Roof slope, also known as pitch, defines how slow or how fast the roof rises over a specified horizontal distance. Quoted as a ratio, a lower number will usually define a flatter roof. A very steep roof will have a high pitch.
Calculating Roof Slope
Roof pitch is a ratio that defines the roof’s vertical rise per 12 inches of horizontal run. For instance, if the roof rises six inches over a 12 inch horizontal length, the pitch is defined as 6/12. A steeper roof will have a higher pitch and a flatter roof will have a lower pitch.
Local building codes will usually define the minimum slope allowed. Typically slope requirements are a result of climate. For instance, in the Northeast, where roofs need to handle heavy snowfall, steeply pitched roofs will allow the snow to slide off rather than build up. In Florida, hurricane winds blow across flat roofs instead of against steep roofs.
Effect of Roof Slope on Cost and Materials
Steeper roofs are typically more expensive than flatter roofs. More material is required to frame and roof it. Installation is more difficult and dangerous. A steeper pitch has more surface area to cover. However, a steeper roof offers better drainage and the shingles will typically last longer than on a flatter roof. Roof slope will also determine what roofing materials are appropriate. For instance, a pitch of 4/12 allows the use of tiles or shingles, a choice that does not work as well on a lower sloped roof. Likewise, a single ply membrane roof does not work well on a steeper pitch.
The minimum slope for a home, if allowed by building codes, is 1/12. This allows some water drainage to the gutters and is a useful slope where roofing materials are very expensive. So-called flat roofs, seen on many homes in the Southwest, are not actually flat, but have a very moderate pitch which is concealed by parapets.