6 Roof Types (and how their structure works)
Posted in Home Improvement on June 21, 2018
On a fall evening, as I sit listening to a cold rain pounding on the roof overhead, I always feel grateful for two things:
- that I'm not out there getting soaked, and
- that my roof doesn‘t leak.
Though the wind-driven rain falls in torrents, my roof faithfully blocks the storm and channels the water to valleys and gutters, which carry it away.
Of course, a roof has to do a lot more. It has to endure hot summer temperatures that can build up to 180 degrees F. That’s hot enough to grill a burger! (Or your feet if you‘re trying to do roof work.) At the opposite extreme, it resists bitter cold and ice. It stands ready to shoulder tons of snow for weeks on end. And it has to shrug off powerful winds, sometimes up to hurricane force. And as if mere survival isn't enough, we expect it to continue to perform almost maintenance-free for decades.
Roofs work in quieter, more subtle ways. too. Frequently they open up enough attic space for that extra room. Where roofs overhang exterior walls, they become built-in awnings that protect siding and windows from the corrosive effects of sun and rain. The angles on many roofs create a natural draft to help ventilate attics. cooling them during summer and flushing out unwanted moisture in winter. And, of course, a roof makes a home look good. It’s an architectural crown whose geometry shapes a home's character.
When you add up all those expectations, you’d think there would be one ideal roof type. But not so. The next time you take an evening walk. Take a look at the roofs in your neighborhood and notice the different sizes and shapes that do the job. Here are the types you're likely to see and a few notes on how and why they work so well.
The GABLE ROOF on this colonial-style home supports three symmetrical dormers, each with a gable roof. Gable framing uses simple but strong triangular geometry.
As you stroll through your neighborhood, you’ll probably see gable roofs everywhere, since they're the most common roof shape throughout the USA and Canada. A gable roof looks like a piece of cardboard folded neatly down the middle and laid over the home's walls (Fig. A). The actual gable is the triangular shape of the wall at either end. When you see that shape, you know you’re looking at a gable roof.
They're the easiest and least expensive to construct. Carpenters use three straight pieces of lumber to form the sides of the triangle — two for rafters and one for a ceiling joist. With the corners securely fastened, this triangular geometry creates a strong, stiff structure that can withstand snow loads, people walking on it, and high winds without collapsing.
Though simple, a gable roof that's designed by an imaginative architect or builder won‘t look dull or boring. You might find a dozen of them on your street, and not one identical to another. The variety comes from skillfully arranged combinations of gables, roof slopes, and trim styles. Though many gable roots look complicated, an experienced builder can layout and frame them relatively easily because of the basic shape of the triangle doesn’t change.
Aside from aesthetics, gables have several practical implications for homeowners thinking about remodeling. First. the steepness of a gable’s slope determines how much attic space you have. Steeper slopes mean more attic headroom. Often you can easily convert these attics into living space. You can put windows in the gable walls and add dormers to the roof if you need more headroom (Fig. A). A shallower slope means less headroom and often only enough space for storage, or perhaps no useable space at all.
Second. gables imply a certain kind of wall framing. The two walls that the roof overlaps support the weight of the roof. These walls are called “bearing walls.” You have to allow for that extra weight by installing a strong beam over any new window and door openings you put into those walls.
0n the other hand. the gable walls, the walls that have the telltale triangles in them, usually aren’t bearing. so you don’t need those extra beams over new openings. (However. sometimes gable walls support the second floor of a two-story home and are therefore bearing below the second floor. When remodeling, always check with your building inspector to make sure!)
Gable roofs have another feature that makes them particularly valuable in colder regions. They are excellent ventilators. In particular, gable roofs with lower slopes resemble and work much like the top of an airplane wing. Wind passing over creates a low-pressure zone that exerts a lift on the roof. That lift sucks the air out of vents near the top of the roof, ventilating the attic better, keeping it cooler and removing excess moisture. In winter this venting helps keep the roof colder, so snow on it won’t melt, run down to the root edge, refreeze. and cause those troublesome ice dams that cause leaks.
But in coastal regions, there's such a thing as too much lift, as hurricane Hugo so clearly demonstrated in 1989. Hugo ripped off more low-sloped gable roofs than other types of roof in South Carolina (though other low-sloped styles didn‘t fare so well either). And if the roof didn‘t go, the gable section of wall often blew off due to the pressure created by the roof. While few of our homes will face wind this extreme, the event illustrates that roof shapes perform differently. The roof design that best withstood hurricane Hugo was the hip roof, because. as we shall see, this shape eliminates those gable ends.
The hip roof on this American foursquare home reduces the bulk and prominence of the roof. The architect or builder was consistent, pulling hip roofs on the dormer, bay, and porch.
Hip roofs look like gable roots with the ends clipped off at an angle (Fig. B). That lowers the profile and makes the roof less visually prominent, especially with steeply sloped roofs. A hip roof also reduces the height of the attic, so you’re likely to bump your head on low, angled ceilings if you want to convert it to living space. You’ll probably have to add dormers to increase the headroom.
The hip roof framing is slightly more complicated than a gable roof, and a bit more expensive to build. It still relies on the triangular structure for strength, but the balance of forces that work through those hip rafters and “jacks” (Fig. B) is less obvious.
The style is a good one, though, so don‘t shy away from building a hip roof if you get a chance. They're easier to assemble than they look. Notice that the hip rafter intersects the corner at a 45-degree angle to each wall. and the jacks meet the hip rafter at 45 degrees, too. So all the rafters are easy to cut. For rafter lengths, turn to a basic framing book. like "Modern Carpentry" by Willis Wagner. Or your framing square might have the lengths stamped right into its metal blade for each common roof slope.
A hip roof has other implications for remodelers, too. Rafters lap over every wall, spreading the roof weight more evenly but making each wall a bearing one. So if you remodel the walls below a hip roof, you'll need to add a beam over each new window and door opening to support the weight.
Hip roofs are the rule in some neighborhoods, especially those built during the 1950s and 1960s when they were commonly featured on long, low. ranch-style homes. But you’ll also see them on both newer and older homes as well. The style is practical as well as aesthetically pleasing when used with wide overhangs Since the roof overhangs all sides of the house, it protects the walls and windows from sunlight and rain better than gable roofs do.
Flat and shed roofs are common in contemporary design and in solar homes, where large, south-facing walls catch maximum sunlight.
Numerous leaks through the roof of my boyhood treehouse taught me to be wary of flat roofs of course, neither my roofing skills nor the material on that project were very good. It turns out that laying a leak-proof flat roof requires professional expertise anyway.
Flat roofs don't shed water like sloped roofs (though they drain water), so they consist of a watertight membrane rather than lapped shingles. The old method in which hot tar was mapped between and over three layers of asphalt-impregnated felt worked fairly well, but it’s giving way to single-layer synthetic coverings, In any case, repair of these roofs is better left to qualified pros.
While they never became very popular for homes, flat roofs are standard on commercial buildings. They make sense to minimize a big building’s profile, especially one that would require a huge sloped roof, They also cut costs by eliminating rafters; builders simply beef up the ceiling joists to carry the roof weight.
A shed roof is really half of a gable roof with the upper and lower sides supported by bearing walls. It’s one of the simplest structures for protection from the elements, so it‘s often an economical, quick solution for an addition or porch roof. You have to build only one bearing wall, since you can use the house wall to support the other side. You’ll see many of them used for dormers, too.
Shed roofs, along with flat roots, are a defining feature of so-called “contemporary” design (Flg. C). Most of these homes were built in the second half of the 20th century. Often you see shed rook in solar designs, where the wall supporting the high end of the roof faces south, so the windows in that wall can catch more sunlight.
Still, flat and shed roof haven’t become very popular. Gable and hip roofs almost completely dominate residential construction these days. Two other types, once common in pre-1950 homes, have virtually disappeared, though they continue to dot many older neighborhoods in most cities — the mansard and gambrel.
Homes with mansard roofs are increasingly rare, though the style often appears on commercial buildings.
In the 17th century, a French architect named Francois Mansart invented the roof that bears his name. Indeed, the design still reminds many of its French roots, perhaps because it looks so unusual and foreign.
The mansard roof has two pitches, a shallow top and steep side, and the roof overlaps and bears on all the side walls (Fig. D). It’s a big-volume roof containing maximum attic space, which is usually used as a second floor. In fact, when some homeowners remodel old homes and convert them to apartment rental units. they remove the gable roof and put on a mansard to get more space.
While practical. mansards virtually disappeared in home construction after the 1940s. However. you might notice remnants of the style in the steeply roofed facades of fast-food restaurants and other small commercial buildings.
Both roof types create easily used living space. The roof framing also frames the walls and railing of the attic room.
The double-gambrel roof on this home creates maximum attic space, virtually an entire second floor.
A gambrel roof is easy to recognize. It will immediately remind you of a barn (Fig. F). Though the style emerged in the USA, during the colonial period, no one knows who invented it or why. Perhaps its popularity as a barn structure gives us a clue; it offers more interior volume. The wider root enlarges the attic and makes it a natural living space.
It has an advantage from a framing standpoint, too. You don‘t need such long and perhaps hard-to-find rafters. You can substitute two lighter, more easily handled, short rafters instead.
Gambrels are efficient, compact houses. They incorporate the maximum amount of useable living space in the smallest, least prominent exterior shell by literally cramming more under the roof. Notice in Fig, E that the roof framing also frames the walls and ceiling for finishing off the interior. Home builders typically punctured the lower roof plane with dormers and windows to better light and ventilate what essentially becomes a second floor, rather than an attic.
Gambrel roofs are a bit more complicated to assemble and their attics more difficult to ventilate than gable roofs. While that makes them slightly more expensive, I suspect the design has fallen victim to a greater homogenization of our current building styles. More spacious suburban lots and bigger homes make their compactness, the main advantage of a gambrel, unnecessary. In browsing through several current books oi house plans, I couldn’t find a single gambrel roof. So unless interest in gambrels revives, this style will soon die out.
Two Final Notes
While we deal with each roof style separately in both our text and illustrations, you'll often see several types on the same home. Architects and builders commonly combine them. So you'll find gable and shed dormers on hip roofs, hip-roofed porches on gabled homes, and shed roofs attached just about anywhere.
But roofs have become plainer in the last 30 years. That’s because many builders now use trusses for the roof structure. Trusses rely on a system of smaller-width rafters and ceiling joists reinforced by a web of crosspieces. They’re factory-built and erected at the home site.
Unfortunately, the cross-pieces render the attic space unuseable, except perhaps for lightweight storage, because you can’t cut and remove them to open up attic space without weakening the entire roof structure. The result is plainer, less interesting roofs on newer homes because, without attic living space, dormers and windows in the roof serve no purpose.