Length of a Roof Slope

Defintion

• A roofs slope refers to the distance that a roof rises over a certain unit of horizontally length, called the run. The length normally used to calculate the run is 12 inches. The slope is expressed as the relation of the rise to the run with the rise expressed first. For example, a roof that rises 4 inches for every 12 inches of run would have a slope of 4 in 12. The greater the rise, the steeper the roof.

Material

• The steepness of a roof slope plays a role in determining what types of roofing materials might be appropriate. Flat roofs, which typically have a slope of 1/8 in 12 or 1/4 in 12, need built-up roofing materials. Shallow roofs, such as those with slopes higher than flat roofs but under 4 in 12, often use roll roofing. Normal roofing, such as asphalt shingles and wood shakes, works best on roofs with slopes from 4 in 12 to 12 in 12.

Measure

• You can measure the roof slope from the outside, but perhaps an easier method is to simply go up to your attic with a spirit level and a measure. You need to mark off 12 inches on the level with a pencil. Hold the level out horizontally from the bottom of a roof rafter and note the 12 inch mark. For the rise, extend the tape measure up from the 12 inch mark and observe where it meets the rafter.

Confusion

• The terms slope and pitch can cause confusion, as some experts use them interchangeably, while others use them to refer to different measurements. Those who refer to pitch as a distinct calculation, use the width, or span, of the house as the run. So a house that rises a total of 10 inches, from the bottom of the rafters to the roof ridge and has a span of 24 feet, has a pitch of 10 in 24, or 5/12.