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Solar passive overhang design on skillion/clerestory roof?

Melaniebethscott | Posted in Energy Efficiency and Durability on

My husband and I are in the design phase with an architect in Washington, PA (hour south of Pittsburgh..Stiller country). We are attempting to build a solar passive home with SIPS. Our architect has been great. He admitted early on to not having done many solar passive homes, which is expected, Pittsburgh is not necessarily a hub for efficient home design. So I’ve been doing alot of the calculations (which is fine by me, makes me feel useful).

The design is fairly basic: rectangle, with 4/12 pitch on the south, clerestory windows and a 6/12 pitch on the north. Ive attached a section for better orientation. Obviously, we can do 2′ overhang on the south of the 4/12. We’re questioning how effective a 2′ overhang/extension of the 6/12 would be for the clerestory. Does that upward pitch affect the calculations and how the windows are shaded? Or will the summer sun shine on the windows regardless of the extension?

I had the architect place the windows up higher in an attempt to shade them in winter, but I’d prefer them lower if an overhang/extension will work. I’ve been trying to use some of the online overhang design tools, but again, they don’t factor in the possibility of an upward pitch.

PS. What is the technical name for this kind of roof design??? Skillion? Shed? Leanto? Single pitch? Clerestory roof?


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  1. morganparis | | #1

    The overhang on the upward sloping roof will have about zero shading effect on the loft windows. Supplementary external shading is pretty hard to incorporate in that roof format also - about the only workable option is an exterior venetian style blind. Otherwise you'll need low SHGC glazing in those windows or you'll cook.

    I'm pretty amazed that your architect couldn't advise you on this. Fundamental solar control issues have been pretty basic stuff in the curricula of most architecture schools for generations.

  2. charlie_sullivan | | #2

    What matters on how the shading works is just where the edge of the overhang is relative to the window. So an overhang that slopes up can work, but just like one that is mounted higher up, you need it to go out further. When you calculate, use the height of the endpoint--the edge of the overhang, and its horizontal extension, not its diagonal length, or its starting height right above the window.

    Personally, I like the look of an overhang there going down, parallel to the lower roof, rather than having it go up, continuing the line of the higher roof. That gives you more shade for a given overhang distance, but a longer distance at a greater height, as you get with the upward pitch, gives you better summer/winter differentiation.

  3. Melaniebethscott | | #3

    At first he said we're not able to do the overhang, then when I pressed why, he mentioned that he didn't think we could get any shade from it. (so he probably knows, just trying to appease me). I was just doing my own research. This type of roof seems to be popular with solar passive design,I find it hard to believe there are no shading benefits to the upper windows.

    I'm not too worried about cooking. We spend alot of time outdoors in the summer. Its the winters that tick me off.

  4. Melaniebethscott | | #4

    That looks like it'd require some pretty fancy math, Charlie. So, basically, that overhang needs to be much "longer" in order to do the shading. It'd be nice to put a south slope overhang on that roof, but I'm watching the dollars and the windows are sucking them all up.

  5. Expert Member

    Do'n forget overhangs have several functions. Keeping your walls dry is always a good idea.

  6. bunney | | #6


    A wonderful source for solar calculations, including in this case roof overhangs:

    The challenge for homeowners is that we have to sort out building science (where findings often disagree) from good old-fashioned bias and, sometimes, from misinformation. Somehow through it all we have tov arrive at conclusions that work for our particularly circumstances.

    For instance above, one contributor writes "The overhang on the upward sloping roof will have about zero shading effect on the loft windows" ... and that "you'll need low SHGC glazing in those windows or you'll cook."

    Whereas another contributor writes that "What matters on how the shading works is just where the edge of the overhang is relative to the window. So an overhang that slopes up can work, but just like one that is mounted higher up, you need it to go out further."

    What's a homeowner to do with such conflicting information? The best answer that I received is "Leave no rock un-turned!" Research, research, research. Approach the topic like a post-graduate course with required readings every night and weekends.

    Only then, in my case, after about one year's study, did I know enough to at least question the builder or architect when I saw design decisions that looked off base for what we are seeking to achieve with our passive solar cabin. It still seems like an impossible task..

    I'll conclude, Melanie, by noting that the passive solar consultant that we hired for our project, which is a similar design as you have illustrated, noted that our clerestory roof overhang, based upon our site's latitude and building orientation, will provide shade during summer solstice. Moreover, we will have high SHGC in our clerestory windows, based upon the ratio of all southern windows to floor area and thermal mass for storing the sun's wonderful and free heat.

    Did we make the right decision? Time will tell.

  7. morganparis | | #7

    Apologies for my earlier, overly hasty response. I hadn't looked closely enough at your building section and had assumed a much taller window than is actually shown. The 2' high window fairly close to to the roof slope will be fairly easy to shade. In your latitude the the midday midsummer solar altitude is about 72 degrees from the horizontal. Trace that angle from the bottom of the window to the projected slope of the upper roof line and you will probably find an overhang of about 18" or so (measured on the horizontal) will be indicated. Extend that further, using say a 60 degree solar altitude, and you will get good shading for a greater part of the day and for a longer period of the year. Bear in mind that the hottest days of the year come well after June 22. Ultimately the balance between desired shade and desired solar gain is a judgment call based on your personal preferences and your usage of the room as well as the available thermal mass on the interior to soak up excessive gain.

  8. GBA Editor
    Martin Holladay | | #8

    Charlie gave the correct answer: "What matters on how the shading works is just where the edge of the overhang is relative to the window. So an overhang that slopes up can work, but just like one that is mounted higher up, you need it to go out further."

    I would also like to second Randy's suggestion to use the online tools at Sustainable Design.

    Finally, a note of caution: If your designer is not familiar with passive solar design, be cautious. Most textbooks on passive solar design repeat rules of thumb developed in the 1970s, and many of these ideas have been found wanting by subsequent research. In general, the idea that you can heat a house by installing oversized south-facing windows is flawed in your climate. In most cases, it makes more sense to invest in good air sealing details, extra insulation, and high-performance windows, rather than adding more south-facing glazing.

    South-facing glazing can lead to overheating in some seasons, and always results in heat loss at night. It's usually best to size windows according to your need for natural light and the view, not passive solar principles.

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