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The perfect roof?

bluesolar | Posted in Energy Efficiency and Durability on

Hi all,

Several companies are working on solar roofs that imitate traditional roof esthetics, e.g. Tesla’s solar shingles, RGS Powerhouse, CertainTeed’s Apollo II, etc. It occurred to me that we could flip it. Instead of imitating shingles, why not make the entire roof out of standard solar panels?

I mean a combination of real PV panels and fake ones. We’d typically need some fake ones because 1) it would cost a fortune to cover an entire roof in real panels (and to procure the mega inverter you’d need for the system), and 2) you only need about 720 sq ft to have a 10 kW system, so there’s not much motivation to spend more.

A key motivation here is cost savings compared to an imitation roof. The imitation roofs, at least Tesla’s, cost massively more than normal solar panels.

Another motivation is that this would look much better than a typical patchy solar install. If you took one of the more attractive PV panels available, I think it would look great. (SunPower and First Solar panels strike me as fairly attractive.) Call it solar chic. What do you think?

Part 2

Solar panels are usually built like tanks. They’re surprisingly tough and robust, stand up to hail and so forth. Why not make them your roofing material instead of propping them up over a traditional roof?

The imitation roofs do this – the solar shingles replace traditional shingles. They don’t fit them on top of a traditional roof – they are the roof.

So I think the same could be done with traditional PV panels, and it would greatly help with the esthetic. The way they’re installed now typically leaves a roof with an ugly, patchy, wounded look. Do you think PV panels are tough enough? Am I overestimating their toughness?

Eager for your thoughts…

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Replies

  1. user-723121 | | #1

    In the land of the ice and snow (Minnesota), snow often lays on roofs, even the southern exposure for long periods of time. Have to wonder about the practicality of solar shingles here. I have been noticing large solar panel installations along Interstate 94 being covered by snow for days. Seems an internal heat strip added to these solar panels would help keep the panels clear in the snowy months.

  2. joenorm | | #2

    While this idea may inevitably be the direction solar panels go, I think its still pretty far off, for good reason.

    I agree that certain installs look "patchy" and bad. A chopped up array is installed on a house with a chopped up roof structure. Houses with simple rooflines can have very clean looking arrays that, in my opinion, look very cool.

    It's worth considering that panels are more efficient the cooler in temperature they are, so having the inherent air gap underneath them in a traditional install is beneficial.

    There are also NEC requirements that make not having electronics up there with the panels almost impossible. Rapid Shutdown is what I am thinking of, if you're not familiar with the concept a google search will explain it.

    Another issue is warranty claims. What happens when a cell goes bad(not common, but not impossible). Do you have to take apart your roof now to fix the problem?

    There are so many benefits to keeping them separate from the roof itself. As an installer I just wish architects would design roofs with solar panels in mind, so the south facing roof can accommodate a nice, attractive rectangular bank of panels

  3. Andrew_C | | #3

    If it's a roof, you have to be able to walk on it. Even if solar panels are quite durable, that's a big ask. I also seem to remember that for firefighting purposes, you have to leave about 3' around the edge of a roof that's safe for walking.

    I agree with Norm that bad-looking arrays are the result of bad (complex) roof design, and there sure is a lot of that going around.

    The engineer in me thinks that keeping the solar array separate from the roof is a good idea. Even if I had a simple south-facing roof, I'd prefer a ground mounted array if possible. I also wonder with the rapidly changing solar power net-metering arrangements if it isn't cheaper and more robust to move toward utility scale solar and wind, rather than putting sub-optimal arrays on individual houses.

    1. bluesolar | | #5

      Interesting point about the firefighting requirements. I didn't know about that. As far as walking on the panels, I wonder if it might be possible. I've heard amazing stories about how tough solar panels are, so I wonder what their PSI or lbs. per sq ft might be. The Tesla tiles are supposed to be virtually indestructible, so I assumed conventional panels can be made as tough as those.

  4. Trevor_Lambert | | #4

    This has actually been done before. We actually gave some thought to it during our design phase. The biggest issue in my mind is making sure the roof is waterproof. You'd be relying on sealant between the panels. A traditional roof doesn't depend on sealant, because it almost inevitably fails at some point. On top of that, the logistics would be challenging. Making sure the sheathing was completely dry, getting all the panels installed and sealed up very quickly before it rains, etc. These things are not small or light.

    1. bluesolar | | #6

      I wondered about the decking. I could see going with a conventional plywood deck combined with normal underlayments or felt. In that case the solar panels would simply replace the top roofing material – the tiles or shingles.

      It would be ideal to have a high strength fiberglass roof deck, a single monolithic structure that would never rot, warp, or leak. Strong and thick enough to jump up and down on. The screws attaching the panels to the fiberglass could be covered with plastic or metal caps, maybe two layers of caps sealed with epoxy or something. That kind of roof might not even need joists to support it – high strength fiberglass could be self-supporting, with beams only along the edges and maybe the roof peak or apex. That would be amazing, but fiberglass is surprisingly expensive.

      1. Expert Member
        BILL WICHERS | | #7

        There is a company called fibergrate that makes materials like this, they primarily sell into marine, industrial chemical, and food service industries. I’ve never heard of their stuff going into roofs. They make grid-like walkways, and fiberglass structural extrusions (I beams, channel, etc). The stuff is NOT cheap, but it is readily available.

        What about putting a membrane roof down, with a few pipe-style penetrations and then building a steel frame over the membrane roof to support the panels? The pipe-style supports would limit the number of roof penetrations, and are relatively easy to seal, the membrane would be the actual roof for weather protection purposes, and the steel frame would give flexibility in terms of getting the required structural configuration to support all the solar panels. You could probably build most of the frame with strut and use just a few welded pieces of structural steel for the main runners.

        Bill

    2. Expert Member
      Akos | | #8

      I recently saw a commercial building roofed with solar panels. The roof was pitched.

      What they did is put a membrane roof with perforated horizontal metal Z grits. The membrane looked to lap over the Z grits so all the fasteners were covered. I didn't see how the panels were ultimately attached to the Z grits, but I can't see why it could't just be bolted to it directly.

      Seemed to be a pretty simple and clean install. The panels protect the membrane but still have a full properly lapped membrane forming the drainage surface.

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