How many shingles do I need?

Why the pitch changes everything

Roofing is the one material where the ground measurement is not the answer. A roof seen from above covers a certain footprint, but the actual surface you have to cover is longer, because it slopes. The steeper the roof, the bigger the gap between the two numbers.

The correction is a single multiplier derived from the pitch. Pitch is written as rise over run — a 6/12 roof climbs 6 inches for every 12 inches horizontally. The multiplier is the hypotenuse of that triangle: the square root of 1 plus the rise divided by 12, squared. A 6/12 roof works out to 1.118, so a 1,200 ft² footprint is really 1,342 ft² of shingles. On a 12/12 roof the multiplier is 1.414 — you would be short by more than 40% if you ignored it.

From there, roofing is sold by the square, which is 100 ft² of finished roof, and most three-tab and architectural shingles run 3 bundles to the square. Suppliers deal in whole squares, so round up.

Budget 10% waste on a simple gable roof and closer to 15% on anything with valleys, dormers or hips, since every one of those means cutting shingles at an angle and throwing away the offcut. None of this includes starter strip, ridge cap, underlayment or drip edge, which are ordered separately.

A worked example: a gable roof with overhangs

Take a house that measures 40 × 30 feet with a 7/12 gable roof and a 1-foot overhang all around. The overhang is the first thing people forget: the roof footprint is 42 × 32 = 1,344 ft², not 1,200. The 7/12 pitch multiplier is 1.158, so the roof surface is 1,556 ft². Add 10% waste and you need 1,712 ft², which is 17.12 squares, or 18 squares and 52 bundles.

The shingles are not the whole order. A gable roof also needs:

• Underlayment: synthetic underlayment rolls commonly cover about 10 squares each, so this roof takes 2 rolls.
• Starter strip along the eaves and rakes, roughly the perimeter of the roof: two 42-foot eaves plus four rakes of about 18.5 feet each, or about 158 feet here.
• Ridge cap for the 42-foot ridge. Coverage per bundle varies by brand, often 20 to 35 linear feet.
• Drip edge around the same perimeter, sold in 10-foot lengths.

Pitch multipliers

If you do not know your pitch, measure it from the attic: hold a level horizontally against a rafter, mark 12 inches along it, and measure straight up to the underside of the rafter. That number is the rise.

Mistakes that cost money

• Measuring the house instead of the roof. Overhangs add 5 to 10% to most roofs. Measure the eave-to-eave dimensions, or add the overhang to the house size.
• Using one waste figure for every roof. 10% suits a simple gable. Hips, valleys and dormers push it to 15% or more.
• Roofing over an old layer without checking. Many codes allow only two layers, and a second layer adds weight and hides rotten decking. Tearing off costs more but often saves a second job.
• Ordering exactly the count. Return policies on unopened bundles are usually generous. One extra bundle on site is cheaper than a second delivery.

What it costs

Architectural shingles often cost about $35 to $55 per bundle in the US, which is roughly $105 to $165 per square in shingles alone. For the 18-square roof above, that comes to about $1,820 to $2,860 before underlayment, starter, ridge cap, flashing, nails and disposal. Three-tab shingles cost less, and prices vary by brand and region.