1. Measuring the slope of a roof.
A roof’s slope is determined by the rate at which it rises, and is expressed as inches of vertical rise for each foot of horizontal run. In example, let’s say that a roof that rises 3 inches vertically at the same time covering 12 inches of the house underneath it has a slope of 3 in 12. To measure a roof slope from outside a house, use a one foot level or mark off 12 inches on a level and put the end of it at the rake of the house. With the level aligned horizontally under the rake, use a ruler to measure the vertical distance between the 12 inch mark and the rake, from inside the house, measure along the rafter rather than the rake.
2. Measuring the area of a roof.
Start by making a blueprint or plans of the ground space covered by each surface of the roof, including the overhangs at the gables and eaves. Write the ground space down into rectangles and then calculate the area of each rectangle (for example, a house can have 2 rectangles covered by two different slopes of the roof). To estimate the amount of roofing material you will need for each roof slope, take the total number of roof areas, add 10% to this total for double layering of shingles at the ridges, hips and eaves, and round the total to the next higher square, or next 100 square feet, of material.
To convert the flat ground areas into slanted roof areas, use the following conversion table. Find the number in the right hand column that corresponds to the slope of your roof, then multiply this figure by the ground area to get the total area of this section of the roof.
- slope:1 to 3 in 12 conversion factor: 1.03
- slope: 4 in 12 conversion factor : 1.06
- slope: 5 in 12 conversion factor: 1.09
- slope: 6 in 12 conversion factor: 1.12
- slope: 7 in 12 conversion factor : 1.16
- slope: 8 in 12 conversion factor : 1.20
- slope: 9 in 12 conversion factor: 1.25
- slope: 10 in 12 conversion factor: 1.30
- slope: 11 in 12 conversion factor: 1.36
- slope: 12 in 12 conversion factor : 1.45
Roofing is a trade that can take years to master, and can be overwhelming to a novice tradesman just starting out. There are many types of roofing, from asphalt shingles, to tiles and roll roofing, slate roofing, and more. But they all follow this formula to determine how much you will need for your project, and you can avoid that annoying task of returning those extra pallets of shingles to home depot!