How to Calculate a Roof

Surface Area

• 1

  Sketch out the outline of your home as if you were in the air looking straight down at it.

• 2

  Measure the length of the home's exterior walls and record the measurements in the corresponding locations on the sketch.

• 3

  Measure the width of the overhang along each wall and record it in the proper location on the sketch. This is the area where the roof line extends past the walls.

• 4

  Measure odd-shaped areas on the roof, such as a dormer. Disregard objects such as skylights and vents.

• 5

  Review the shape of the roof on the sketch. If it has oddly shaped sections, break the roof into smaller sections to determine the surface area.

• 6

  Multiply the length and width of each individual roof section and add the results for each section together.

• 7

  Multiply the length and width of the overhangs and add these totals to the surface area of the roof.

• 8

  Multiply the total area of the roof surface by the roof pitch multiplier to calculate the roof's size in square feet. This information can be found online.

Slope

• 9

  Climb to the peak of the roof on a ladder.

• 10

  Hold a level on the peak of the roof and balance it until it's level.

• 11

  Measure the distance between the end of the level and the surface of the roof.

• 12

  Calculate the slope using the length of the level and the distance between the roof and the level. For instance, if the distance between the roof and the level is 12 inches and the level is 24 inches long. The base run is 12 inches long, so you need to reduce the length of the level to 12 inches. To do that, divide both numbers by two, so the slope is reduced from 12 in 24 to 6 in 12. This means that for every 12 inches of run, the roof rises 6 inches.