Every House Needs Roof Overhangs
Adequate roof overhangs help shade windows in hot weather and reduce the amount of rain that hits your siding, windows, and doors
Many residential designers pay too little attention to roof overhangs. Roof overhangs have several important functions: they can protect exterior doors, windows, and siding from rain; they can shade windows when solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. is undesirable; and they can help keep basements and crawl spaces dry. A house with improper overhangs can overheat in the summer, can suffer from water entry problems at windows and doors, and can have premature siding rot.
The most common design error is to make roof overhangs too stingy. It’s also possible (although much rarer) for roof overhangs to be too wide.
A typical gable roof has two kinds of roof overhangs: eave overhangs and rake overhangs. Because it’s easier to frame a wide eave overhang than a wide rake overhang, problems from stingy overhangs are more common at rakes than eaves.
Keeping water off of walls
Perhaps the most important function of wide roof overhangs is to help keep water off siding, windows, and doors.
[Photo credit: Tom Arstingstall, Tromler Construction]
While it’s impossible to stop all wind-driven rain from reaching your walls, wide roof overhangs make a big difference — especially if there is just one story under the overhang.
Of course, an overhang that is trying to protect two or three stories is much less effective at keeping the wall dry. (Fortunately, there are solutions to this problem — notably the inclusion of a “brow roof” above the first floor. I will discuss brow roofs in more detail later in this article.)
[Photo credit: Lynnette Hartwig, gettingbestprice.com]
Walls with stingy roof overhangs get regularly soaked. These repeated wetting episodes cause a variety of problems. Although these problems are worse in high-rainfall climates than low-rainfall climates, almost all North American homes are built in regions where it makes sense to protect walls from the full force of wind-driven rain.
[Photo credit: Olek Lejbzon & Co. / oleklejbzon.com]
Protecting siding. A house without roof overhangs leaves siding unprotected and vulnerable, like an orphaned lamb released near a pack of wolves. Unprotected walls suffer high rates of water entry, premature failure of any paint or stain, and premature siding failure.
[Photo credit: Martin Holladay]
Protecting windows and doors. Windows and doors can be protected either by roof overhangs, by recessing windows and doors in thick walls, or by including head casing and head flashing that are designed to be significantly proud of the siding plane. If you look at older buildings, you’ll often notice that the casing on window heads and door heads is substantial, and is often capped by a protruding ledger. These features help deflect rain.
[Photo credit: Five Star]
There are two reasons that all exterior doors need to be protected by a roof: to prevent jamb rot, and to keep visitors dry until the homeowners answer the doorbell. Although these principles are obvious, a significant percentage of exterior doors are inexplicably unroofed.
[Photo credit: John F IV / Flickr]
Getting rain to drip away from the foundation
Another function of wide eave overhangs is to ensure that roof water doesn’t drip near the foundation. Keeping the eaves-drip away from the house helps keep your crawl space or basement dry. (For more information on this topic, see Fixing a Wet Basement.)
[Photo credit: Martin Holladay]
Keeping the eaves-drip away from the house also limits the damage caused by splashback. Splashback is a common cause of siding rot.
Shading your windows
Roof overhangs can help shade your windows. In cold weather, any shade on your windows is probably unfortunate; in hot weather, shade is almost always welcome. Since shade is sometimes good and sometimes bad, window shading strategies are usually a balancing act.
Climate matters. If you live in Fairbanks, Alaska, you will probably welcome solar heat gain through your windows on almost any day of the year. However, if you live in Phoenix, Arizona, you may prefer all of your windows to be shaded in every season.
North windows. In the northern hemisphere, north windows don’t get much sun — so roof overhangs don’t really affect north windows.
[Image credit: © 2014 Home Power Inc. - http://www.homepower.com. Reprinted with permission. ]
South windows. Roof overhang length matters more on the south side of a house than it does on any other orientation. If you intend to follow traditional passive solar design principles — and it usually makes sense to do so — you will probably size the roof overhang so that south-facing windows are fully shaded at noon on June 21st (when the sun is high in the sky) and receive full sun at noon on December 21st (when the sun is relatively low in the sky).
While this describes the design goal, the result is not ideal. For one thing, in July and August the sun traces a lower path through the sky than it does in late June, so more sun may enter the house during the late summer than is desired, even though the weather is still hot.
Moreover, the sun’s path through the sky on March 21 (when weather may still be cool) is identical to its path through the sky on September 21 (when the weather may still be hot). Any attempt to balance solar heat gain through a building’s windows with outdoor temperatures faces an insurmountable obstacle: the planetary flywheel effect. This effect introduces a delay between peak insolationAmalgamation of the words “incoming solar radiation” that means the total amount of solar energy that strikes a given surface in a given time. It is commonly expressed in kilowatt-hours per square meter per day. and high outdoor temperatures.
A related problem: in all seasons, the sun is lower in the sky at 10:00 a.m. and 2:00 p.m. than it is at noon. So even if south windows are fully shaded at noon, they might receive unwanted sun at 2:00 p.m.
In spite of the fact that it’s impossible to achieve perfect shading of windows, it’s still worth following traditional passive solar principles when designing roof overhangs. A useful online tool to help designers with this task is Overhang Design. (The free software program is provided by a Seattle company called Sustainable By Design.)
If you are trying to achieve a good balance between solar heat gain and shading, you can tweak your south overhang to achieve the results you prefer. In a hot climate, you might want to lengthen your roof overhang somewhat, to favor shading during the shoulder seasons; and if you live in a cold climate, you might want to shorten your roof overhang.
East and west windows. Sunlight hits east and west windows when the sun is low in the sky, and it’s hard to control this nearly horizontal sunlight.
The larger the area of east-facing and west-facing windows, the more likely that these windows will cause overheating during the summer. As a first step, east-facing and west-facing windows should usually be fitted with low-solar-gain glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., especially in hot climates. (In colder climates, where summer mornings are often cool, a little heat gain through east-facing windows may be welcome, so high-solar-gain glazing may make sense for east windows in some climates).
[Photo credit: Rose and Joe Schlatter]
Overheating matters more in late afternoon than in the early morning, so west-facing windows are the most challenging — especially in a hot climate. In a very hot climate, west-facing windows should be minimized or eliminated entirely.
It’s hard for roof overhangs to be wide enough to shade west-facing windows, especially when the west side of the house is one of the gable ends. If you want to shade your west-facing windows, the best approach is to include a deep porch on the west side of your house.
Roof design strategies
Having listed some of the reasons why roof overhangs are important, we can now address roof design strategies.
[Photo credit: TransylvaniaDreaming.eu]
Strategies for protecting doors. There are at least four strategies for protecting doors: making the overhang of the main roof wide enough to protect the door; integrating a gable or cricket with the main roof to protect the door; creating a recessed entry; and attaching a “roofette” to the wall above the door. (“Roofette” is my personal nickname for a small gable or shed roof that protects an exterior door.)
After a few years, the lower sections of the jambs of unprotected exterior doors begin to rot. Many carpenters and handyman make a good living repairing these rotten door jambs.
[Photo credit: ActiveRain.trulia.com]
If you don't want to protect your exterior doors with a roof overhang, at least make sure that the doors are recessed from the exterior plane of the wall.
[Photo credit: Martin Holladay]
“Brow” roofs. The roof overhangs on two-story homes are at least 17 feet above the lowest courses of siding, so they often do a poor job of protecting walls from rain. That’s why two-story homes often need a roof for every floor.
The usual solution is a so-called “brow” roof — a narrow shed or hipped roof attached to the wall at the level of the first-floor ceiling. If a brow roof is sized to shade south-facing windows in late June, it can meet passive-solar design principles while helping to protect the wall from rain.
Gable-end overhangs. Gable-end overhangs — that is, rake overhangs — extend to the peak of the roof. Since the peak is a long way from the base of the wall, these overhangs are less effective than eave overhangs.
[Photo credit: Casual Country Log Homes]
One solution is the prow roof overhang. While this type of gable-end overhang does a better job of protecting the walls than most rake details, some people don't appreciate the prow roof aesthetic.
[Photo credit: Martin Holladay]
A safer strategy is simply making a conventional rake overhang wider than usual.
[Photo credit: FloridaDisaster.org]
Sometimes it makes sense to frame wide rake overhangs with ladder-style outriggers. If the gable end is two stories tall, a wide rake overhang can be supplemented with a brow roof between the first and second floor.
[Photo credit: Tom Auger / Rever & Drage]
In the middle of the 20th century, architects who embraced modern design principles often promoted buildings with flat roofs and no roof overhangs. Unfortunately, this severe aesthetic has not yet died.
[Photo credit: Ciiwa.com]
It's worth noting, however, that it's possible to embrace the modern aesthetic in a way that retains generous roof overhangs.
Can roof overhangs be too wide?
While it’s far more common for roof overhangs to be too stingy than too wide, some designers have gone too far and ended up with roof overhangs that are oversized.
For example, homes with porches on two or three sides can be dark and gloomy. Especially in cold climates, it’s good to let some sun come in through your windows.
Finally, remember that wide roof overhangs need to be properly engineered to resist wind uplift. In most cases, so-called hurricane clips (steel clips that attach rafters or trusses to top plates) are better than toenailed birds’ mouths. When in doubt, talk to an engineer.
Martin Holladay’s previous blog: “South-Facing Skylights: Threat or Menace?”
- Image #1: Static.panoramio.com
- Image #2: Allstate Exteriors / Roofinestimatorprowebsites.com
- Image #3: www.lovebeinganonny.com
- Images #4 and #6: Kenthillwebblog.blogspot.com
- Image #5: Syracuse Home Remodling / Allcraft
- Image #7: G. Mills / www.frontiernet.net/~gmills/garage.htm
- Image #8: Square Jer / Jerry H.
- Image #9: BungalowRestore.blogspot.com
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- Image #15: Building Science Corporation / http://www.buildingscience.com/documents/insights/bsi-063-over-roofing
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