Roofing Calculator

The formula

A roof estimate is three steps: get the true surface area, convert it to squares, then price each line separately.

1. Surface area. Measured on the ground, your footprint is smaller than the roof above it. Multiply by the pitch factor:

• Roof area = footprint × pitch multiplier
• 4/12 → 1.054 · 6/12 → 1.118 · 8/12 → 1.202 · 12/12 → 1.414

2. Squares. A roofing square is 100 sq ft. Squares = roof area ÷ 100. Everything in roofing is priced this way.

3. Waste. Cuts at valleys, hips and rakes are the losses. A simple gable runs 10%, a hip roof with dormers 15%, and a cut-up roof with multiple valleys 20% or more.

Bundles follow from squares: three bundles per square for standard asphalt, four for heavier designer shingles.

Worked example

An 1,800 sq ft footprint at 6/12, architectural shingles, moderate complexity, one layer to tear off.

1. Roof area = 1,800 × 1.118 = 2,012 sq ft 2. Squares = 2,012 ÷ 100 = 20.1 squares 3. With 15% waste = 23.2 squares to order 4. Bundles = 23.2 × 3 = 70 bundles 5. Material at $135/sq ≈ $3,130; labour at $215/sq × 1.15 ≈ $4,970 6. Tear-off at $55/sq ≈ $1,105, plus a dumpster

Accessories — underlayment, drip edge, starter, ridge cap, valley metal and nails — typically add 10 to 15% on top of the shingle price and are the line most often left out of a homeowner's own estimate.

Where the money actually goes

Labour is usually the largest single line on an asphalt re-roof, ahead of material. That surprises people who price shingles at the supply house and assume they now know the job.

Three things move labour hard:

Pitch. Anything above 6/12 stops being walkable. Above that the crew needs roof jacks, planks and fall protection, work slows, and material has to be carried rather than slid. Budget 20 to 30% more labour on a steep roof, and more again above 9/12.

Complexity. Every valley is two cut edges and a flashing detail. Every dormer is four. A simple rectangle with two planes and one ridge goes down fast; the same square footage split across nine planes takes half again as long and wastes far more material.

Height and access. A second storey with a tight side yard means a longer dumpster carry and possibly a conveyor. Three storeys usually means staging.

Tear-off, and what is underneath

Most jurisdictions allow two layers of asphalt shingles, and a few allow three. Roofing over saves the tear-off line, and it is almost always the wrong call. You cannot inspect the deck, you cannot install ice-and-water shield properly at the eaves, the added weight is real, the roof runs hotter and the new shingles wear faster, and the next re-roof costs more because two layers come off instead of one.

Assume some deck replacement. On a roof over 20 years old, or anywhere there has been a leak, a few sheets of OSB will need replacing at around $85 each installed. A good contract states a per-sheet price up front so it is not negotiated with the roof open.

Reading a quote

Ask any bid to break out squares, waste percentage, underlayment type, ice-and-water coverage, ventilation, and the deck-replacement rate. Two bids that differ by thousands usually differ on those lines — synthetic underlayment versus felt, ice-and-water at the eaves versus none, a proper ridge vent versus reusing the old one. A cheap roof is often a normal roof with the accessories deleted, and those are the parts that keep water out at the details where roofs actually fail.