National “average roof cost” numbers are not very useful, because your price depends on your roof size, pitch, material and location. Roofers estimate with a simple formula. Once you know it, you can check any quote in two minutes.
The roofer’s formula
Total cost = squares × installed price per square + extras
- Squares = roof area × (1 + waste %) ÷ 100. One square is 100 sq ft of roof surface, not floor area.
- Installed price per square includes shingles, underlayment, labor, standard flashing and cleanup. Get it from local quotes.
- Extras are the items that move the final number most.
Step 1: get your squares
| Home | Typical roof squares (with waste) |
|---|---|
| 1,000 sq ft, 1 story | 13 to 15 |
| 1,500 sq ft, 1 story | 19 to 22 |
| 2,000 sq ft, 1 story | 26 to 29 |
| 2,000 sq ft, 2 stories | 13 to 15 |
| 3,000 sq ft, 2 stories | 19 to 22 |
These assume a 4/12 to 8/12 pitch, a 12 inch overhang and 10% waste. For your real number, enter your house in the roof square footage calculator.
Step 2: multiply by a price per square
The table shows the math at a few example price points. Use the price from your own quotes.
| Squares | at $400 / square | at $550 / square | at $700 / square | at $900 / square |
|---|---|---|---|---|
| 15 | $6,000 | $8,250 | $10,500 | $13,500 |
| 20 | $8,000 | $11,000 | $14,000 | $18,000 |
| 25 | $10,000 | $13,750 | $17,500 | $22,500 |
| 30 | $12,000 | $16,500 | $21,000 | $27,000 |
Step 3: add the extras
| Extra | How it is usually priced |
|---|---|
| Extra layer of old shingles to tear off | Per square, per layer |
| Rotten or damaged decking | Per sheet (4 × 8) |
| Steep pitch (about 8/12 and up) | Per square surcharge |
| Second or third story | Per square or flat fee |
| Ice and water barrier | Per roll or per linear foot |
| Ridge vent, new flashing, pipe boots | Per item or per foot |
| Permit and disposal | Flat fee |
Decking is the big unknown on old roofs. Ask every bidder for their price per sheet so you can compare fairly.
Worked example
A 40 × 30 ft ranch, 12 inch overhang, 6/12 pitch, one layer to tear off.
- Roof area: 42 × 32 × 1.118 = 1,503 sq ft
- Squares with 10% waste: 16.53
- At $500 per square installed: 16.53 × 500 = $8,265
- Extras: 6 sheets of decking, ridge vent and permit, priced from the quote
Open this estimate in the calculator and type your own price per square to update the cost live.
How to read a roofing quote
- Check the squares. If a quote lists far more squares than your calculated number plus 10% to 15% waste, ask why.
- Compare the same scope. Underlayment type, ice barrier length, ventilation and drip edge should match across quotes.
- Look for per-sheet decking pricing and the number of sheets included.
- Confirm the warranty for both materials and workmanship.
Shingle quantities are explained in how many bundles of shingles in a square. Considering other materials? Compare metal roof replacement cost and flat roof replacement cost.