How to calculate roof area
roof area = (length + 2×overhang) × (width + 2×overhang) × pitch multiplier
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): 1.118
- Roof area: 1,344 × 1.118 = 1,503 sq ft
- Squares: 1,503 ÷ 100 = 15.03 → order 16 squares (add 5–10% waste)
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. Measure from the attic by holding a level against a rafter and measuring the drop 12 inches out.
| Pitch | Angle | Multiplier |
|---|---|---|
| 2/12 (Low slope) | 9.5° | 1.014× |
| 3/12 (Low slope) | 14° | 1.031× |
| 4/12 (Standard) | 18.4° | 1.054× |
| 5/12 (Standard) | 22.6° | 1.083× |
| 6/12 (Most common) | 26.6° | 1.118× |
| 7/12 (Steep) | 30.3° | 1.158× |
| 8/12 (Steep) | 33.7° | 1.202× |
| 9/12 (Steep) | 36.9° | 1.250× |
| 10/12 (Very steep) | 39.8° | 1.302× |
| 12/12 (45° pitch) | 45° | 1.414× |
Roofing materials comparison
| Material | $/square | Lifespan | Notes |
|---|---|---|---|
| 3-Tab Asphalt | $90–$100 | 20–25 yrs | Cheapest; easy install |
| Architectural Shingles | $100–$150 | 25–30 yrs | Best value; dimensional look |
| Metal (Standing Seam) | $300–$500 | 40–70 yrs | Long life; energy efficient |
| Clay Tile | $400–$600 | 50–100 yrs | Very long life; fireproof |
| Slate | $600–$1500 | 75–200 yrs | Longest life; premium look |
| Wood Shake | $350–$500 | 20–40 yrs | Natural look; good insulation |
Material cost only — add $150–$400/square for labor depending on material complexity. Asphalt is the cheapest and most common choice; architectural shingles offer the best value.
How many shingles do I need?
Standard asphalt shingles come 3 bundles per square. To calculate:
- Bundles: (roof area ÷ 100) × 3 bundles/square
- Waste factor: add 10% for simple gable roofs, 15% for hip or cut-up roofs
- Underlayment: 1 roll covers ~4 squares (400 sq ft). A 20-square roof = 5 rolls
- Ridge cap: 1 bundle per 35 linear feet of ridge and hip lines
- Nails: ~1.5 lbs per square (4 nails per shingle)
- Drip edge: measure all eaves and rakes; sold in 10-ft sections
Average roofing costs
National averages
- Asphalt (standard): $8,000–$15,000
- Architectural shingles: $10,000–$20,000
- Metal standing seam: $20,000–$40,000
- Clay tile: $25,000–$50,000
- Slate: $35,000–$100,000+
Based on a 2,000 sq ft roof, fully installed.
Add-ons to budget for
- Tear-off old roof: $100–$150/square
- Plywood decking replacement: $70–$100/sheet
- Flashing & chimney work: $300–$1,500
- Permits: $100–$500
- Dumpster rental: $300–$600
- Gutters replacement: $1,000–$3,000
When to replace vs repair
Signs you need a full replacement:
- Roof is 20+ years old (asphalt shingles)
- 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. If repairs will cost more than 30% of a replacement, just replace.
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. Small residential reroofs range from 15–30 squares; larger homes can be 35–50 squares.
Frequently asked questions
How do I calculate my roof size?
Measure the building footprint (length × width in feet), then multiply by the roof's pitch factor to get the actual sloped area. For a 40×30 ft house with a 6/12 pitch: 40 × 30 = 1,200 sq ft footprint × 1.118 (pitch multiplier) = 1,341 sq ft of roof. Add for overhangs (typically 1 ft on each side) and 5% extra for hip roofs. Divide by 100 to get 'squares' — the unit roofing materials are sold in.
How many bundles of shingles do I need?
For standard 3-tab or architectural shingles, 3 bundles = 1 square (100 sq ft). For a 2,000 sq ft roof: 20 squares × 3 = 60 bundles. Add 10–15% for waste (cuts, starter course, ridge cap). Total order: 66–69 bundles. Ridge cap shingles are separate — plan 1 bundle per 35 linear feet of ridge/hip. Underlayment is another addition — 1 roll per 4 squares.
What is a roofing square?
A roofing square equals 100 square feet (10 ft × 10 ft) of roof area. It's the industry-standard unit for pricing and ordering roofing materials. A typical US house roof ranges from 15 to 30 squares (1,500–3,000 sq ft). When a contractor quotes '$400 per square,' that's $4/sq ft. This doesn't include tear-off, underlayment, or flashing — ask what's included in the per-square price.
How much does a new roof cost?
The US national average for a complete asphalt shingle reroof is $8,000–$15,000 for a typical 2,000 sq ft roof. Mid-grade architectural shingles run $4–$7 per sq ft installed. Premium materials: metal $12–$25/sq ft, tile $15–$30/sq ft, slate $30–$75/sq ft. Tear-off of the old roof adds $100–$150 per square. Permits, dumpster rental, and flashing/drip edge typically add $500–$1,500 to the project.
How does roof pitch affect cost?
Steeper roofs cost more because: (1) they have more actual surface area per square foot of footprint — a 12/12 pitch adds 41% more area than a flat roof; (2) labor is slower and requires safety harnesses; (3) very steep roofs (10/12+) often command a 'steep charge' of 20–50% above base labor rates. A 6/12 pitch roof on the same house will cost roughly $2,000–$4,000 more than a 3/12 pitch.
How long does a roof last?
Lifespan depends on material: 3-tab asphalt 20–25 years, architectural shingles 25–30 years, metal roofing 40–70 years, clay tile 50–100 years, slate 75–200 years, wood shake 20–40 years. Climate matters — hot sun, freeze-thaw cycles, and hail all shorten life. Proper attic ventilation can extend shingle life by 20–30%. Most residential roofs in the US need replacement at 20–25 years.
What is the cheapest roofing material?
3-tab asphalt shingles are the cheapest at $90–$100 per square for materials. Installed with labor, expect $3.50–$5.50 per sq ft total. Rolled roofing (asphalt roll) is cheaper per square but only suitable for low-slope or utility buildings — it lasts just 5–10 years. Architectural shingles cost 10–30% more than 3-tab but last longer and look better — they're usually the best value overall.
How do I measure roof pitch?
From inside the attic, hold a level against a rafter and measure vertically down 12 inches from the level's end to the rafter — that drop in inches is the rise. A 6-inch drop over 12 inches of run = 6/12 pitch. From outside, use a level and tape on a gable end or against a shingle. Alternatively, a smartphone app with a tilt sensor can measure pitch angle directly.
Should I tear off the old roof or go over it?
Tear-off is almost always better. Pros of tear-off: inspect decking for rot, proper underlayment install, full manufacturer warranty, no weight concerns. Cons: $100–$150 per square more. Overlay (second layer) is allowed by code in most areas but: adds weight, traps heat, shortens new shingle life, hides problems, and most manufacturers void warranty on a third layer. If the existing roof has any leaks or soft spots, always tear off.
How many squares is a typical house roof?
A typical US single-family home has a 15–25 square roof (1,500–2,500 sq ft). Breakdown by house size: 1,200 sq ft ranch ~18 squares, 2,000 sq ft two-story ~22 squares, 2,500 sq ft house with garage ~28 squares, 3,500+ sq ft large home ~35–45 squares. Roof size is usually 10–30% larger than living area because of pitch, overhangs, and multi-level roofs.