How to calculate roof area
roof area = (length + 2×overhang) × (width + 2×overhang) × pitch multiplier
pitch multiplier = √(1 + (rise ÷ 12)²)
squares = roof area ÷ 100
Example: 40×30 ft gable roof, 6/12 pitch, 1-ft overhangs
- Footprint w/ overhangs: 42 × 32 = 1,344 sq ft
- Pitch multiplier (6/12): √(1 + 0.5²) = 1.1180
- Roof area: 1,344 × 1.1180 = 1,503 sq ft
- Squares: 1,503 ÷ 100 = 15.03; with a 10% waste allowance (a convention, not a rule), order 17 squares
Roof pitch explained
Pitch is expressed as rise over run, the vertical rise in inches for every 12 inches of horizontal run. A "6/12" pitch means 6 inches of rise for each foot of run. The steeper the pitch, the more actual roof area per square foot of footprint. A roof whose planes all share one pitch, gable or hip, has the same area: footprint × multiplier. A gambrel or mansard has two pitches, so work each pitch's share of the footprint separately and add them. Measure from the attic by holding a level against a rafter and measuring the drop 12 inches out.
| Pitch | Angle | Multiplier |
|---|---|---|
| 2/12 (Low conventional) | 9.5° | 1.014× |
| 3/12 (Low conventional) | 14.0° | 1.031× |
| 4/12 (Standard residential) | 18.4° | 1.054× |
| 5/12 (Standard residential) | 22.6° | 1.083× |
| 6/12 (Standard residential) | 26.6° | 1.118× |
| 7/12 (Standard residential) | 30.3° | 1.158× |
| 8/12 (Steep) | 33.7° | 1.202× |
| 9/12 (Steep) | 36.9° | 1.250× |
| 10/12 (Steep) | 39.8° | 1.302× |
| 12/12 (Very steep / dramatic) | 45.0° | 1.414× |
Roofing materials comparison
| Material | Sold by | Notes |
|---|---|---|
| 3-Tab Asphalt | Bundle (typically 3 per square) | Usually the lowest-priced shingle; flat look |
| Architectural Shingles | Bundle (typically 3 per square) | Dimensional look; heavier than 3-tab |
| Metal (Standing Seam) | Square, panel or piece | Sold as panels or sheets |
| Clay Tile | Square, panel or piece | Heavy; framing must be able to carry it |
| Slate | Square, panel or piece | Very heavy; framing must be able to carry it |
| Wood Shake | Square, panel or piece | Natural look; untreated wood burns and rots |
This page lists no prices or lifespans: no official source publishes them, and they vary by region, product and installer. The manufacturer's data sheet and warranty give a product's figures, and local quotes give the prices to type into the Cost tab.
How many shingles do I need?
The figures below are typical trade and packaging figures, not official ones. The label on your product governs. To calculate:
- Bundles: (roof area ÷ 100) × bundles per square, typically 3 for asphalt shingles
- Waste allowance: a convention you choose; roofers commonly allow more for hip and cut-up roofs than for a plain gable
- Underlayment: a felt roll typically covers about 4 squares; synthetic rolls often cover more
- Ridge cap: typically about 1 bundle per 35 linear feet of ridge and hip lines
- Nails: typically about 1.5 lb per square; the shingle maker's nailing pattern sets the count
- Drip edge: measure all eaves and rakes and buy by the section length your supplier stocks
What a roofing quote covers
No government source publishes roofing prices, so this page quotes none. Ask each roofer to itemize, then enter their figures in the Cost tab. Lines that often appear on a reroof quote:
- Material, per square
- Labor, per square, sometimes with a surcharge for steep or high roofs
- Tear-off and disposal of the old roof, per square, and a dumpster
- Replacement of rotten decking, usually per sheet
- Flashing, chimney and skylight work, drip edge and vents
- The building permit
When to replace vs repair
Signs you need a full replacement:
- Shingles near the end of the manufacturer's stated life
- Widespread curling, buckling, or cupping shingles
- Heavy granule loss, bald spots or lots of granules in gutters
- Multiple leaks or stained ceilings in different areas
- Sagging rooflines (structural issue, get professional help)
- Daylight visible through attic boards
Repair makes sense for isolated damage, a few missing shingles, a single leak, or storm damage affecting one slope. When repair quotes start to approach the price of a new roof, compare the two with a roofer.
Roofing squares explained
One roofing square equals 100 square feet of roof area (10 ft × 10 ft). Nearly all roofing materials, shingle bundles, underlayment, metal panels, are sized and priced by the square. A 2,400 sq ft roof = 24 squares. Contractor quotes are typically given as "price per square" for both material and labor.
Frequently asked questions
How do I calculate my roof size?
Take the outside length and width of the house, add the overhang on every side, and multiply to get the plan area. Then scale it up for the slope with the pitch multiplier, √(1 + (rise ÷ 12)²). With no overhang, a 40 by 30 ft house under a 6/12 roof has 1,200 sq ft of plan and about 1,342 sq ft of roof. Roofers count in squares of 100 sq ft, so that is 13.4 squares before any waste.
How many bundles of shingles do I need?
Asphalt shingles usually come 3 bundles to a square, but the wrapper on your product is what counts. For 2,000 sq ft of roof with a 10% waste allowance you would order 66 bundles. Cap shingles for ridges and hips are bought separately; a bundle typically covers about 35 ft. A roll of felt underlayment typically covers about 4 squares, and synthetic rolls often cover more.
What is a roofing square?
It is the trade's unit of roof surface: 100 sq ft, a 10 by 10 ft patch. Shingles, underlayment and most contractor quotes are counted per square. To compare a per-square quote with a per-foot one, divide the per-square price by 100. Ask whether the quoted figure covers tear-off, underlayment and flashing or only the shingles.
How much does a new roof cost?
No government source publishes roofing prices, and they swing widely with region, material, access and season, so this page does not quote any. Get written quotes from local roofers, then type their per-square material and labor prices, any tear-off rate and lump sums such as permits into the Cost tab to see the total and the price per square foot side by side.
How does roof pitch affect cost?
Pitch changes how much surface there is to cover. A 12/12 roof has 41% more surface than its footprint, and a 6/12 roof has 8.5% more than a 3/12 roof over the same house. Steep roofs are also slower and need more fall protection, which roofers usually price into labor.
How long does a roof last?
It depends on the material, the installation, attic ventilation and the weather it faces: sun, hail and freeze-thaw all shorten it. The manufacturer's warranty states what it covers and for how long, and an inspection tells you more about a particular roof than its age does.
What is the cheapest roofing material?
Three-tab asphalt shingles usually have the lowest material price per square, with architectural shingles next. Roll roofing is cheaper still but is meant for low-slope and outbuilding roofs. Compare quotes for the same roof area rather than list prices, since labor and tear-off can outweigh the material difference.
Should I tear off the old roof or go over it?
Tearing off lets the roofer inspect and repair the decking, install new underlayment properly and keep the shingle warranty straightforward. Laying a new layer over the old one saves the tear-off charge but adds weight and hides any damage. Your local building code limits how many layers are allowed, and a leaking or soft roof should be stripped.
How many squares is a typical house roof?
There is no typical figure: footprint, overhang and pitch all change it. As one worked case, a 50 by 30 ft hip roof at 5/12 with 1 ft overhangs comes to about 1,803 sq ft, or 18.0 squares. Put your own measurements into the Area tab.