How many shingles do you need?

How do I calculate how many bundles of shingles I need?

Measure the roof's footprint from above, overhangs included, and multiply by the pitch factor to get the real area of the slopes. Add 10% for waste, divide by 100 to get squares, and multiply by 3 bundles per square. A 40 by 30 ft house with a 6/12 roof: 1,200 × 1.118 = 1,342 square feet of roof; with 10% extra, 1,476 square feet — 14.8 squares, or 45 bundles.

What is a square in roofing?

A square is 100 square feet of roof. Most three-tab and architectural shingles come 3 bundles to the square, so a bundle covers about 33.3 square feet. Some heavy premium shingles take 4 or 5 bundles per square — the wrapper says which; add that bundle size as your own row.

How do I find the pitch of my roof?

Pitch is how many inches the roof rises for every 12 inches it runs. Hold a level flat against the underside of a rafter or on the roof, mark 12 inches along it, and measure straight up from that mark to the roof: 6 inches means a 6/12 pitch. A steeper roof has more area over the same footprint — 6/12 adds about 12%, 12/12 about 41%.

How much extra shingles should I order?

About 10% for a simple gable roof and 15% for a hip roof, where every hip and valley means angled cuts. For a roof with dormers and many valleys, plan on 20%. Starter strips and ridge caps are sold separately and are not part of this count.

How much underlayment do I need?

The same area as the roof itself. Synthetic underlayment usually comes in rolls of 10 squares (1,000 square feet) and #15 felt in rolls of 4 squares. In cold climates, codes also call for ice and water shield along the eaves and in valleys.

The pitch factor

A sloped roof is longer than its footprint. For a rise of r inches per 12 inches of run, the slope length is the hypotenuse of a right triangle, so the area is multiplied by √(1 + (r ÷ 12)²). The factors for common pitches are:

• 3/12 (about 14.0 degrees): × 1.031
• 4/12 (about 18.4 degrees): × 1.054
• 6/12 (about 26.6 degrees): × 1.118
• 8/12 (about 33.7 degrees): × 1.202
• 10/12 (about 39.8 degrees): × 1.302
• 12/12 (about 45.0 degrees): × 1.414

The pitch is a ratio, not an angle. A 6/12 roof is about 26.6 degrees, not 6, and a 12/12 roof is 45 degrees. The factor applies to the footprint measured to the outer edge of the eaves; measuring only the walls leaves out the overhang, which can be a foot or more on every side.

Worked example: a hip roof at 8/12

The footprint is 36 by 28 ft including the eaves, so 1,008 square feet. At 8/12 the factor is 1.202, so the roof surface is 1,008 × 1.202 = 1,211 square feet. A hip roof needs about 15% extra: 1,393 square feet, or 13.9 squares. At 3 bundles per square that is 41.8, so order 42 bundles. Underlayment covers the roof surface itself, 12.1 squares: 4 rolls of #15 felt at 4 squares per roll, or 2 rolls of synthetic at 10 squares.

What the calculator does not count

Starter strips along the eaves and rakes, ridge and hip caps, drip edge, flashing, nails, and ice and water shield are separate items, and each is measured in linear feet, not square feet. Measure the ridge and hips from the plan and check the coverage on the package. Tear-off and disposal are separate too: a second layer of shingles is a heavy load, and local codes limit how many layers a roof may carry, so check with your building department.

Common mistakes

• Using the attic floor area or the floor area of the house instead of the footprint with the overhangs.
• Forgetting the pitch factor. At 8/12 it adds 20% to the area; at 12/12, 41%.
• Treating a roof with dormers, valleys or several hips as a plain rectangle. Each cut adds waste, so choose the higher allowance or split the roof into planes and add them.
• Mixing squares and square feet: one square is 100 square feet, and a bundle covers a third of a square.

Sources: the pitch factor is plain geometry (the Pythagorean theorem). Square (100 square feet), bundles per square and underlayment roll sizes are the usual manufacturer and retailer packaging for asphalt shingles; check the wrapper on your product. Related tools: roof pitch calculator, rafter length calculator, square footage calculator and gutter calculator.