When you start any home improvement project, it helps to know the lingo. Here are some common terms to help you talk the talk when you’re in the market for a new roof.
First things first -- roofs are measured in squares. A roofing square is simply an area that equals 100 square feet. Many roofs have a surface area of approximately 15 to 20 squares, or 1500 to 2000 square feet. This measurement helps a contractor determine how many bundles of shingles or other materials are needed to cover the surface of the roof.
Next, there’s the incline of the roof. The incline of a roof can be expressed in one of two ways - either as the slope or the pitch. The slope is the ratio of the vertical rise to the horizontal run and is expressed as inches per foot. So, a roof with a slope of “8 in 12” or “8/12” rises up eight inches for every 12 inches across. You may hear contractors talk about low-slope roofing products. In general, a roof with a slope of less than 4/12 is considered low-slope and may need special materials to help protect it from ice and water build-up.
Pitch is similar to slope in that it measures the incline of the roof, but it is expressed as a fraction, dividing the vertical rise by the entire horizontal span from exterior wall to exterior wall. To find your horizontal span, you add your vertical rise and your horizontal run. So, a roof with a vertical rise of eight feet and a horizontal run of 12 feet would have a horizontal span of 24 feet. Dividing eight into 24 gives us a pitch of one-third.
Styles: Gable, Hip, Gambrel, Mansard, Shed
Two of the most common roof styles you’ll see are gable roofs and hip roofs. A gable roof has two sloped surfaces that meet along the top ridge, forming a triangle or gable out of the exterior walls on each side. A hip roof also has a top ridge, but it has four surfaces that slope downward from the ridge to meet the exterior walls on all sides.
Other styles you may come across include gambrel, mansard and shed roofs. The gambrel or “barn-style” roof is similar to a gable roof, except it changes slope partway up on each side, creating an exterior wall that is shaped like the front of a barn on each side. A mansard roof is another unique style that has four sloping sides, each of which becomes very steep part of the way down. This European style typically features dormer windows protruding through the very steep sides. Finally, a shed roof is the simplest style; it has one sloped surface and is seen on many sheds and some dormers known as “shed dormers.”
Anatomy of a Roof
In addition to roof measurements and styles, it’s helpful to know the different parts of the roof and which materials will go where.
The foundation of the entire roof is called the decking or sheathing. It’s typically made of plywood or oriented strand boards (OSB boards) that are attached to the rafters of your attic. Next, the underlayment is rolled out and installed on top of the decking before the shingles go on. This base layer was originally made of a thick felt-like material but today there are synthetic options that provide extra water-resistance underneath the shingles.
The horizontal edge that runs along the very top of a roof’s peak is called the ridge. This is where a ridge vent will be installed along the entire length of the ridge to help ventilate the attic of the home. Older homes may have gable and soffit vents instead of a ridge vent, so it’s important to choose a contractor who can explain how the various types of vents work together and which ones are best for your home.
Hips and valleys are the next edges you’ll see as you work your way down the roof. A hip is an edge created by two surfaces sloping down and away from each other, whereas a valley is more like a crevice that’s created when two surfaces slope down and towards one another. If you poured water on a hip, it would run off the hip and down the the adjacent surfaces; if you poured water on the valley, it would run down the length of the valley. Because valleys are runways for water, it’s important that these areas are protected with a waterproofing barrier called ice and water shield and with valley flashing before the shingles are installed.
At the bottom edges of the roof, you’ll find the eaves. The eaves are the sections of the roof that run just above the gutters and are a common place for ice and snow to collect in the winter. For this reason, ice and water shield must be installed along all eaves (six feet up the eaves is recommended) to protect against water penetration. In addition, a strip of material called drip edge is installed between the eaves and the gutters to help guide rainwater off the roof and into the gutter system.
Finally, any penetrations of your roof such as chimneys, pipes or skylights need to have flashing installed around them. Flashing is a metal material such as lead or copper that comes in a variety of shapes to help seal these vulnerable openings from leaks.