Ideal roof overhang length?
I think I have decided on the wall assembly that I want to use but by the sounds of it it spend time concentrating on the wall but very little time on the roof.
I have read several books on sustainable design and affordable or low tax housing. My general plan was to build a rectangular house with a gable end roof on a 4/12 pitch. I had intended to use either a 16” or 24” raised heel truss with 2’ overhangs. I intended to use through fastened metal roofing. My design goals are one and only one roof line.
After reading Martin’s suggestions for roofs I think I favor upgrading to a 12/12 pitch to help shed water and provide a less likely to leak roof. I would be willing to pay for the extra ~30% roofing area knowing that the likelihood of any leaks causing damage would be diminished.
My questions roof overhangs. I still haven’t decided if i’m going to go with 8’ ceilings and a duct wall or 9 foot ceilings and drop the ceiling 1’ over the bedrooms for HRV ducts. This will factor into window sizes and placement for passive solar tempering.
I am looking to build in Sanilac County Michigan and the average rainfall is around 30” and 49” snowfall per year.
What is the ideal overhang for this climate? I was planning on 2’ overhangs but am interested in longer overhangs if possible if it doesn’t interfere with passive solar.
What is the ideal overhang for the rake on the gable end? Since the overhang on the gable end side is at varying higher elevations should this overhang be larger than at the eaves to provide more protection from precipitation and the sun?
Raised Heel or over sized truss? I have found very little information on the web on this topic. Is there a significant difference in thermal bridging between running raised heel vs over sized trusses? When using steeper pitches like 9/12 or steeper the over sized would have plenty of space for full depth insulation on the attic floor. I think over-sized trusses would be stronger and provide a surface for vented soffit.
Thanks,
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Replies
Shane,
I am working on a new design that uses a simple double pitch roof with a 7:12 pitch. I will use a metal roof, so I don't think there is any reason to go to a steeper pitch. The roof overhang will be 36 inches at the eaves measured horizontally, and 36 inches at the gable ends. There is a certain logic in extending the gable overhang further the higher you get, as you mention, but that complicates the roof, and I don't like the look of it. Another approach might be to have a longer overhang at the gables than at the eaves. But again, I think that would look awkward. With the 36" overhang, I will use roof support brackets. I like the look of a generous overhang, and the support brackets.
I don't think there are any hard and fast rules. The pitch, and overhang are a balancing act to get the most performance with the least cost. A large overhang keeps some of the water off of the windows and siding, but not all of it. But the larger the overhang, the more water is prevented from wetting the windows and walls.
So I concluded that what I think looks best is equal horizontal overhang all around. But even that is somewhat subjective to assess in terms of appearance because the overhang is measured horizontally for the horizontal gable overhang, and measured horizontally for the pitched eave overhang. So the two features are sort of apples and oranges.
The house I am in now has about a 44" overhang all around. I only used support brackets at the four corners because the cantilever is longest there when you consider the diagonal line (in plan view) running between the corner of the house and the corner of the roof.
The roof support details offer a lot of opportunity for architectural expression, but they also require some careful design to transfer their loading between the roof overhang and the wall frame. Their spacing must be made to coordinate with the window locations to some extent. They can only run down the walls to a certain extent in order to stay above the windows.
Shane,
Why not the design the overhang together with window size and placement? Based on compass alignment and latitude, it ought to be possible to design a combination of window placement and overhang length that gives you desired shading in the summer months, while also getting sun below the overhang in the winter.
Sketchup (from Trimble, previously Google) includes a feature that lets you locate your building (using Google Earth), and then will calculate shadows for any day of the year. I've been using this software (its free to download, btw) to design the house I am planning to build in Macomb County, Michigan (I'm interested in you wall construction conclusions).
For example (your mileage may vary), I've got 2' horizontal overhangs on the south side of the house, with 48" tall windows located 18" below the overhang. With this arrangement, at noon on June 21, I get full shading of all of the windows on this wall. On Dec 21, I get no shading from the overhang.
However, I don't know if this is the best answer. August tends to be hotter, and February colder, so it might be better to try to get the overhang/window size and position balance to extend the shading to cover the solstice +/- 30 days...same thought on the winter solstice. I'm afraid that might lead to the windows being too small, too low on the wall though.
Also, I'm planning on a 10/12 pitch. This gives me a roof angle that is slightly lower than my latitude. This would bias a roof mounted PV system to provide more power in the summer at the expense of some theoretical power in the winter - when we don't see the sun anyway.
Go for lots of overhang, imo. I have 3' on the eaves of my house (built in '80), primarily to help keep water away from the foundation. On my new shop, the overhang is 3' all around. The gable truss and the first truss inboard are approx 5.5" shorter to accomodate 2x6 fly rafters. It is springy when you nail out there, but I don't see any problem with it structurally, nor did the truss man. On my house, I will likely go with the flies 16" OC instead of 24", just for giggles. And slope that ground like crazy. I have ditches lined w/ visqueen and filled w/ rocks instead of gutters, as I hate gutters. It all seems to have worked. With long overhangs you may find that the truss is high enough over the wall for your insulation, too. Are you concerned about a 4:12 pitch leaking? Do you have excessive winds? 18 degrees is a pretty good drop, imo, but I am no roofer. The more the merrier, I guess, unless you are the guy roofing it. Check w/ your HRV guy, but mine said a 2x6 was plenty ofroom for ducts. A full foot sounds generous.
Shane,
First of all, I recommend this free online calculator for determining the size of a roof overhang for a passive solar design: Overhang Design.
Second, a wide overhang is always better than a stingy overhang, as long as (1) the overhang isn't so wide that it interferes with solar gain in winter, and (2) the overhang doesn't make the house more vulnerable to damage in a hurricane or a high-wind event. This latter problem can be addressed by the use of hurricane clips and consultation with an engineer.
I'm a fan of wide gable overhangs, but they can be tricky to build. One possibility is to install "ladder rafters" at the rakes, perpendicular to the ordinary rafter orientation. These ladder rafters are twice as long as the overhang is wide, extending into the attic for structural strength.
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Shane,
Have you considered a hip roof?
If the roof will be framed with trusses, a hip roof with a single ridge isn't really any more difficult to put together than a simple gable roof and you get the advantage of having the same overhang all the way around.
The roof on the house I'm building is a "Dutch gable" - essentially a hip roof with gables that peek out at each end of the ridge - with a 6/12 pitch and 32" overhangs.
Highly recommend either checking out the program Martin linked to or a program like Sketchup so that window size and placement can be modelled holistically with overhang design.
You can always install a shed roof and corbels on windows located under a gable or on the first floor of a two-story house.
Oh, so that is what "ladder rafter" means. I have heard that, but I always call them "fly rafters". By either name, I think that is the way to go. It is so easy to do and (I think) structurally sound.