How we worked it out
Updated as you change the numbers.
- Footprint = 40 ft × 30 ft = 1,200 sq ft.
- Pitch factor for 6/12 = √(1 + (6 ÷ 12)²) = 1.118.
- Roof area = 1,200 × 1.118 = 1,341.6 sq ft.
- Squares = 1,341.6 × 1.1 ÷ 100 = 14.76.
- Bundles = 14.76 squares × 3 = 44.27, rounded up to 45.
Method and assumptions
A sloped roof has more area than the ground it covers. For a roof whose planes all have the same pitch, gable or hip, the roof area is the footprint (including overhangs) times the pitch factor √(1 + (rise ÷ 12)²). That is the length of the rafter for each foot of level run.
Roofing is sold in squares of 100 sq ft. The calculator adds your waste allowance, converts to squares and multiplies by the bundles per square, rounding up to whole bundles.
Starter strips, hip and ridge caps, underlayment, drip edge and flashing are sold separately and are not included.
Tips
- To find the pitch, hold a level horizontally against a rafter or the roof edge, measure 12 in along it, and measure straight up to the roof. That height is the rise.
- If parts of the roof have different pitches, work them out separately and add them up.
- Roof work is dangerous. For anything beyond a small repair, use a licensed roofer.
Questions
How many bundles of shingles do I need for a 2,000 sq ft roof?
2,000 sq ft is 20 squares. With 10% waste that is 22 squares, and at 3 bundles per square, 66 bundles.
What is a 6/12 roof pitch in degrees?
About 26.6°. The roof rises 6 in for every 12 in across, and its pitch factor is 1.118, so it has about 12% more area than its footprint.
Sources
- GAF: shingle installation instructions (slope limits and coverage)
- International Residential Code, R905.2: asphalt shingles
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.