Formula
Roof area is the footprint times the pitch multiplier, the same hypotenuse as the roofing calculator, plus any extra planes. A square is 100 square feet. Material squares round up to a whole bundle, then divide back by the bundles per square, so the shingle order is a whole number of bundles. Labor and tear-off use the measured squares, with no waste. Gable rake length on this page is the span times the multiplier, which is the two slopes of one gable end, matching the worked example. The roofing takeoff counts both gable ends, twice that length. Edit the drip-edge length if your measurement includes both ends. Hip ridge and hip lengths are the roofing calculator’s lengths.
area = footprint × √(1 + (rise/12)²)
bundles = ceil(squares × (1 + waste) × bundles per square)
tax = (shingles + underlayment + ridge + drip) × rate
What moves a bid is the material, the pitch, the cuts, how many layers come off, access, local labor, the permit, and whether the deck needs boards. Two quotes differ when those lines differ. Walk a quote against this table and ask what is missing: decking allowance, underlayment, flashing, drip edge, disposal, warranty, and the permit.
Worked examples
40 × 30 ft gable, 6/12, 10 percent waste, example prices. 13.42 squares, 45 bundles, 15.0 squares of shingles. Shingles $1,800, underlayment $375, labor $2,683.28, tear-off $804.98, disposal $450, ridge $160, drip edge $283.85, flashing $225, permit $150. Total about $7,798.
25 squares entered directly, no waste. 75 bundles.
The prices in that example are test inputs. They are not a cost survey.
Reference
A flat roof, zero rise, has a multiplier of 1. The area is the footprint.
Questions
How do I calculate the cost to replace a roof?
Turn the footprint and the pitch into squares, round the shingles up to whole bundles, then multiply each line by the price you were quoted. Labor and tear-off use the measured squares. Shingles use the squares after waste.
How many squares is my roof?
A square is 100 square feet of roof surface, not of the house footprint. A 40 by 30 foot footprint at 6/12 is about 13.4 squares before waste.
How much waste should I add?
Ten percent is the opening allowance for a simple gable, and 15 percent for a hip. Both are planning allowances. The bundle count on the package still wins. GAF packages Timberline HDZ at 3 bundles per square.
Does the roof pitch change the cost?
It changes the area. A steeper roof has more surface on the same footprint, and it can change how the crew works. This page only changes the area. It does not add a steep-slope adder unless you type one into the labor price.
What is included in a roof replacement quote?
Ask whether the quote replaces the underlayment, the flashing, the drip edge, and the ridge cap, whether it tears off every layer, whether disposal and the permit are in the number, and what happens if the deck needs boards. Compare those lines with the table here.
How much does tear-off add?
Tear-off is the measured squares times the layers times the price per square you type. One layer on the example roof is the squares times that rate. A second layer doubles that line. Decking repair is not in the line.
Should I get more than one quote?
Yes. Two bids can differ because of access, the deck, the layers, and what is included. This total is a planning estimate from your prices, and the band around it is the uncertainty percent you set.
Sources
- The pitch multiplier is the hypotenuse of 1 and rise/12, shared with the roof pitch calculator and the roofing calculator on this site.
- GAF, “How many bundles of shingles are in a square.” Most standard asphalt shingles, 3-tab and architectural, are three bundles per square, and a square is 100 square feet. The Timberline HDZ data sheet lists 3 bundles per square. Some specialty shingles take more. The package wins.
- Waste of 10 percent on a gable and 15 percent on a hip is a planning allowance on this page, not a figure from that GAF page. No cost survey is cited, because the prices are yours.