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Rooftop Solar Looking Up

Posted July 31, 2016 12:00 AM by Engineering360 eNewsletter

Are modern rooftops already a lot stronger than we think? When the U.S. Department of Energy (DOE) tallied up results from its Solar America Cities program, at least one city had reported that "concerns about structural integrity" were making it difficult to obtain construction permits for rooftop solar installations. Sandia National Laboratories didn't believe it. So with two first-of-their-kind studies funded by DOE's SunShot Initiative, Sandia stressed wood rooftop structures to the point of failure, then compared that data with allowable loads identified in the International Residential Code and the National Design Standard. Engineering360 looks at what they found.


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#1

Re: Rooftop Solar Looking Up

08/01/2016 8:11 AM

I would assume (here at least) that building code would have been substantial enough for "snow loads" in many areas. Which should handle the units.

In areas where there is no chance ever of having snow, maybe the roof design including the roof angles, might not be stable enough, but what about other extreme weather, tornados and the like?

Garage roofs might be an alternative (cheaper to beef up?), or even mounting them in a corner of a large garden, on the ground.

Interesting problem.

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#4
In reply to #1

Re: Rooftop Solar Looking Up

08/01/2016 10:38 AM

Here in the great upper Midwest the need to replace them every few of years is the problem. If you have ever seen what hail can do to autoglass you can imagine what it will do to a solar panel. Three years ago a hail storm took out my all but two pieces of glass on my minivan, made the metal look like the surface of the moon, cause my condo to be resided and roofed and took out my AC condenser. That storm lasted 15 minutes. Insurance companies are not happy about that and are really not thrilled about insuring solar panels on roofs. That to me that is the biggest draw back to solar panels now days. At least in my area.

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#13
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Re: Rooftop Solar Looking Up

08/06/2016 9:43 PM

When you build trusses or have them engineered they are built to sustain approx 350lbs per foot. This is what a building inspector told me. Now if you use decking and put limited amount of nails tacking it down - then you do face a problem. I usually put 6 nails in my decking per 4'. That is 8" spread which nothing will moved or sag. Solar panels are NOT heavy. Maybe it is just me but as my wife says " I BUILD TO OVERKILL".

I have wondered if it is possible to have the panels inside with a small magnified 6" to 12" mirror exposed to the elements be used to produce enough power to do the same as the regular panels. With today's technology - I would think that anything is possible. Secondly why could you not use them on each side of your house or if you had a second floor attached to the siding where the sun would hit it in the morning as well as the evening direct. Would this not work? I am asking-not up on exact scientifics about how much sun or time is needed.

"There's one thing I know - is "You Never Know"!

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#14
In reply to #13

Re: Rooftop Solar Looking Up

08/07/2016 7:13 AM

For your first query, I would say in my opinion only, are that its totally impractical and would be very inefficient.

Not to mention the problems of getting everything watertight and keeping it that way.....

A roof is almost like boat, it needs to have as few openings as possible.....and all properly sealed.....

Furthermore, I never use nails, for the simple reason that their "hold" on the wood, relies often on some rust later on....They are for cheap and fast building, not "good" building. "Clenching" can make the hold better, but a lot of work extra.

I simply don't like them, except for the ones my ex Brother in Law sold many years ago, which were ribbed and had heat sensitive glue in the ribbing, which melted when they were hammered in.

Also they were made rust proof by a zinc (possibly) type of coating. They were shot into place with a nailing gun. After insertion, they were as good as impossible to remove, so you had to be sure that what you built did not ever need to be taken apart!!

I found the only "clean" way to remove them was to grind off the head, remove the wood and then cut off the rest of the nail flush and leave them in!!

Using anything else to force remove them, simply crushed the wood and made a mess.....it did not remove the nail!!!

I think that both the nails and the gun were made in the USA and imported into the UK, but its a long time ago, I may be wrong!!

Otherwise, whatever I build (inside or out) if it is made of wood or board, is done with Pozidrive, wood screws, plated against rust. Simply because, at a pinch, I can dismantle it if needed.

Though I admit that occasionally, for some special parts, I also glue, usually after a "test" build to fit everything together accurately.

Now that is "BUILDING TO OVERKILL".

The same as the floors I have built in our house, with supporting framework (I have a house over 100 years old, with incompatible floor levels that I made to be the same height!), twice as many supporting struts, only 30 instead of the recommended 60cm apart......all screwed together....since then, no squeaks or movement....

Nails can never do it for me personally, though I know many use them for speed of building......I almost hate them.....if you can "hate" an inanimate object!! The same as I hate the form of drywalling used by many around the world....

Nails are for me a way to possibly make a "temporary" structure, but thats just me I suppose.....

This picture is of similar nails that my B.I.L. sold in the UK many years ago. The main difference to the ones I used, were that the ones I used were proportionally longer and thinner.....and they never split wood as these in the picture might possibly do, in my opinion....

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#17
In reply to #14

Re: Rooftop Solar Looking Up

08/07/2016 1:43 PM

I can see what you are saying. When I said decking I meant putting plywood down on trusses. If I was putting it down as flooring - yes I do use screws. Decking shot by nail also have ridges around the neck area that holds unless you buy knock offs. I built several houses and you can tell well built and the ones that sag. The GC that I worked under was very adimate about quality. His houses were never more than 1/4 " out of square. He was a perfectionist and sold for about $110 a ft. $400-$500K. We also built on bluffs and slopes which required extensive prep. In fact the ONLY blueprint he had he never pulled out of his truck for measurements. He knew it very well and he could flip it around garages and bedrooms on opposite side and still knew the plans. Most of the time the house sold before we were even half way through because the people knew the quality and the name. That is how I learned and am glad to have worked with him. Made me respect quality and when you go into other houses - well, you could tell the difference. What about solar on the sides of houses that I had mentioned. Is that a possibility??

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#18
In reply to #17

Re: Rooftop Solar Looking Up

08/07/2016 1:48 PM

Arrg. CR4 is a gossip column for people to conjecture on things they don't know anything about. Entertainment?

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#19
In reply to #18

Re: Rooftop Solar Looking Up

08/07/2016 2:05 PM

You too it would seem!!!

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#2

Re: Rooftop Solar Looking Up

08/01/2016 9:05 AM

Preface this by saying I was a registered engineer, but never did buildings, but we did do steel mezzanine floors. Remember that wood is quite variable due to grain, knots, and other stuff I don't know.

Due to safety factors (factors of ignorance!), roofs may be stronger than we expect, and might be a bit stronger than that if they upped the design to hit standard (cheaper) sizes. However, adding undesigned loads does use up part of the safety factor. The roof overstrength will also depend on whether they used a prescriptive design or CAD. We designed floors to a uniform load, which is easy. Concentrated loads means worries about "where is it?" Remember also that adding rooftop items such as air conditioners, especially on older buildings, will be using up part of the safety factor built into the design.

Solar panels are fairly light (compared to other loads) and would probably be calculated as a uniform load; therefore the chances of being OK are good.

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#3

Re: Rooftop Solar Looking Up

08/01/2016 9:09 AM

Yes, if the racking attachments to the roof are well distributed, no framing members are severely compromised (cut) and some bozo does not stack 15 panels on a centrally located spot on the roof. The real loading problem is wind load when panel layout enhances lateral or uplift loads, especially at ridges and near edges of the roof.

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#5

Re: Rooftop Solar Looking Up

08/01/2016 12:36 PM

The OP points out a clear issue that most of us in the industry face. Permit processes that require engineering documentation of load capability of solar racking systems have a dismal track record of utilizing real data to back them up. Almost every roof in the world is installed by humans, many of them of the 200 lb variety. Point loads generated by installation are far in excess of documented design allowances. The typical roof is far more robust than permit issuers, supported by elemental load tables, are prepared to accept. It is a problem that is being largely over engineered, because the engineers don't know if it is safe.

Requiring 3 or 6 or 9 feet of edge relief on an array is effective, at both minimizing wind loads and improving installation safety, but marginal at any real load safety impact. And because it reduces size, it decreases return for the owner. Those edge areas are the most robust and stiffest sections of the roof plane anyway. Our company has installed over 1000 systems, more than 3/4 did not require permitted engineering, but were in fact engineered, by us. We recently had a 100 year very wet 3 foot snow load on a difficult to permit/engineer flat roof that was installed with ballasted PV, and declared marginal, but approved, with an extensive and expensive review process. We measured almost no deflection. My point is that there is indeed a problem with roof structure analysis, and that it is almost always way too conservative. It is bearable, though, and the solution is to improve analysis, not reduce compliance responsibility.

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#6
In reply to #5

Re: Rooftop Solar Looking Up

08/01/2016 2:32 PM

That smells strongly of common sense, thanks.

I really think that some sort of "emergency Blanket", that can either manually or automatically be deployed to give a certain level of safety to those expensive modules, maybe some sort of double skinned pool cover,that reduces heat loss might be good enough....

It could be deployed even every evening once the sun goes down for example....Up at sunrise!

It cannot be difficult to build a sensor that "notices" hail during the day, to automate safety during daylight hours....

Should I patent this idea Guys?

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#7
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Re: Rooftop Solar Looking Up

08/05/2016 3:39 PM

Probably should. I am sure the thought has occurred to others before, but they took no action on the idea. Flexible solar panels are on the way, I suspect, and these could roll up during bad weather, or at night, and not deploy at all when snow load is high (or something else like an obstacle is present).

The flexible solar will be thinner, and would have to have critical tensioning in two dimensions to maintain something resembling a flat surface. It presents a point for problems to arise. A hail cover such as plastic cellular foam with smooth upper and lower skin seems reasonable, but how the heck to store it. Failure to deploy would be an issue for liability concerns.

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#8
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Re: Rooftop Solar Looking Up

08/05/2016 3:55 PM

I was assuming that it could be made to roll up, exactly as those pool covers do....

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#9
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Re: Rooftop Solar Looking Up

08/05/2016 4:25 PM

Suppose you could only certify efficacy up to a given mass range of hailstone? Perhaps those less than 1 Kg?

The questions then arise as to terminal velocity of hailstones, damage threshold for impact energy of the solar elements, and the impulse reduction imposed by the multicellular foams.

Could a foam product be made to have sufficient resilience that it could be compressed by 2/3 to 1/3 original thickness when rolling it up? Or something else equivalent to that? If the product only has to deploy once in its lifetime (not likely), how about using something similar to airbag technology, so the inflation also deploys the cover over the solar panels, and does so fairly quickly?

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#10
In reply to #9

Re: Rooftop Solar Looking Up

08/06/2016 5:00 AM

If they are that big, (certainly within the realms of possibility in some areas, just!), then there would be so much damage to roofs, cars and even people dead, I am pretty sure that a normal insurance would HAVE to fix things up.

Assuming insurance has been taken out!

My guess is that something like a pool blanket, would probably offer some considerable extra safety for hail say up to a couple of inches thick......which is the worst I have personally seen here in Germany and the UK. It dented a lot of cars!!

Also, why does it have to be compressed when rolled, the pool blankets we have here remain basically the same thickness, rolled or not!

Or even individual units for each solar panel?

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#12
In reply to #10

Re: Rooftop Solar Looking Up

08/06/2016 10:06 AM

I lost an '89 Chevy Cavalier to a Central Florida hail storm, but it was interesting to see the difference in damage among the various vehicles in the parking lot. E. g., a Chevy Suburban of the same vintage as my car and parked close to it suffered comparatively minor damage from which a body shop could effect a ''make whole'' recovery without breaking the bank.

The Florida Solar Energy Center and other test venues test and certify for hail damage resistance of PV panels, those passing sporting something equivalent to tempered glass or better. But there is always the bigger chunks of ice that will do in the panels.

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#20
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Re: Rooftop Solar Looking Up

08/08/2016 9:26 AM

Here in Texas, the larger hailstones do not appear as smooth round balls of ice, but very hard almost jagged odd-shaped ellipsoids with protrusions of ice as long as one inch.

When one of these bad boys strikes home, it will leave a mark. Cows have been killed, people too, rarely. I think at least here where thundernoggins reach exceptionally high altitudes (everything is bigger in Texas, including my imagination), you will want a more cushiony blanket.

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#11
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Re: Rooftop Solar Looking Up

08/06/2016 9:49 AM

Yes, the stomping around load on the edges of 1/2 and 5/8 inch plywood or particle board (OSB) set on 2x rafters spaced at 600 mm with little aluminum ''H'' clips somewhere along the abutting panel edges challenges the waterproofing integrity of the roof that may otherwise hold up just fine under the stomping load. I give clients the option for 2x4 edge blocking, the method used in the early days of paneled roof decking before the roof industry (and realtors, bankers, etc.) got their fingers in the mix to cut costs with the ''H'' clips. Not surprisingly, the owners choose the ''H'' clips with ''But they meet code, right''?

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#15

Re: Rooftop Solar Looking Up

08/07/2016 11:32 AM

I am not sure where this is going, but we don't need new roll up solar technology to protect our roofs and solar panels from hail, and we don't face sheathing attachment issues.

Solar mounting brackets are attached through sheathing with load spreading brackets (each is about three square inches) into structural members with lags, typically. This dramatically increases sheathing strength, and in my opinion, while there is some increase in load, (about less than 4 lbs per square foot, or as much as 35 lbs. per point), the box plane structure created increases stiffness by a considerable amount, possibly improving the structure, by reducing deflection.

The state of solar panel strength is very robust, and has very high insurance research data. It simply is not an issue. Have solar panels been broken by hail? We install in five states, for over ten years, over 10,000 panels from 15 manufacturers. Never had a single panel hail damaged.

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#16
In reply to #15

Re: Rooftop Solar Looking Up

08/07/2016 11:58 AM

Perhaps you need to address the post #4 and #9 if I remember correctly.

It seems that you have been really lucky with that total lack of breakages. I do not have the units on my house, but I know people that have suffered (insured) breakages.....here it apparently happens more often!

I found a company in the USA who tests for hail breakage in this manner:-

Hail impact test. Replicates a natural hail storm by dropping a 1.1-pound, one-inch steel ball onto solar panels from a height of 13 feet. Then we repeat this up to 20 times in the same place on at least 11 different points of impacts.

On this website:-

http://www.solarworld-usa.com/why-choose-solarworld/the-solarworld-standard

It seems quite a good test to me for the majority of hail storms.....pretty tough panels.

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#21
In reply to #15

Re: Rooftop Solar Looking Up

08/08/2016 9:28 AM

What kind of plastic and/or glass are you folks using? Trade secret?

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#22
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Re: Rooftop Solar Looking Up

08/08/2016 12:19 PM

Almost all PV is covered with 2-3 mm borosilicate, (Pyrex) with a highly mat textured finish (to assure maximum radiation harvest, with minimal reflection away from the cells below). 1 inch hail will decimate many rooves, but probably will not harm your panels. There is an unlimited youtube stream of tests if you are curious.

Some panels tend to be less textured than others, particularly the thin film models.

BTW, we don't design them, or assemble them, we just point them and watch them glow.

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#23
In reply to #22

Re: Rooftop Solar Looking Up

08/08/2016 1:42 PM

cc. All seems to be in order then, better than we have a right to?

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#24
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Re: Rooftop Solar Looking Up

08/08/2016 2:47 PM

Back to the OP's point, Permit departments are not accustomed to the process of structure analysis, I don't blame them. It is CYA. But they could make it less expensive and time consuming, and recognize what most people know. In a code built house, using manufacturers instructions for PV design and installation, 5 lbs a square foot is usually acceptable. Roofers don't even get a permit to add a second layer of shingles, in my experience.

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#25
In reply to #24

Re: Rooftop Solar Looking Up

08/08/2016 4:01 PM

Why, PFR, my friend, I think you have it right, sir!

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