Roofing Calculator

Tip: do not measure on the roof if you can avoid it. Measure the house footprint from the ground, add the overhangs, and let the pitch factor do the rest.

Worked example

A simple gable roof covers a footprint of 50 ft by 30 ft including the overhangs, which is 1,500 square feet. The roof rises 6 inches for every 12 inches of run, written as 6/12.

The pitch factor for 6/12 is the square root of 1 + 0.5 squared, or 1.118. The actual roof surface is 1,500 x 1.118 = 1,677 square feet. Adding 10% for waste and dividing by 100 gives 18.45 squares.

Most architectural shingles come three bundles to a square, so you need 18.45 x 3 = 55.3, which rounds up to 56 bundles. You would also need two rolls of synthetic underlayment and 16 pieces of 10 ft drip edge for the 160 ft perimeter.

What is a roofing square?

Roofers measure in squares. One square is 100 square feet of roof surface. Shingles are priced and sold per square, though you buy them in bundles. Most three-tab and architectural shingles take three bundles per square. Some heavier designer shingles take four or five, so check the wrapper.

Because a sloped roof is bigger than the ground it covers, you cannot use the house's floor area directly. You need the roof's actual surface area, which is where pitch comes in.

Pitch factors

Pitch is the rise in inches for every 12 inches of horizontal run. A steeper roof has more surface over the same footprint. Multiply the footprint by the pitch factor to get the surface area:

• 3/12: 1.031
• 4/12: 1.054
• 5/12: 1.083
• 6/12: 1.118
• 8/12: 1.202
• 10/12: 1.302
• 12/12: 1.414

The formula is the square root of 1 + (rise / 12) squared. To find your pitch, hold a level horizontally against a rafter or the roof edge, measure 12 inches along it, and measure straight up to the roof surface. The number of inches is the rise.

Measuring the footprint

Measure the outside of the house and add the overhang on each side. A 46 ft by 26 ft house with 2 ft overhangs all around has a roof footprint of 50 ft by 30 ft. Include attached garages and porches that share the same roof. For an L-shaped house, split it into rectangles and add their footprints.

This method works for gable and hip roofs with a single pitch. For roofs with sections at different pitches, calculate each section with its own pitch and add the squares together. Dormers add surface area and many more cuts, so measure them separately and raise the waste allowance.

How much waste to allow

• Simple gable roof: 10%.
• Hip roof: 15%, because every hip and valley creates angled cuts.
• Complex roofs with valleys, dormers and many facets: 15% to 20%.

Starter strip and ridge cap shingles are usually bought separately. Measure the eaves and rakes for starter, and the ridges and hips for cap. Leftover bundles can often be returned unopened, so rounding up is safer than coming up one bundle short.

Underlayment, drip edge and nails

Synthetic underlayment rolls typically cover 10 squares. Traditional #15 felt covers about 4 squares per roll, and #30 felt about 2. Ice and water shield goes along eaves and in valleys in cold climates and is sold separately.

Drip edge runs along the eaves and rakes, in 10 ft pieces. The calculator uses the footprint perimeter, which matches a gable or hip roof with drip edge on all edges. Plan on about 320 roofing nails per square for four nails per shingle, more in high-wind zones that require six.

This estimate is for planning and comparing quotes. A roofer will also check the decking, flashing and ventilation, which can change the job.

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Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes.