points are installed while the roof goes on, that's much better than a retrofit later. If this is a commerical-style roof, I like to use the rubber boots that can be cut for different size pipes. Use a round steel tube for the support penetration, have the roofers flash in the metal base for the boot, then install the boot around the support tube and clamp it in place in the usual manner. These rubber boots can flex and give, which makes them less like to leak due to a cracked caulk seal (or similar sealing means used on a rigid attachment). I've had very good luck with those boots for sealing all kinds of roof penetrations as long as they are round things :-)
Make sure the interior side of that support tube is securely anchored to the framing, and remember that it needs to handle uplift forces too. If, for example, the support tube was attached to the side of a rafter or truss (if it's a truss, it's best to tie it to BOTH the top AND the bottom chords), it's a good idea to secure that rafter or truss with steel straps or hurricane ties to anchor it to the wall for uplift forces. Wind can do a lot of damage, and one downside of rooftop solar is that it's like putting a sail on your roof. The more distance between the solar panels themselves and the roof surface, the more portential wind issues you can have too.
Note that my preference for a support tube would be schedule 40 steel pipe with a welded plate, ideally with a flange, that bolts into the rafter or truss. The flange would go along the underside of the rafter or truss chord as a catch so that the bolts are entirely in shear for uplift forces. The lower attachment (in the case of a truss) would have the flange opposite pushing down on top of the lower chord. This would be about as a secure as you could get without designing an entire dedicated support system. Note that more supports means less force exerted on each support too.
This is something that makes sense to overbuild a bit to be extra safe. I've seen commerical stuff (usually TV satellite dishes) that have blown off of roofs 3+ stories up and crashed into cars in the parking lots below. Luckily no one has ever been injured in any of these incidents that I've seen, but the potential for serious injury is very real. Rooftop stuff needs to be secured properly for safety.
BTW, now that I think about it.... You're in DC as a recall. If rooftop decks are a thing in DC the way they are in the Federal Hill area of Baltimore, you might check with some REPUTABLE installers of those decks (I've seen some pretty scary roof decks) -- they may have some good ideas as to how to secure stuff to roofs of structures out there.
Bill
https://www.greenbuildingadvisor.com/article/designing-for-solar-panels
https://cobodesigner.com/solar-pv-systems/
we use HDPE water filled module for flat roof
Marco / Sales Director
Suzhou Foner Solar Power Co. Ltd.
Tel: +86 512 63617333
Fax:+86 512 63628355
Wechat/Whatsapp: +86 15829465152
Add:No.558,Fenhu Avenue,FOHO High-tech Industrial Development Zone Suzhou, China 215211
Mail: [email protected], [email protected]
http://www.fonersolar.com
we use HDPE water filled module for flat roof
Marco / Sales Director
Suzhou Foner Solar Power Co. Ltd.
Tel: +86 512 63617333
Fax:+86 512 63628355
Wechat/Whatsapp: +86 15829465152
Add:No.558,Fenhu Avenue,FOHO High-tech Industrial Development Zone Suzhou, China 215211
Mail: [email protected], [email protected]
http://www.fonersolar.com