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Community and Q&A

Has anyone built a south-facing roof on a building that’s 45 degrees off?

Andy CD Zone 5 - NW Ohio | Posted in Plans Review on

I have the opportunity to add an attached garage to my house, and I’d like to provide a plaftform for solar PV on the roof (Zone 5, NW Ohio). The existing structure (and the entire town) is oriented 45 degrees off; that is to say, the street runs northeast/southwest. I’d be attaching the garage on the side of the house that faces southeast.

I’ve attached here a couple sketches of an odd roof arrangement on top of a 24×24 footprint. Has anyone tried something like this before? That proposed south-facing slope doesn’t extend all the way from corner to corner; there’s a ridge which results in gables on the northwest and northeast walls. That’s 650 square feet of due south-facing roof. My other goal is to NOT use trusses, allowing a funny-shaped but creative space in the attic for my wife to use as a studio (that’s her and her desk).

This is the brainstorming stage, and I’m seeking feedback from a design and structural perspective. I know it’s completely odd looking–let’s get that out of the way first. But I’m the builder and the property owner, I’m game to try unusual things to maximize my solar dollar, and FWIW I am subject to no code oversight. Just curious if and how other people have handled this situation. Or is this much-ado about nothing; would the southwest slope of a simple 5/12 gabled roof be perfectly adequate for solar PV?

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Replies

  1. User avater
    Stephen Sheehy | | #1

    Andy: Some people suggest that a southwest facing solar array will become the preferred location, because late afternoon solar power may be more advantageous to the grid, which tends to need peak output in late afternoon.
    Our roof is about 20° west of the South and production is pretty much what would have been predicted for a due South facing roof. I suspect your orientation is probably fine, but maybe a solar expert can comment.

  2. Malcolm Taylor | | #2

    Andy,

    i don't know anything about the solar implications of the orientation, i'll confine myself to comments on the structure and design.

    i don't think it's odd at all, although I don't know how it meshes with the existing house.

    The roof is a bit harder to frame for two reasons:

    - The span is such that without trusses you are probably looking at two beams, one at the peak and another unfortunately at the widest point. Depending on your roof assembly they would probably have to be dropped for continuity of venting and insulation.

    - Second, as all the exterior walls are load-bearing, they need to be framed before the roof, which makes them a lot harder to layout with a consistent slope. Nothing insurmountable, but fiddly. You might find it a head-scratcher if you aren't an experienced framer.

    To handle the runoff a meal roof would be easier than shingles.

  3. User avater
    John Semmelhack | | #3

    https://pvwatts.nrel.gov/pvwatts.php

    Ballpark: 60kWh annual difference per installed kW of capacity.

  4. User avater
    Stephen Sheehy | | #4

    John's estimate is about 5% of what you'd otherwise expect if oriented directly south. It would be cheaper to add another panel or two than changing the roof framing.

  5. Andy CD Zone 5 - NW Ohio | | #5

    Thanks for the answers thus far. I'd like to engage Malcolm's ideas on the structure, but this whole thing is a non-starter if I gain only 5% by rotating my array 45 degrees from SW to S. Obviously, the framing is FAR more complicated for a roof oriented on the bias.

  6. Trevor Lambert | | #6

    A 5% difference sounds about right, from what I remember. There are sites where you can plug in different numbers and get relative differences specific to your location. You can probably make just as big a difference by adjusting the tilt.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Andy,
    John Semmelhack had the best answer. If you are planning to install a PV system, familiarize yourself with the PVwatts tool. The online software is free. It was developed at NREL with your tax dollars. Use it. It's simple, and it provides accurate and quick answers to this type of question.

    (So we don't have to guess or speculate...)

  8. Andy CD Zone 5 - NW Ohio | | #8

    The PVWatts tool is great, thanks all for the reference! And yes, I ran my numbers and the difference in output is closer to 4%, which means that an avant-garde roof is simply not worth the cost.

  9. User avater
    Armando Cobo | | #9

    Another option is you can design a modern house with a "flat" roof... or use one of these babies, ha! http://www.smartflowersolar.com

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