How to measure a roof for shingles

You can measure a roof from the ground, as long as you correct for the slope. Here is the method, the multiplier table, and a worked example you can follow with your own numbers.

Step 1: measure the footprint

Measure the building as if you were looking straight down at it — length by width, in feet. Do not try to measure along the slope. For a simple gable roof on a 40 by 30 foot house, the footprint is 1,200 square feet.

If the roof has several sections, break it into rectangles, measure each, and add them together.

Step 2: find the pitch

Pitch is written as rise over run: how many inches the roof climbs for every 12 inches it travels horizontally. To measure it, hold a level against the roof surface, mark 12 inches along it, and measure straight down from that mark to the roof. A 6 inch drop is a 6/12 pitch.

From inside the attic works just as well, against a rafter, and you do not have to get on the roof at all.

Step 3: apply the multiplier

The sloped surface is always larger than the footprint. The multiplier is the hypotenuse of the pitch triangle:

• 4/12 pitch → 1.054
• 6/12 pitch → 1.118
• 8/12 pitch → 1.202
• 12/12 pitch → 1.414

Our 1,200 square foot footprint at 6/12 becomes 1,200 × 1.118 = 1,342 square feet of actual roof. Ignore this step on a steep roof and you will be short by a third.

Step 4: add waste, then convert to squares

Add 10% on a simple gable roof and 15% once there are valleys, hips or dormers, because those force angled cuts that cannot be reused. 1,342 + 10% = 1,476 square feet.

Roofing is sold by the square, which is 100 square feet. That is 14.76 squares, so you order 15 squares — suppliers deal in whole squares. At the usual 3 bundles per square, that is 45 bundles.

What the square count does not include

Squares cover the field of the roof only. Ordered separately, and all by the linear foot rather than the square:

• Starter strip along the eaves
• Ridge cap along every ridge and hip
• Drip edge at the eaves and rakes
• Underlayment, sold by the roll and rated in squares
• Valley flashing, ice and water shield, and pipe boots

Measure the ridges, hips, eaves and rakes with a tape on the plan or from the ground, and price those separately.

A note on layers

Most codes allow two layers of asphalt shingles on a roof. If there are already two up there, you are paying for a tear-off as well, and that changes the job far more than the shingle count does. Count the layers at a rake edge before you price anything.

Measuring without climbing on the roof

You do not have to walk the roof to get a good estimate. Three safer ways:

• From the ground and the attic: measure the house footprint outside, add the overhangs, and measure the pitch from the attic with a level and a tape. That is everything the calculator needs.
• Count the shingle rows: architectural shingles are usually laid with about 5⅝ inches showing. Count the rows from the eave to the ridge on one slope (binoculars help) and multiply by 5⅝ inches to get the slope length. For example, 38 rows × 5.625 inches is about 214 inches, or 17.8 feet.
• Use plans or an aerial report: house plans list the roof pitch, and roofing suppliers can often order an aerial measurement report for a modest fee. Contractors use these to quote.

Hip roofs and valleys

The pitch multiplier works for hip roofs too, as long as every face has the same pitch: footprint (with overhangs) × multiplier gives the full roof area, hips included. What changes is the waste. A hip roof has more angled cuts along each hip, so use 12 to 15% instead of 10%. Every valley adds cuts on both sides, and valleys also need valley liner or ice-and-water membrane, measured in linear feet along the valley.

Safety and permits

Roofing is one of the most dangerous DIY jobs. If you do go up, use a properly secured ladder, a roof harness on anything steeper than a walkable pitch, and never work on a wet or frosty roof. Many towns require a permit for a re-roof, and some limit roofs to two layers of shingles, so check before you order.