How to use the roofing calculator
- Enter the building length and width, including the roof overhangs.
- Enter the roof pitch as rise in inches per 12 in of run. If you do not know it, use the roof pitch calculator.
- Choose a waste allowance. Hips, valleys and dormers create more offcuts than a plain gable.
- Enter the bundles per square printed on the shingle wrapper.
Check the coverage printed on your shingle bundles before you buy; Decoratoradvice uses the figure you enter, and it varies between products.
Roofing formulas
Pitch factor = √(1 + (pitch ÷ 12)²)
Roof area = length × width × pitch factor
Squares = roof area ÷ 100 × (1 + waste)
Bundles = squares × bundles per square, rounded up
A roofing square is 100 sq ft of roof surface. When every roof plane has the same pitch, the footprint × pitch factor gives the true area for gable and hip roofs alike.
Worked example
A 40 × 30 ft footprint with overhangs, 6/12 pitch, 10% waste, three bundles per square:
Pitch factor: √(1 + 0.5²) = 1.118
Roof area: 40 × 30 × 1.118 = 1,342 sq ft
Squares: 13.42 × 1.10 = 14.76
Bundles: 14.76 × 3 = 44.3, so 45 bundles
Pitch factors
| Pitch | Factor |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
What else to order
This calculator covers the shingles. You will also need underlayment, starter strip, ridge cap, drip edge, flashing and nails, all of which depend on the roof's edges, ridges and valleys. Roof decking is sheet goods; the plywood calculator counts sheets from the roof area.
Tips
- Measure the footprint at the drip edge, not the walls, so overhangs are included.
- Calculate roofs with different pitches as separate sections, then add them.
- A tear-off can reveal rotten decking that must be replaced before new shingles go on. Hold money back for it; the renovation contingency calculator suggests a reserve.
- If you are choosing between shingles and steel, compare with the metal roofing calculator.
- Plan gutters and attic vents at the same time with the gutter calculator and the attic ventilation calculator.
Common mistakes
- Using the floor area. A sloped roof has more surface than the house footprint.
- Leaving out overhangs. A 1 ft overhang all round adds noticeably to a small roof.
- Assuming three bundles per square for every product. Check the wrapper.
Frequently asked questions
How many squares is my roof?
Roof area ÷ 100. A 1,342 sq ft roof is 13.42 squares before waste.
How do I calculate roof area from the ground?
Measure the footprint including overhangs and find the pitch, then multiply the footprint by the pitch factor in the table.
How many bundles of shingles do I need?
Squares (with waste) × bundles per square from the wrapper. For 14.76 squares at three per square, 45 bundles.
How much waste should I allow?
Simple gable roofs need less than roofs with hips, valleys and dormers. Ten percent is a common planning figure for a simple roof; add more as the roof gets more complex.
Does a hip roof have more area than a gable roof?
Not if both have the same footprint and the same pitch on every plane. The area is the footprint × pitch factor either way, though a hip roof has more edges and ridges to finish.
What is a pitch factor?
The ratio of sloped length to horizontal run. Multiply any horizontal measurement by it to get the length along the roof.
Key takeaways
- Roof area = footprint (with overhangs) × pitch factor.
- One square = 100 sq ft.
- Bundles = squares with waste × bundles per square from the wrapper.
- Use the same method for each section of a roof with mixed pitches.