Roof Area Slope Factors

Basics

• A roof's slope is more commonly known as its pitch, and it is the way the roof surface angles from horizontal. It is normally described as the number of inches the roof rises for every 12 inches it moves horizontally. A bare roof and a level are used for the most accurate measurements, though the ratio can be estimated. The calculation begins with measuring a point near the bottom of the roof line in inches from the ground; for example, it may be 144 inches. Move horizontally 12 inches, and then measure the second point's distance from the ground; that distance may be 148 inches, for example. Subtracting the second point from the first point results in the difference between the points. Using the examples, the difference is four for every 12 inches, which means the slope is 4:12.

Categories

• A roof with a slope of 2:12 or under is considered flat, and is not only simple to install but can provide a surface that serves as outdoor space for relaxing. At 3:12 to 5:12, a roof has low pitch, which is considered walkable and still simple to work on when necessary. A medium slope ranges from 6:12 to 9:12 and is considered non-walkable. It requires equipment such as planks and roofing jacks for repair and installation. The steepest slopes range from 10:12 to 12:12, which is a 45-degree angle. It is the most difficult and dangerous pitch to work with, and the workers require fall arrest and special protection equipment.

Multipliers

• Special multipliers can be used to determine how slope affects roofing area. When the slope is greater, the area is greater, and more roofing material is needed for a greatly sloped roof than a flat roof of similar length and width. Multipliers for low-sloped roofs range from 1.15 to 1.25. For medium pitches, it’s 1.25 to 1.4, and for steep pitches it’s 1.41 to 1.7. Assume, for example, that a roof measures 20 by 50 feet, or 1,000 square feet. Using the appropriate multipliers makes that square footage 1,150 to 1,250 for roofs with low slopes, 1,250 to 1,400 for roofs with medium slopes and 1,410 to 1,700 for roofs with high slopes.

Costs

• Roofs with high slopes mean high costs because of the increase in roofing materials needed and the hazards of working on the roofs. Other factors that can affect cost include the type of roofing material; number of slopes, gables and other roof structures; the weather during installation or repair; whether or not previous roofing material has to be removed and hauled away; and the roof's accessibility. If, for example, a house has a flat roof and is in a desert environment and the weather is fair, then its roof is less expensive to install and repair than the roof of a house that has a steep roof and several tree branches hanging over the roof and it is winter.