How to use the roofing calculator
A roofing order starts with one number: the true area of the roof surface, measured on the slope. This calculator gets there from the plan dimensions you already have on the drawings. It adds the overhang on every side, multiplies the plan area by the slope factor for the pitch, converts the result to roofing squares and then breaks it down into shingle bundles, underlayment, ridge cap, drip edge, starter strip and nails.
- Pick US or metric units and the roof type: gable (two slopes and two gable ends), hip (four slopes) or shed (one slope).
- Enter the building length and width at the wall line, and the overhang at the eaves and rakes. The ridge of a gable or hip roof runs along the length.
- Enter the pitch as rise per 12 or in degrees. If you only know rise and run, convert them first with the roof pitch calculator.
- Set the waste and bundles per square. Use about 10% for a gable and 15% for a hip roof as a starting estimate.
- Choose the underlayment and ridge cap coverage printed on the products you plan to use, then read the results.
Roofing formulas
Slope factor = √(1 + (rise ÷ 12)²)
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × slope factor
Squares = roof area ÷ 100 ft² (or ÷ 9.29 m²)
Bundles = ⌈squares × (1 + waste) × bundles per square⌉
Hip length = √(2 × run² + rise²), where run = half the width incl. overhang
The slope factor works for any plane at a single pitch, so it gives the right area for gable, hip and shed roofs alike. A hip roof has the same surface area as a gable roof of the same pitch and footprint; what changes is the edge lengths. A gable roof has two eaves and four rakes, a hip roof has four eaves, one shorter ridge and four hips, and a shed roof has one eave, one high edge and two rakes.
| Pitch | Angle | Slope factor | Area added |
|---|---|---|---|
| 3/12 | 14.0° | 1.031 | +3% |
| 4/12 | 18.4° | 1.054 | +5% |
| 6/12 | 26.6° | 1.118 | +12% |
| 8/12 | 33.7° | 1.202 | +20% |
| 10/12 | 39.8° | 1.302 | +30% |
| 12/12 | 45.0° | 1.414 | +41% |
Worked examples
A 40 × 30 ft gable roof at 6/12
With a 1 ft overhang the roof covers 42 × 32 = 1,344 ft² of plan. At 6/12 the slope factor is 1.118, so the roof surface is about 1,503 ft², or 15.03 squares. Adding 10% waste gives 16.5 squares, which is 50 bundles at three per square. The ridge is 42 ft long, so two bundles of ridge cap at 30 ft each will do, and the eaves and rakes need 84 ft and about 72 ft of drip edge.
The same house with a hip roof
The area stays at about 1,503 ft², but with 15% waste the order rises to 17.3 squares, or 52 bundles. The ridge shrinks to 42 − 32 = 10 ft, while each of the four hips runs √(2 × 16² + 8²) = 24 ft, so the ridge and hips together total about 106 ft and need 4 bundles of ridge cap. All 148 ft of perimeter is eave.
A metric shed roof at 15°
A 6 × 4 m garden studio with 0.3 m overhangs covers 6.6 × 4.6 = 30.4 m² of plan. At 15° the slope factor is 1 ÷ cos 15° = 1.035, so the roof is 31.4 m², or 3.4 squares. That pitch is just over 3/12, so shingles would need double underlayment, and many designers would choose a metal or membrane roof instead.
Who this calculator helps
- Architects and designers checking roof areas and material quantities for cost plans and specifications.
- Roofing contractors and estimators producing a quick first takeoff before a site measure.
- Builders and developers comparing the cost of gable and hip options at different pitches.
- Homeowners and self-builders checking a reroofing quote or ordering materials for a garage or shed.
- Students learning how pitch, plan area and edge lengths relate on a real roof.
Practical tips
- Measure the eaves and rakes on site if you can; overhangs on drawings often differ from what was built.
- Order starter strip for the eaves, and for the rakes too if the shingle maker calls for it.
- In cold climates, plan for an ice barrier at the eaves; IRC R905.1.2 calls for one extending at least 24 in inside the exterior wall line where ice dams are a known risk.
- Run L-shaped or cross-gabled roofs as separate rectangles and add 3–5% extra waste for each valley.
- Fold the material totals into an early budget with the construction cost calculator, and size the roof framing with the joist and rafter span calculator.
Weighing up shingle colours or a metal roof? Show the client options on a photo of the house with Exterior Redesign.
