Advice for residential flat roof installation in New England

[J.P.M.]

Ampster and JPM. Are you both done pissing on each other? If not please look into another hobby or I might get pissed myself at both of you.

As I wrote, my last response to Ampster.

[austinb]

I definitely appreciate a good discussion of semantics and defining terms (being in academia), but I still would love to get advice or hear personal experience that others have had with these types of installations. Thanks again to anyone who is willing to share their thoughts on the level of risk associated with low-pitch roof installations!

[Mike90250]

I definitely appreciate a good discussion of semantics and defining terms (being in academia), but I still would love to get advice or hear personal experience that others have had with these types of installations. Thanks again to anyone who is willing to share their thoughts on the level of risk associated with low-pitch roof installations!

If you have the space for a pole mount or ground mount for the array, I would do either before resorting to a array on a low-pitch roof.
1) no leak worries
2) fewer firefighter walk way restrictions
3) easier to set the proper angle for best harvest
4) better angle & better snow shedding
5) possibly easier troubleshooting and maintainace

[Ampster]

I have experience with two flat roof installations.

One is a townhome which had a TPO membrane. In that case the roofer did the flashing using boots that fit over the standoffs. The boots were glued to the membrane using the same glue used for the other seams of the membrane roof. The standoffs were fastened to the rafters by the solar installer then flashed by the roofer. I think I paid the roofer $50 per boot for his work but that relieved the installer of any liability for leaks. In that case the panels were mounted horizintal and my only regret is that I did not ask for some slope so that the panels would more easily stay clean. I can go into more details if needed.

The other installation was also over a TPO membrane but using ballast mounted racks which provided approximately a 30 degree tilt. (Latitude 34

[austinb]

Ampster: Given your experiences with both systems, do you have a preference? Have either developed any problems?
Mike90250: Unfortunately, we have no space for a ground mount array.

[solarix]

We install on "flat" (near level) roofs all the time. I say that if you have water ponding around a solar mount - it will leak sooner or later. What we do is use a post mount with a long SPAX screw down through the middle into the roof framing that makes a compression seal using Chemlink M1 under the mount. Thats the first line of defense. Then we put a 3/4" high 4" diameter plastic ring around the mount and fill it with elastomeric roof sealer to absolutely prevent the water from ponding around the hole and also be a secondary seal. All told, not that expensive and so far great success with no roof leaks against us. You do need to work with whatever roofer holds the warranty (if any) on the roof as they will absolutely claim it violates their warranty. We've paid roofers to do the mounts just so the liability is on them, and we've paid roofers to come do whatever they want to our mounts so they will maintain their warranty.
I've done ballasted mounts and think that is a lot of weight on your roof (although this method is very common with commercial buildings). Ditto on the low tilt panels being not a good idea. Any array less than 15deg tilt is going to get dirty and stay dirty as it will not self clean in the rain.

[Ampster]

Ampster: Given your experiences with both systems, do you have a preference? Have either developed any problems?
Mike90250: Unfortunately, we have no space for a ground mount array.

The ballast mount was a 100kW on a commercial building at a Community Center. The roofing that preceded the solar was a complete tear off with some repairs. The solar bid was the lowest bid and met all the specs of that city. I was on the bid approval committee. As solarix pointed out, they are common in commercial. I wouldn't want one on the roof over my bedroom.. But a lot has to do with the economics.

How much life do you think your roof has remaining? Is there ponding now?

[austinb]

We do get some ponding. The roof is about 4 years old, so I'd guess that it has another 10 years in it. At this point, I still don't know if the installer is going to propose a ballasted system or a penetrating system.

[J.P.M.]

I definitely appreciate a good discussion of semantics and defining terms (being in academia), but I still would love to get advice or hear personal experience that others have had with these types of installations. Thanks again to anyone who is willing to share their thoughts on the level of risk associated with low-pitch roof installations!

Since you seem to be asking for thoughts, I've designed power and process equipment and systems, and modified a number of low pitched roofs/area coverings to support various types of process equipment, some of it HVAC in nature, and some other types - cryogenic vaporizors and a few small cooling towers for a couple examples - some of it weighing several tons and usually vibrating a fair amount, but all of it having external loads that included wind and seismic considerations, but no PV installations. I have done a few smaller solar thermal design reviews/mods for placement on low pitched roofs with no roof mods, and moats around supports, but no PV.

Given that and what I think I might know about roof mounted PV systems, I'm pretty sure a lot of the design criteria and requirements and driving considerations are similar. On low pitched roofs, my experience is that it's not about risk as much as about foreseeing and avoiding problems to the greatest extent possible at the design stage - where you are now. For one thing, checking/modifying roof design structural particulars for any required modifications is a required first step. For another, IMO and as previously stated, ballast systems are a poor idea. Unless the ballast is anchored (or not) by more than friction between the supports and the roof which usually means blocking or penetrative anchorage and so probably (but not always) roof penetrations, the avoidance of which was probably the intent of using a ballasted system in the first place, it'll move/creep over time due to external forces and thermal dimensional changes.

What I've seen in use and designed for placement on roofs or elevated structures which indeed may well be mostly/more horizontal than smaller mostly horizontal roofs on dwellings if they are large in area, sometimes with built in drainage with gentle sawtooth slopes, are almost exclusively solid penetrative anchorages.

Part of any roof modification, beyond checking/modifying the existing structure for the application, may also and often result in beefing up of the roof around supports and building by elevations or "humping" to add (slight) changes in roof slopes at the equipment anchorages so that equipment mountings themselves are on a slightly raised/sloping portion of the roof. Then, equipment supports would often be placed in what we'd somewhat indelicately call puss pockets - basically a containment filled with tar or epoxy in a way not unlike what Solarix describes. The anchorages would then be anchored to the roof and then the containment would be filled, thus sealing it. The roof membrane would then be applied and sealed over all the equipment supports and the immediately adjacent roof, (hopefully) making a tight(er) seal and hopefully a (more) water impervious barrier that had all it's sharp profile changes at corners/changes in direction generally above the surrounding roof and hopefully, by design, above any inevitable standing water/ponding. Thus, the roof sealing system would be unbroken and with equipment support sites relatively easily at least viewable and hopefully accessible.

I don't expect any residential PV system would be quite so involved. Probably some system such as Solarix describes would be as or better fit for purpose for residential or smaller commercial applications.

Overall, and in summary, to help ensure few/no leaks for a long time, good design on low pitched roofs with respect to avoiding water problems often starts with the idea that water won't usually flow uphill, so keeping seams or possible leak paths or breaks or sharp changes in profile in a roof membrane at a level above any possible high water levels is one of the first lines of defense, and then probably providing rain protection/cover for those seam/break areas. Another useful attribute, because no support or seam on a roof or membrane will be leak tight forever, is to keep all supports within sight and accessible for inspection/service. Binoculars often come in handy for inspection such times.

One other point I thought about relative to the Ecofoot system you're looking at: It still has roof penetrations for the wind skirt attachment. As the product blurbs state and as I thought, the wind skirt seems necessary for wind loading considerations. So, while fewer in number, it seems that system will still have roof penetrations.

Take what you want of the above. Scrap the rest.

[austinb]

Well, I don't think we want to get into making modifications to the roof or roof supports... I must admit that this is all giving me some second thoughts. I just got the final design plans and they are indeed planning on the ballasted system. Though, either approach seems like it could be trouble.

I love the idea of having a solar system. I teach and research on coastal climate change risk and adaptation, so also feel that "walking the walk" is important. Plus, our state has a grant program that will get the system installed for an ROI of about 10% with net metering, so I'm happy enough with that. But, spending the next 5-10 years worrying about roof leaks and sagging does not sound fun, nor does an actual roof leak or sag.... it seems like nobody has a ton of actual experience with residential installations on low-pitch roofs, so it does seem like a lot of speculation from the solar companies who all say "no problem!" and the others who I've encountered who say "don't do it!"

I've got a call into the roofing company that installed the roof 4 years ago and may see if I can get them involved in helping me assess this. The info here is super helpful and highly appreciated!

[bcroe]

As mentioned a flat array covering the roof will collect as much energy as tilted up panels, I would
expect with a lot less weight per square foot. That because (as I see it) the energy hitting the roof
is about the same as very efficiently placed tilted up panels. The first problem is it takes a lot more
panels. Snow will be near impossible to clear, but that is not so different from other arrays on roofs.
good luck, Bruce Roe

[J.P.M.]

Well, I don't think we want to get into making modifications to the roof or roof supports... I must admit that this is all giving me some second thoughts. I just got the final design plans and they are indeed planning on the ballasted system. Though, either approach seems like it could be trouble.

I love the idea of having a solar system. I teach and research on coastal climate change risk and adaptation, so also feel that "walking the walk" is important. Plus, our state has a grant program that will get the system installed for an ROI of about 10% with net metering, so I'm happy enough with that. But, spending the next 5-10 years worrying about roof leaks and sagging does not sound fun, nor does an actual roof leak or sag.... it seems like nobody has a ton of actual experience with residential installations on low-pitch roofs, so it does seem like a lot of speculation from the solar companies who all say "no problem!" and the others who I've encountered who say "don't do it!"

I've got a call into the roofing company that installed the roof 4 years ago and may see if I can get them involved in helping me assess this. The info here is super helpful and highly appreciated!

Some reasons why you might not see a lot of comment from low pitch roof installed PV owners:
1.) There are fewer of them than PV owners who installed on greater sloped roofs.
2.) Because PV installations on low slope roofs usually involve rows of sloped panels, the cost/installed STC W usually goes up.
3.) However, the lower STC W area density brought on by the panel row spacing required for installation on a flat roof, and as previously discussed, limiting array annual output to something like that of a low sloped array, covering the entire roof, make things less practical and less able to meet the required duty with respect to output.

NOMB, but walking the walk can start by using less energy. An isolated reference point: I live in a 3,200 ft.^2 home and use 6,500 - 7,500 kWh/yr. and don't think I live like a hermit. If you live in an environment similar to what you teach and research about, you ought to find it relatively easy to get lower usage than that number. If you do, and because smaller usage usually, but again not always will cost less/delivered kWh, you'll probably find it harder (although not impossible) to get a residential PV system that's cost effective, especially with a ballasted system on a low sloped roof. Not buying/using something is just about always cheaper than getting more of it. It's also easier on the planet. Just sayin'.

Reality: Solar companies make money by putting their equipment on your roof, not necessarily saving you money in terms of long term cost effectiveness, or caring about any long term consequences, physical or financial, their equipment may have on your property. Caveat Emptor.

As for the roofing company, since they have nothing to gain beyond some potential problems, my guess is they may be of the "don't do it" camp. I'd also think if the roof warranty period is > 4 years, there is a more than zero probability they'll tell you that putting anything on their product/installation will void the rest of any warranty on that product. I'd read the contract for the roof work to be sure.

One last thing: After spending some time looking at the product lit. for that ballasted system you're considering, I bet, since it's all linked together but still not really anchored to the roof, that the collector frames are what will give most of the dimensional rigidity to the array/system rather than the racking providing support to the panel frames. That'll stress the panel frames and cause possible problems that are less likely than racking of more standard design. Also, and probably because of thermal expansion/shrinkage, I'd think that without some positive anchorage, the array will tend to creep over time, probably in the direction of the roof gradient.

[J.P.M.]

As mentioned a flat array covering the roof will collect as much energy as tilted up panels, I would
expect with a lot less weight per square foot. That because (as I see it) the energy hitting the roof
is about the same as very efficiently placed tilted up panels. The first problem is it takes a lot more
panels. Snow will be near impossible to clear, but that is not so different from other arrays on roofs.
good luck, Bruce Roe

Bruce: it's not the array weight that causes structural concerns and required checking - that weight is close to trivial. It's the external wind loading, compressive or uplift and perhaps some tangential loadings that need looking at. Fewer tilted panels will weigh less than an array with ~ the same output that a sawtooth array would have at, say, a 45 deg. tilt, but as your own photos suggest, the extra racking involved will probably add about as much weight as what's saved by using fewer panels.
l
I do most heartily agree that clearing snow from a tightly packed array oriented parallel to a low sloped roof can be an exercise in futility.

[bcroe]

Bruce: it's not the array weight that causes structural concerns and required checking - that weight is close to trivial. It's the external wind loading, compressive or uplift and perhaps some tangential loadings that need looking at. Fewer tilted panels will weigh less than an array with ~ the same output that a sawtooth array would have at, say, a 45 deg. tilt, but as your own photos suggest, the extra racking involved will probably add about as much weight as what's saved by using fewer panels.
l
I do most heartily agree that clearing snow from a tightly packed array oriented parallel to a low sloped roof can be an exercise in futility.

I was hoping, flat mounted panels would have about the same uplift as the bare roof, and spread the
weight of less hardware but more panels, more evenly over the entire area. Is that futile? Bruce Roe

[J.P.M.]

I was hoping, flat mounted panels would have about the same uplift as the bare roof, and spread the
weight of less hardware but more panels, more evenly over the entire area. Is that futile? Bruce Roe

Bruce:

Most of the uplift on a roof mounted array comes from air movement behind the panels. On a low sloped roof in applications of arrays parallel to the roof, and with small roof to array clearance, there will probably not be a great deal of uplift until buffetting and gust factors are considered, and even then the particulars of the application will come into play as will the judgement of the designer. To my knowledge, ASCE-7-16 does not address that particular case specifically. Still, my experience is that the uplift on a roof due to wind forces on an array parallel to that roof need to be checked if for no other reason than CYA.

For uplift: There can be some uplift on an unoccupied roof (for this conversation that means no PV arrays) of any orientation in the form of Bernoulli forces - the same forces that, among other things, are responsible for chimney draw - but for an array not parallel to a roof those forces are usually something like an order of magnitude or two less than the possible wind induced uplift forces on an array that get transferred to racking and so to a roof, making the analysis slightly different - a bunch of point loads vs. a distributed load.

The compressive forces on roofs from wind forces on arrays not parallel to the roof can be equally substantial.

The magnitude of downward (compressive) wind induced forces on a roof from an array parallel to a roof are usually about the same as a roof would see if the array were not there. However, the distribution of the wind loads to the roof from the racking supports will be via a set of point loads at roof attachment points and so may need somewhat different analysis/treatment than the same magnitude of approx. equally distributed forces.

To your specific question, I don't think it's futile. I don't think the uplift or compressive loads on a roof of any practical orientation from wind forces on an array parallel to that roof will be substantially more than on the same roof with no array on it - but the uplift will be a bit different on the array due to stagnation pressures as wind is slowed down when it gets under the array similar to something called velocity head losses. The operative word there is "substantial". The forces and moments will be different and need to be checked.

For the case of an array that's not parallel to a roof of any orientation, the forces and moments that are wind induced on the array and then transferred to the roof can and will most likely be substantially different and larger than wind wind loads on the same roof with no array on it.