Material
A grade can define the durability of some roofing materials, according to the American Society for Testing and Materials, or ASTM International. Though many grade levels exist, lower numbers generally indicate better quality. For example, Grade S1 roofing slate will last more than 75 years. Grade S3 slate lasts from 20 to 40 years. For clay tiles, Grade 1 resists severe frost and Grade 3 provides negligible resistance. One exception is the grading for Type II insulating fiberboard for roofing. In this case, Grade 1 is primed and used under a built-up and polymer-modified bitumen system. Though Grade 2 is unprimed, it is denser and has higher-quality physical properties for use under a single-ply roof system.
Slope
Grade is also used interchangeably with pitch, gradient, angle or slope for roofing, and it defines how much a roof is slanted from the horizontal. Higher grades designate steeper roofs, which use up the most materials and enable the roof to dominate the appearance of a home. They are also harder and more expensive to install. Lower grades designate flatter roofs and are more common on modern architecture. They allow the surrounding landscape and the house design itself to dominate and are easier and cheaper to install.
Estimating
A quick way to estimate the grade of a roof is to calculate how many feet a roof rises versus how long it runs horizontally. For example, a roof that rises 3 feet for every 12 feet of length has a 3:12 grade. This ratio or less is considered a low grade, is walkable and is easy to install. A roof with medium grade has a range from 5:12 to 8:12 and requires special equipment like roofing jacks for installation. Grades from 10:12 and higher are considered steep and require special protective gear for the installers.
Area
Using the roof grade to create multipliers lets you estimate the total square footage of a roof, which in turn determines the amount of roofing materials you’ll need and their cost. For example, assume a roof measuring 40-by-50 feet. Multiplying both numbers yields a product of 2,000-square-feet, which is accurate only if your roof is flat. For low-pitched roofs, use a multiplier of 1.15 to 1.25 on the product, which increases the square footage to between 2,300 and 2,500. For medium-pitched roofs, use a multiplier of 1.25 to 1.4. For a steep roof, use a multiplier of 1.41 to 1.7, which bumps the square footage up to between 2,820 and 3,400.