To do this, measure the length and width of each plane on the roof, including dormers. Then, multiply length x width to get the square footage of each plane. Finally, calculate your roof’s total square footage by simply adding the square footage of each of the planes together.
How to Determine Your Roof Pitch
The next step is to measure the pitch (also referred to as the roof slope) of the roof deck. You’ll need to figure out the vertical distance (also known as the rise) over a 12” horizontal segment (also known as the run). Record these values as roof rise over run, with the vertical measurement listed first and the horizontal second.
It’s important to keep in mind that the roof pitch will be dramatically different for homes with a flat roof than homes with a steep roof.
Roof pitch affects the actual area of the roof. Depending whether the roof area is measured horizontally (possibly from a drawing or photograph), a correction factor is necessary to determine the actual area of the roof. Given pitch and a horizontal area measurement, multiply the horizontal area by a correction factor corresponding to pitch, provided in the table below, to determine the actual area of the roof to be used in the Roofing Material Calculator. While it is possible to estimate the amount of necessary materials using only the total roof area measurement, as can be seen from the table, depending how large the pitch of the roof, the actual area of the roof can differ by up to 2.236 from the measured total area at a pitch of 24/12. As such, while it can be cumbersome, measuring the area and pitch of each part of the roof and multiplying by the corresponding correction factor will result in the most accurate estimate of necessary roofing materials.
Typical Slope Correction Factors:
| Pitch | Angle | Multiply By | Pitch | Angle | Multiply By | |
| 1/12 | 4.8° | 1.003 | 2/12 | 9.5° | 1.014 | |
| 3/12 | 14.0° | 1.031 | 4/12 | 18.4° | 1.054 | |
| 5/12 | 22.6° | 1.083 | 6/12 | 26.6° | 1.118 | |
| 7/12 | 30.3° | 1.158 | 8/12 | 33.7° | 1.202 | |
| 9/12 | 36.9° | 1.250 | 10/12 | 39.8° | 1.302 | |
| 11/12 | 42.5° | 1.357 | 12/12 | 45.0° | 1.414 | |
| 13/12 | 47.3° | 1.474 | 14/12 | 49.4° | 1.537 | |
| 15/12 | 51.3° | 1.601 | 16/12 | 53.1° | 1.667 | |
| 17/12 | 54.8° | 1.734 | 18/12 | 56.3° | 1.803 | |
| 19/12 | 57.7° | 1.873 | 20/12 | 59.0° | 1.944 | |
| 21/12 | 60.3° | 2.016 | 22/12 | 61.4° | 2.088 | |
| 23/12 | 62.4° | 2.162 | 24/12 | 63.4° | 2.236 |
CALCULATING THE AREA
If you want to get the exact area measurement for your roof, you can follow these steps:
- Once you have your roof’s pitch, divide the number by 12. (For example, if your roof’s pitch is 4 in 12, you would divide 4 by 12. This would yield 1/3.)
- Next, square your result. (If your number is 1/3, squared would yield 1/9.)
- After that, add 1 to your number. (1/9 + 1 = 10/9.)
- Next, figure out the square root of your new number. (The square root of 10/9 is 1.05.)
- Next, use your measuring tape to measure the length of your house. (Be sure to include overhangs.)
- After that, measure the width of your house. (Be sure to include overhangs.)
- Multiply your house length by your house width to get the area. (For example, 40 feet x 30 feet = 1,200 square feet.)
- Next, multiply the area by your roof’s pitch. (1,200 x 1.05 = 1,260 square feet.)
- To allow for hips, ridges, and waste, add 10% of your final number for a gable roof and 17% of your final number for a cottage roof. (Your total number would be either 1,386 or 1,474 square feet.)
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