Understanding your result
The headline is the roof surface area, with the squares and the squares to order beneath it. The stats show the squares, the pitch factor, the footprint the calculation started from and the squares with waste. The breakdown is a chain: footprint with overhangs, pitch factor, main roof area, dormers, total, squares, the ratio of roof to footprint, and the waste suggestion for the shape you chose.
The number to remember is the roof area: it is the input to the shingle calculator, the underlayment calculator, the tile, slate, shake and metal calculators and every cost calculator on the site. The row “roof area per unit of footprint” is the one to quote when someone asks why the roofer’s squares do not match the house’s square footage.
The waste suggestion is a starting point. Ten percent for a gable, 15 for a hip and 20 for a cut-up roof are the figures estimators use; the waste guide explains where each comes from and when to depart from them.
How we calculate this
The multiplication by the pitch factor works because every plane of a roof with one pitch is tilted by the same angle, and the area of a tilted plane is its horizontal projection divided by the cosine of the angle, which is exactly the pitch factor. It does not matter whether the roof is a gable, a hip or an L-shape: as long as every plane has the same pitch, the sloped area is the footprint times the factor. A roof with two pitches (a 4/12 porch on a 6/12 house) must be run as two footprints, and a dormer, whose faces are small and whose cheeks are vertical, is easier to add as a lump sum.
The pitch calculator converts a measured rise and run or an angle into the pitch this page needs, and the pitch factor table lists the factor for every pitch from 1/12 to 24/12.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Overhang | 12 in on all sides | Common US eave overhang; measure yours (rakes are often shorter) |
| Pitch | 6/12 | A typical US house pitch; measure yours with a level or the pitch calculator |
| One pitch for all planes | yes | The pitch factor method; run other pitches separately |
| Roof area per dormer | 40 sq ft | A small gable dormer (two faces plus cheeks); set yours |
| Waste by shape | 10% gable, 15% hip, 20% cut-up | Roofing estimating practice; see the waste guide |
| Square | 100 sq ft | The roofing industry’s unit of coverage |
Assumptions last reviewed October 7, 2026.
The calculator does not subtract chimneys and skylights (their area is small and the shingles around them are cut anyway), does not model a roof whose planes have different pitches, and does not measure anything: the footprint and the pitch come from you. The guide to measuring a roof from the ground covers the tape, the level and the satellite view, and why a roof is bigger than the house shows the arithmetic for several house shapes.
Two worked examples
A ranch house measured at the walls
A 50 by 32 foot house, 12-inch overhangs, 6/12 pitch, plain gable, no dormers.
- Footprint with overhangs: 52 × 34 = 1,768 sq ft
- Pitch factor: √(1 + (6 ÷ 12)²) = 1.118
- Roof area: 1,768 × 1.118 = 1,977 sq ft, 19.77 squares
- Roof per unit of footprint: 1.118, 12% more than the footprint
- Suggested waste for a gable: 10%, so 21.74 squares to order
Against the 1,600 square feet of floor, the roof is 24 percent bigger: 10.5 percent from the overhangs and 11.8 from the pitch.
A hip roof from a footprint area with two dormers
A 2,000 square foot footprint (including overhangs) at 8/12, hip roof, two gable dormers at 40 square feet each.
- Pitch factor at 8/12: 1.2019
- Main roof: 2,000 × 1.2019 = 2,404 sq ft
- Dormers: 2 × 40 = 80 sq ft
- Roof area: 2,484 sq ft, 24.84 squares
- Suggested waste for a hip: 15%, so 28.56 squares to order
The 8/12 pitch alone adds 20 percent to the footprint; the dormers add another 3. Twenty-nine squares of shingles against a footprint that a floor-plan estimate would have called 20.
Where to find your inputs
Length and width. Outside of the walls, measured with a tape at ground level on the eave side and the gable side. For an L-shaped house, measure the two rectangles separately and add the footprints, remembering that the inner corner is only roofed once.
Overhang. Under the eave, horizontally from the wall to the drip edge. If the rake overhang differs from the eave overhang, use the eave figure for the long sides; the difference on the short sides is a few square feet.
Pitch. From a level and a ruler on a rake board, from the plans, or from the pitch calculator.
Dormers. Count them from the street. A small gable dormer is about 40 square feet of roofing; measure a large shed dormer as its own rectangle and add it as extra area.
Common mistakes
- Using the living area. The roof covers the footprint, overhangs included, not the heated square footage. A 2,000 square foot two-storey house may have a 1,000 square foot footprint.
- Forgetting the overhangs. They add 10 to 20 percent to the footprint on an ordinary house and far more on a small building.
- Skipping the pitch factor. The floor plan is the horizontal projection. At 6/12 the roof is 12 percent bigger; at 12/12, 41 percent.
- Counting a hip roof as bigger. Same footprint, same pitch, same area. The difference is waste and cap, not area.
- One pitch for a mixed roof. A shallow porch roof or a steep front gable needs its own factor. Run them separately and add.
- Measuring a roof plane by eye from a satellite image. Satellite tools measure horizontal projections; the slope still has to be applied unless the tool says it already has.
Questions people ask
- How do I calculate roof area from the square footage of a house?
- Start with the footprint, not the living area: the length and width of the building at the eaves, with the overhangs added to each. Multiply for the footprint area, then multiply by the pitch factor, √(1 + (rise ÷ 12)²). A 50 by 32 foot house with 12-inch overhangs has a 1,768 square foot footprint, and at 6/12 the roof is 1,768 × 1.118 = 1,977 square feet. A two-storey house uses the footprint of the top floor only.
- How many squares is a 2,000 square foot roof?
- A square is 100 square feet of roof surface, so 2,000 square feet of roof is 20 squares. If 2,000 square feet is the house footprint rather than the roof, the roof is larger: about 22.4 squares at 6/12 and 24 at 8/12, before overhangs. Add 10 to 20 percent for waste before buying shingles.
- Is a hip roof bigger than a gable roof?
- Not for the same footprint and pitch. The triangular hip ends replace the triangular gable walls, and every plane is tilted by the same pitch, so the area is identical. What differs is the waste: cutting every course along four hips wastes about 15 percent against 10 for a plain gable, and a hip roof has more ridge and hip length to cap.
- How much do dormers add to roof area?
- A small gable dormer adds its two roof faces and its two cheek walls, roughly 40 square feet of roofing and siding for a 4-foot-wide dormer; a shed dormer over a bedroom can add 100 square feet or more. The calculator adds a per-dormer figure you can set, since dormers vary more than anything else on a roof.
- Does the overhang really matter?
- Yes. Twelve inches on every side adds 2 feet to each dimension: a 50 by 32 house becomes 52 by 34, and the footprint grows from 1,600 to 1,768 square feet, 10.5 percent. On a 24-inch overhang it is 22 percent. The overhang is roofed and shingled like the rest, so it belongs in the footprint.
- What is the pitch factor for roof area?
- The pitch factor is the ratio of sloped to horizontal length, √(1 + (rise ÷ 12)²): 1.054 at 4/12, 1.118 at 6/12, 1.202 at 8/12, 1.414 at 12/12. Multiplying the horizontal footprint by it gives the sloped area of every plane at once, provided they share one pitch. The pitch factor table lists every common pitch.