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.
.jpg)
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 |
.jpg)
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.
- Square your result.
- Add 1 to your number.
- Take the square root of your new number.
- Measure the length and width of your house (include overhangs).
- Multiply the length by the width to get the area.
- Multiply the area by the roof’s pitch factor.
- Add 10% for gable roof or 17% for cottage roof for waste.