What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Building length (footprint): 40 ft.
- Building width (footprint): 30 ft.
- Eave and rake overhang: 12 in.
- Roof pitch (rise per 12 of run): 6 /12.
- Roof type: Gable (two planes).
- Waste allowance: 10 %. About 10% for simple gables; 15% or more for hips, valleys and dormers.
- Bundles per square: 3 (most architectural shingles).
What happens, step by step
- Underlayment rolls over both roof planes first, from the eaves up.
- Drip edge trims the eaves and rakes, then shingles start at the bottom edge.
- A square is 100 sq ft of roof; 3 bundles cover one square.
- Ridge cap finishes the peak: 42 ft of ridge.
The answer: squares to buy (with waste) is 16.53 squares.
How it’s calculated
- Slope factor = √(1 + (pitch ÷ 12)²)
- Roof area = (length + 2 × overhang) × (width + 2 × overhang) × slope factor
- 1 roofing square = 100 sq ft
- Hip length = (width ÷ 2) × √(2 + (pitch ÷ 12)²); hip-roof ridge = length − width
- Gable drip edge = 2 eaves + 4 rakes; hip drip edge = perimeter
Worked example
40 × 30 ft footprint, no overhang, 6/12 gable roof, 10% waste.
Inputs
- Building length (footprint)
- 40 ft
- Building width (footprint)
- 30 ft
- Eave and rake overhang
- 0 in
- Roof pitch (rise per 12 of run)
- 6 /12
- Roof type
- Gable (two planes)
- Waste allowance
- 10 %
- Bundles per square
- 3 (most architectural shingles)
- Underlayment coverage per roll
- 1000 sq ft
- Ridge cap coverage per bundle
- 30 lin ft
Results
- Squares to buy (with waste)
- 14.76 squares
- Roof area
- 1,341.6 sq ft
- Shingle bundles
- 45
- Underlayment rolls
- 2
- Ridge + hip length
- 40 ft
- Ridge cap bundles
- 2
- Drip edge length
- 147.1 ft
- Drip edge pieces (10 ft)
- 15
- Slope factor
- 1.118
Assumptions and limits
- Rectangular footprint with all roof planes at the same pitch.
- Dormers, valleys, skylights and chimneys are not modelled — add waste for them.
- Hip roof ridge assumes equal pitch on all four sides.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
What is a roofing square?
One roofing square covers 100 sq ft of roof surface. Most architectural shingles come 3 bundles per square.
How do I measure a roof without climbing on it?
Measure the building footprint on the ground, add the overhang on each side, and multiply by the slope factor for your pitch. This works for any roof where all planes share one pitch.
How much waste for shingles?
About 10% for a simple gable. Hip roofs, valleys and dormers create more cutting, so 15% or more is common.