What Goes Into the Cost of a New Roof?
Roofs are priced by the square
A roofing square is 100 sq ft of roof surface, and almost everything on a roofing bid is quoted per square: shingles come three bundles to the square, labour is bid per square, tear-off and disposal per square per layer. The square count comes from the house footprint (including overhangs) multiplied by the slope factor for the pitch — a 42 × 32 ft footprint at 6/12 is 1,344 × 1.118 = 1,503 sq ft, or 15 squares of actual roof and about 16.5 squares of material once 10% waste is added.
The eight lines
| Line | Priced by | What it covers |
|---|---|---|
| Tear-off and disposal | square × layers | Stripping old roofing, dumpster, tipping fees |
| Decking repair | per sheet | Rotten plywood found after tear-off |
| Underlayment and accessories | square (with waste) | Synthetic felt, ice and water shield, starter strip, ridge cap, nails |
| Roofing material | square (with waste) | Shingles, metal panels or tile |
| Flashing and penetrations | flat or per item | Drip edge, step and valley flashing, pipe boots, vents |
| Installation labour | square × factors | The crew, scaled for pitch, height and complexity |
| Permit | flat | Where required; often includes an inspection |
| Contingency | percentage | What tear-off reveals |
Where the money goes on a typical shingle roof
Using the placeholder prices in the roof replacement cost estimator on that 15-square roof, before contingency:
| Line | Share of subtotal |
|---|---|
| Installation labour | 37% |
| Shingles | 26% |
| Tear-off and disposal, one layer | 18% |
| Flashing, drip edge, vents | 7% |
| Underlayment and accessories | 7% |
| Decking allowance | 4% |
The proportions are the useful part, not the dollar figures — your prices will differ, but the shape holds: labour and tear-off together are more than half, shingles about a quarter. It follows that a premium shingle is a smaller upgrade than it looks (it changes a quarter of the bill), and that the labour factors — pitch, storeys, complexity — are where two bids for the same house part ways.
What raises the price per square
- Pitch. Steeper than about 7/12 the crew needs roof jacks or harnesses and slows down; above 9/12 expect staging. Pitch also adds area: a 12/12 roof is 41% more surface than its footprint.
- Height. Two and three storeys mean more ladder time, more staging and slower material handling.
- Complexity. Hips, valleys and dormers waste shingles (15–20% instead of 10%) and every cut course takes longer.
- Layers. A second old layer doubles tear-off and disposal.
- Access. A drive the dumpster and boom truck can reach saves hours of carrying.
- Decking. Unknown until the old roof is off; ask how extra sheets are priced.
Reading a roofing bid
Ask every bidder for the same lines: squares, material by brand and grade, underlayment and ice shield extent, tear-off layers, decking price per sheet, flashing items, ventilation, permit, and warranty on labour separately from the manufacturer’s. A bid that is one number is not comparable to anything. Run your own figures through the estimator first, so you know which line a surprising bid is surprising on.
Turn a house footprint and pitch into roofing squares, then into an itemised re-roof estimate — tear-off, shingles, underlayment and accessories, labour, flashing, decking repairs and contingency — priced with numbers you collect locally rather than a national average.
Frequently asked questions
What is the biggest cost in replacing a roof?
Labour. Installation and tear-off together are usually more than half of a shingle roof bill, with the shingles themselves about a quarter and underlayment, flashing, decking and disposal making up the rest. That is why pitch, height and complexity, which all slow the crew, move the price more than the shingle grade does.
What is a roofing square?
One hundred square feet of roof surface. Shingles are sold three bundles to the square, and roofers quote labour, tear-off and disposal per square. A 1,500 square foot roof is 15 squares, and you order about 16 to 17 squares of material to cover waste at hips, valleys and rakes.
Why is a roof quoted per square instead of per square foot of house?
Because the roof is bigger than the house. The slope factor for the pitch adds 12 percent at 6/12 and 41 percent at 12/12, and overhangs add more. Two houses with the same floor area can have very different roof areas, so the trade prices the roof surface it actually has to cover.
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