Roofing Calculator
A 40 by 20 foot footprint with a 1 foot overhang and a 6/12 pitch is 1,033.06 square feet, 10.33 squares, and 11.36 squares with 10 percent waste.
Last updated . Footprint and overhang start in feet. Rise and run start in inches. Switch them to meters or millimeters. Waste starts at 10 percent. No price.
Enter the footprint, the overhang, and the pitch. Roof area is the plan times the pitch factor. One square is 100 square feet.
Pitch only: roof pitch calculator. Bundles: shingle calculator. A paved asphalt mat: asphalt calculator.
Roof area and squares
The overhang is added on every side before the pitch factor. A 6 in rise and a 12 in run are 6/12. The factor is sqrt(1 + (rise ÷ run)²). It fits a roof of one pitch. It does not draw dormers. Waste starts at 10 percent. No price.
Results
How the squares are counted
Plan length is the building length plus the overhang on both ends. Plan width is the same. A 40 by 20 foot footprint with 1 foot of overhang is 42 by 22, which is 924.00 square feet. The pitch factor is sqrt(1 + (rise ÷ run)²). For 6/12 that prints as 1.118. Roof area is 1,033.06 square feet, 95.97 square meters.
Squares = roof area ÷ 100, so 10.33 squares before waste. Ten percent makes 11.36. The factor fits one pitch over the whole plan. It is the same factor the roof pitch page prints for 6/12. Bundles are not counted here.
Worked example
Leave every field as it opens: 40 ft by 20 ft, overhang 1 ft, rise 6 in, run 12 in, waste 10 percent. Plan reads 924.00 ft² · 85.84 m². Factor reads 1.118. Roof area reads 1,033.06 ft² · 95.97 m². Squares read 10.33. With waste reads 11.36.
| Pitch | Overhang | Waste | Roof area | Squares | With waste |
|---|---|---|---|---|---|
| 6/12 | 1 ft | 10% | 1,033.06 ft² | 10.33 | 11.36 |
| 4/12 | 1 ft | 10% | 973.98 ft² | 9.74 | 10.71 |
| 6/12 | 0 | 10% | 894.43 ft² | 8.94 | 9.84 |
| 6/12 | 1 ft | 15% | 1,033.06 ft² | 10.33 | 11.88 |
Frequently asked questions
How many squares is a 40 by 20 roof at 6/12?
Leave the footprint at 40 ft by 20 ft, the overhang at 1 ft, the rise at 6 in, and the run at 12 in. Plan reads 924.00 ft² · 85.84 m². Factor reads 1.118. Roof area reads 1,033.06 ft² · 95.97 m². Squares read 10.33. At 10 percent, with waste reads 11.36.
What is the pitch factor?
The factor is sqrt(1 + (rise ÷ run)²). A 6 inch rise and a 12 inch run are 0.5, and sqrt(1.25) prints as 1.118. The roof area uses the full factor, then rounds the square feet. One square is 100 square feet, so 1,033.06 square feet is 10.33 squares.
What does a 4/12 pitch change?
Set the rise to 4 and leave the run at 12. Factor reads 1.054. Roof area reads 973.98 ft² · 90.49 m². Squares read 9.74. With waste reads 10.71. The plan stays 924.00 ft² · 85.84 m², because the overhang did not change.
What if there is no overhang?
Set the rise back to 6 and the overhang to 0. Plan reads 800.00 ft² · 74.32 m². Factor stays 1.118. Roof area reads 894.43 ft² · 83.10 m². Squares read 8.94. With waste reads 9.84. The overhang is added on every side before the factor.
What does 15 percent waste change?
Set the overhang back to 1 and waste to 15. Squares stay 10.33. With waste reads 11.88. Ten percent of 10.33 is 11.36. The waste is a planning allowance for hips, valleys, and cuts. It is not a bundle count. Bundles are the shingle calculator.
Does this work for a cut-up roof?
It fits a roof of one pitch, where the sloped area is the plan area times the factor. A gable and a hip of the same pitch use that factor. Dormers, different pitches, and open porches are extra area you add. The page does not draw them.
Is this asphalt paving or shingles?
This page is roof area and squares. Asphalt on this site is a paved driveway mat, not a shingle roof. Field bundles, starter, and ridge cap are the shingle calculator. Rise, run, and degrees by themselves are the roof pitch calculator.