Determining The Roof Slope Of A Home

A roof slope is also known as roof pitch. This is the length that a roof raises vertically over a 12-inch horizontal distance. Some types of roofs, like a gable roof, have several different roof slopes.

Importance of Roof Slope

Slope is a considered a basic aspect of roof design. Many different types of roofs have varying roof slopes, and the pitch of the roof has a significant impact on the exterior design of the home. The materials used for roof construction are often highlighted as a result of roof pitch and slope.

Roof slope can determine the effectiveness of water drainage and even interior volume of the building. If there is a problem with water collection on the roof, it is very likely related to incorrect roof slope.

Determining Slope

The slope of the roof is determined by the vertical rise in inches for every horizontal 12 inch length or 'run.' A common formula is:


This means that for ever 12 inches horizontally, the roof rises x-number of inches. Some of the frequently used roof slopes are:

  • Flat Roof: 2/12
  • Low Slope: 2/12-4/12
  • Conventional Slope Roof: 4/12 to 9/12
  • Steep Slope: 9/12 and higher

The steeper a roof is sloped, usually the more attractive it is considered. A highly steeped roof will often last longer and will prove to be better adapted for geographical regions where rain and snow are common, since the weight of the snow and water easily slides off the roof and prevents damage due to excess water weight. A steeply sloped roof will often cost more to construct because of added costs for lumber and framing.

The slope of a roof may also determine the kind of roofing materials that are most appropriate for a home. Low sloped roofs do not handle heavy tiles or shingles well. However, rolled roofing does much better with flat or low sloped roofs.

Very often, homeowners choose to change the slope of a roof as part of a major home remodeling project.

Calculating the Slope

Using a ladder to get to the edge of the roof peak, place a level on the roof surface lengthwise. You will need to use a level at least 12 inches long. Place the level so that it is on the high side of the roof. Raise the low side of the level until it is completely horizontal and level so that the level bubble is centered.

Use a tape measure, yardstick or ruler to measure the distance from the 12-inch mark on the level to the surface of the roof, making sure that the tape measure you are using is perpendicular to the roof.

The distance from the level of the roof surface is the roof slope. If, for example, the measurement were 4 inches from the roof surface, the roof slope would be 4/12 or a 4 pitch roof.

If you have a roof that has several different planes, you may need to measure the slope in several places to get a completely accurate measurement of the roof slope.