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.
Roofing Calculator
Enter the footprint and pitch — get squares, bundles and the multiplier applied for you.
Open the roofing calculator →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.