More Musings of an Energy Nerd
Although roofs are simple in principle, designing a good roof can be tricky. At a minimum, a good roof should prevent the entry of rain and snow into your house. Ideally, it should direct rain and melting snow to a location far from your walls and foundation, to minimize the chance of water entering your home. A properly designed roof should be pleasing to look at. A good roof may also provide shade to certain windows that might otherwise contribute to overheating.
Moreover, a good roof will be long-lasting and solidly built. If the roof is likely to accumulate snow and ice, it must be able to bear the weight of the expected frozen precipitation. The roof structure and the roofing should be designed to withstand windstorms; in some regions of the country, that means that a good roof will be able to resist hurricane-force winds.
Problematic roofs are fairly common
A badly designed roof can fail in many ways. The most dramatic way, catastrophic structural failure, is fortunately rare. The two most common causes of catastrophic structural roof failure are heavy snow and high winds.
In New England, heavy snow leads to a few catastrophic roof failures almost every winter. These usually occur after one or more snowstorms—especially when the snow is both deep and dense, or when a heavy snowfall is followed by rain. Code-compliant roofs shouldn’t fail under the weight of snow, but a few do—for example, in cases of unnoticed construction errors or rot.
Strong windstorms regularly tear roofs off buildings. After a neighborhood…
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5 Comments
Martin,
If I could add a plea to this: By far the most common reoccurring question on GBA is how to insulate a roof once it has already been framed. Often at that point the options are very limited, and many yield unsatisfactory results. Please think this through at the design stage, where any necessary changes to the structure or details can be made.
A cautionary photo of my own roof, which follows very few of Martin’s suggestions.
Malcolm,
I couldn't agree with you more. In fact, I wrote an article ("Plan Ahead for Insulation") on the topic . Here is the link: https://www.greenbuildingadvisor.com/article/plan-ahead-for-insulation
I'm not willing to post a photo of my own roof, even if I could. (I don't know anyone who owns a drone.) My roof includes many of the flaws I wrote about. I guess that's how I determined that my design choices were flawed.
Martin,
I should have known you had covered it at some point.
Seasons greeting. What I learned about building science from your writings dwarfs what I've taken from anywhere else.
Thank you Martin! I fixed my roof plan by moving the chimney to avoid a cricket. I left the sky light over the central stairway they provide natural light.
Just a note on high wind zones. Hipped roofs of between 4:12 and 8:12 with minimal overhangs experience the lowest wind pressures. So that's a consideration for people in those zones. But of course any properly engineered and constructed roof shape should be up to the task.
In seismic zones the raised heel truss connection to the wall below needs careful consideration, so that horizontal shear transfer requirements are met. The higher the heel the more difficult it is to do economically.
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