How to estimate a roof replacement
Roofing is priced per square, and every supplier, sub and competitor talks in that unit. The number itself is easy. The two places estimates go wrong are before it (measuring the roof off the ground and forgetting the pitch) and after it (pricing from a per-square rule of thumb that does not know your crew, your dump or your overhead). This page fixes both.
1. Footprint is not roof area
You can measure a footprint from the ground or a satellite view. The roof is bigger, because it slopes. The multiplier is pure geometry: the square root of rise² + 12², divided by 12.
| Pitch | Multiplier | Area added | Note |
|---|---|---|---|
| 3/12 | 1.031 | +3% | Low slope |
| 4/12 | 1.054 | +5% | |
| 6/12 | 1.118 | +12% | Typical suburban |
| 8/12 | 1.202 | +20% | Steep: harness and jacks |
| 10/12 | 1.302 | +30% | |
| 12/12 | 1.414 | +41% |
2. Waste is real, and it depends on the shape
Cuts at hips, valleys, rakes and around penetrations throw shingle away. Ten percent on a simple gable, fifteen on anything with hips and valleys, more on a cut-up roof with dormers. Squares to order = roof squares × (1 + waste), rounded up to whole squares.
3. Tear-off: the layer count is the biggest surprise on the job
A second layer doubles the tear-off squares and the tonnage. Architectural asphalt runs about 250 pounds per square per layer, so a 19-square roof with two layers is 38 tear-off squares and nearly five tons: two dumpsters, not one, and an extra half a crew-day. Always count the layers at the edge before you quote.
4. Crew-days, not hours
Roofing crews work in days. Your production rates, squares installed and squares torn off per crew-day, are the two numbers that convert the roof into labour. Time three of your own roofs and correct the defaults; everything about the estimate depends on them. Then labour is crew-days × crew cost per day, where crew cost is people × loaded rate × hours. At $22 an hour with 35% burden (roofing comp is the highest of any trade) a four-person crew is $950 a day.
5. Overhead per crew-day
Monthly overhead divided by billable crew-days per month. $4,200 ÷ 16 = $262.50 on every day the crew is on a roof. Left out, this is the difference between the margin on the quote and the margin on the tax return.
Worked example
48 × 32 house with 180 sq ft of dormers, 6/12 pitch, two layers to tear off, four-person crew.
| Roof squares 19.18, order 22 with 12% waste | |
| Tear-off 38.4 squares, 4.8 tons, 2 dumpsters | $850 |
| Materials: 22 × $138 + $1,070 edge, ridge, ice & water | $4,106 |
| Crew-days: 1.28 tear-off + 0.96 install = 2.24 × $950 | $2,127 |
| Equipment 2.24 days × 8 hrs × $18 | $322 |
| Permit | $250 |
| Overhead 2.24 × $262.50 | $588 |
| Total job cost | $8,243 |
|---|---|
| Quote at 35% margin, rounded up to $25 | $12,700 |
| Price per square / cost per square | $662 / $430 |
A "$550 a square" rule-of-thumb quote on this roof is $10,550, which is a 21.9% margin: just under your 22% minimum, with nothing left for the rotten decking you find on Tuesday.
Common mistakes
- Quoting off the footprint. Two squares on a 6/12, four on a 10/12. It is the most common leak in residential roofing estimates.
- One dumpster by habit. Count layers, do the tonnage, round up.
- Per-square price copied from a forum. It does not know your comp rate, your dump, or that your crew installs 20 a day and not 30.
- No decking allowance. State a per-sheet price for replacement decking in the quote. Never absorb it.
Frequently asked questions
How do you calculate roof squares from the footprint?
Footprint × pitch multiplier ÷ 100. 1,716 sq ft at 6/12 is 19.2 squares. Add 10% to 15% waste for squares to order.
What is the pitch multiplier?
√(rise² + 144) ÷ 12. 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 12/12 = 1.414.
How much should a roofer charge per square?
Price from cost and divide by one minus the margin. On typical numbers an architectural tear-off and replace lands $550 to $750 per square; a second layer adds $60 to $100.
How many dumpsters does a roof tear-off need?
Squares × layers × 250 lb ÷ 2,000 = tons; divide by the dumpster's tonnage and round up. A 19-square two-layer roof is about 4.8 tons, two 3-ton dumpsters.
Every default in the calculator is a placeholder. Material prices, comp rates, production rates and dumpster costs vary by region and by crew. Replace them with your own numbers. This tool does not provide legal, tax, insurance or accounting advice.