Although you may have thought about the color and material you want for your roof, the typical homeowner puts little thought into the actual design of their roof. And roof overhangs specifically, forget about it. Roof overhangs haven’t even crossed most people’s minds.
A roof overhang is simply an extension of the eave or “edge” of the roof (I mistakenly said "house" on the podcast). The overhang can extend beyond the exterior walls of the house many inches or a few feet.
In this week’s mini lesson, I’ll tell you why overhangs shouldn’t just be an afterthought. And why almost every new home should include them, where possible.
Before we get into the meat of the mini lesson, let’s go over a Pro term.
Pro term: Scupper
A roof scupper is a roof drainage system usually used for flat roofs, also called parapet roofs. Scuppers have metal openings on the edge of the roof. From that metal scupper opening, roof water moves into and through a downspout that looks similar to a downspout in a typical gutter system.
Roof scuppers are not the same as roof drains. Roof drains channel water through the floor or deck of the flat roof. Then water from a roof drain enters a hidden piping system that carries it away from the house. Roof drains are typically more expensive than scuppers.
With a scupper, water drains through an opening in the side of the roof edge, down a downspout that is usually on the exterior of the house. Roof drain systems are often hidden, while scuppers are easily seen and can be used in the design of exterior of the house, or can made to be less noticeable by being tucked away in exterior corners.
So that was scuppers, our Pro term this week.
Alright, moving on to this week mini lesson…
The Fine Homebuilding forum says that a roof overhang is “one of the most important basic design features to a well designed building. And it is almost always ignored and misunderstood”.
The most common errors made with designing and building roofs is forgetting about overhangs. A house with absent or small overhangs can overheat in the summer and be damaged by rain all year long.
Rarely, in areas that have long or especially cold winters, roof overhangs could be too wide, creating too much shade on houses that could benefit from the warm sun several months out to the year. But, overhangs that are too small are a much more common problem.
WHY OUR HOUSES SHOULD HAVE ROOF OVERHANGS
1. Keeping water off of exterior walls
Perhaps the most important function of roof overhangs is to help keep water off exterior siding, windows, and doors. Of course it’s not possible to stop all rain, especially wind-driven rain, from reaching your exterior walls, but properly sized roof overhangs can give quite a bit of protection.
Exterior walls with small roof overhangs get wet every time it rains, causing moisture problems, including potential mold growth, rotting and deterioration of paint, stains, structural materials and window and door frames. The risk of moisture problems is greater in high-rainfall regions, but adding roof overhangs to protect a home’s exterior walls make sense for houses in most areas.
2. Keeping rain away from your home’s foundation
Another function of roof overhangs is to ensure that roof rainwater falls away from the foundation. Gutters can help with this too, but gutters work best in conjunction with roof overhangs. Without overhangs, rain that falls on the roof can travel down the home’s exterior walls and settle on the ground right next to the home’s foundation. Overhangs help keep your foundation and basement dry and free of leaks.
3. Reducing splashback
Keeping roof water spillage from settling directly next to the house also limits the damage caused by splashback. Splashback occurs when water hits the ground right beside the house and splashes back against the home’s exterior walls or doors. Splashback can cause rot and deterioration of the lower part of your house over time. Splashback can also cause excess dirt on the bottom of your house, which is especially noticeable on light colored houses.
4. Shading your windows
This is the main reason I want roof overhangs. The most effective way to reduce summer heat gain through windows and glass doors is to block the sunlight before it hits the glass. This can be done with exterior shading devices, including overhangs. Not only does this make the house more comfortable, but it can reduce your air conditioning bill. For this reason, overhangs are a key design feature if you have large windows and live in a region with hot summers.
How much shading you want will depend on where you live. If you live in a place with long, cold winters and fairly comfortable summers, you probably want the sun’s heat to come through your windows almost all year long (so wide overhangs for shading may not be necessary). However, if you live where summers are long and hot, you probably want to reduce solar heat gain through your windows with wider overhangs.
There is no true standard size for overhangs, but typical overhangs that are used for protection from rainfall are 16-18 inches wide. Overhangs wanted for shading should be minimally 24 inches. More commonly, they are 36 inches wide or more.
There’s a software program that house designers, architects, mechanical engineers and curious homeowners can use to determine the ideal width of overhangs for their regions. There’s also a similar online overhang design tool called Sustainable by Design Overhang Design Tool that you can play with for free.
Now let me quickly address 2 and 3 story houses.
Are second floor roof overhangs adequate to protect first floor windows and walls from sun and rain? The short answer is no, not usually. The roof overhangs will protect the upper floor of a 2 or 3 story house, but you need additional overhangs or awnings for the lower floors.
Think about it… the roof overhangs on two-story wall are at least 17 feet above the lowest points of the exterior walls, so roof overhangs, by themselves, often do a poor job of protecting tall walls from rain and sun.
Two and three-story homes often need a covering for every floor. That might come in the form of a second floor balcony to protect the exterior wall below it or an awning or brow roof that can be used above lower floors to shelter walls beneath it.
A “brow” roof is a roof-like projection that’s attached to the exterior wall right above the first floor of the house.
Should you place overhangs on all sides of your house?
Yes and no. 16-18 inch overhangs that are used to protect your house from the elements should go everywhere. But extending overhangs to a width needed for shading is not necessary for all sides of house.
Roof overhang size matters most on the south side of a house. Most people follow traditional passive solar design principles, meaning they have the roof overhang sized so that south-facing windows are fully shaded at noon on June 21st. June 21st is when the sun is highest in the sky (solstice). Using the software program I mentioned, building professionals determine what angle and width your overhangs should be based on the sun’s position in the sky on June 21.
The ideal overhang size and angle will also allow you to receive full sun at noon on December 21st (when the sun is relatively low in the sky).
A good overhang design allows more winter sun to come in than summer sun— giving us more solar heat during the cooler months and more shading in hotter months. This will save energy and utility costs.
Occasionally another date is used depending where you live and how long the summer is in your region. For example, although peak sun angles occur at the solstice on June 21, peak temperature and humidity are more likely to occur in August, so, depending on where you live, you might want to use an August date for the program.
In a hot climate, consider wider roof overhangs, so you get shading not only in the summer, but in late spring and early fall when it continues to be hot outside. If you live in a cold climate, you can shorten your roof overhangs to get less shading throughout the year.
In part 2 of this mini lesson, we’ll go over whether north, east and west windows will benefit from wide roof overhangs for shading. Plus I’ll tell you what you can do to protect your house from the sun’s heat and rain if you either don’t want overhangs or can’t have them because of building codes.
Please remember that the purpose of this podcast is simply to educate and inform. It is not a substitute for professional advice. The information that you hear is based the only on the opinions, research and experiences of my guests and myself. That information might be incomplete and it’s subject to change, so it may not apply to your project. In addition, building codes and requirements vary from region to region, so always consult a professional about specific recommendations for your home.
Thanks for stopping by. I hope you learned as much as I did.