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Community and Q&A

Designing a Roof for Solar Panels

Mauro_Zammarano | Posted in General Questions on

OK so I finally have a floor plan for my new house in Bethesda MD. Architect came up with a complex roof (praire roof style) which I believe is one of the worst possible for solar panels. I mentioned to the architect that I need a simple roof to optimize solar panel installation facing South, South-West but I would appreciate a professional advice. Who should I contact to have a proper roof designed for solar panels? Would installers provide such a service or energy raters?

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  1. Expert Member
    NICK KEENAN | | #1

    If your roof is one plane or two planes you don't have to worry about part of the roof shading another part. There are online calculators where you put in the orientation and elevation and it gives you effective insolation, but I can tell you that orientation matters, elevation angle not so much. Highest insolation is pointing due south and tilted at your latitude from vertical, which is 42 degrees.

    With a complex roof part of the roof can shade another part. Most architectural CAD programs have the ability to simulate the position of the sun at any given time. The architect should be able to simulate the sun at 9, 12 and 6 on the equinoxes and solstices to do a quick check for shading.

    I don't know if it's also true in Montgomery County, but in DC Pepco makes it hard to do a system over 10K Watts. With the new 400 Watt panels that's 25 panels. So you want to have roof area for 25 panels.

    Installation is going to be more expensive if it has to be broken up over several roof sections.

  2. user-6623302 | | #2

    Have you given any though to ground mounted solar panels? Maybe on a shed or carport? Just not your roof.

  3. Mauro_Zammarano | | #3

    There are trees in the neighbor's lot that might impact the solar panels so I really need to have an idea of their potential impact. Solar installers confirmed the potential tree problem but asked me a 3D design to simulate shadows. What I was looking for are guidelines that can help the architect in the design stage of the roof. The lot is not that large to allow ground mounted panels. An option might be to have some of them on top of the covered patio, which is facing SW.

  4. jadziedzic | | #4

    Make the roof as simple as possible, especially the section(s) that face south; a simple gable-end roof is best. Avoid dormers and other "origami" on those section(s). Minimize the number of penetrations through the roof, such as for plumbing vents; have them located near the ends of the roof section(s) to be used for solar panels. Be sure the roof can accommodate the extra load of the racking (mounting) system for the panels and the panels themselves.

    These may be of use:

    There are some requirements for fire fighter access called out by the National Fire Protection Agency (such as clear paths to the peak), but recent changes have minimized their impact.

  5. GBA Editor
    Martin Holladay | | #5

    Joseph is right. What you want, ideally, is a south-facing shed roof or a simple gable roof (without dormers or skylights) with the ridge running in an east-west direction. You don't want any plumbing vents or chimneys on the south-facing slope of your roof.

  6. Expert Member
    Akos | | #6

    As more places move away from netmetering, a south facing roof becomes less ideal. A good option is to go with a west or south west facing array, this way your max production will closer match the peak load from the house. On a yearly basis this produces a bit less energy but not a whole lot.

    You can try out different options using this great tool:

  7. Mauro_Zammarano | | #7

    My neighbor has high growing trees about 15 feet from my building. What happens if in 2 years my roof gets completely into their shadows? Hopefully we will get laws like in CA that protect from these situations

  8. STEPHEN SHEEHY | | #8

    Along with making solar more difficult, complex roofs add significantly to the cost. Often they are complex because the house has multiple corners, bumpouts, etc. That means complicated foundation, framing, exterior finishes, interior finishes. Roofs with hips and valleys usually can't be easily vented. Often harder to air seal and insulate.

  9. Patrick_OSullivan | | #9

    > Who should I contact to have a proper roof designed for solar panels?

    An architect! Sorry to be glib, but if your design professional isn't satisfying what is presumably one of your most important design criteria, they're failing.

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