How to measure a roof for shingles
- Measure the footprint. Take the length and width of the roof as seen from above, from the outside edge of the fascia on one side to the other. If you measure the walls instead, add the overhang on both sides.
- Find the pitch. Use the roof pitch calculator with a rise and run measured in the attic or on the gable end.
- Pick the roof style. A gable roof has two sloped planes and triangular ends. A hip roof slopes on all four sides. For L-shaped or mixed roofs, split them into rectangles and add the results.
How the estimate is calculated
Roof area = length × width × pitch factor
Squares = roof area ÷ 100
Bundles = squares × (1 + waste) × bundles per square, rounded up
Gable rake = width ÷ 2 × pitch factor (four of them)
Hip length = √(2 × (width ÷ 2)² + (width ÷ 2 × pitch ÷ 12)²) (four of them)
The footprint method works because every roof plane at the same pitch is its plan area times the same pitch factor, no matter how the planes are arranged. That is why the result is identical for gable and hip roofs; only the edge lengths differ.
Worked example
A 40 by 30 foot gable roof at 6/12 has a pitch factor of 1.118, so the roof area is 1,200 × 1.118 = 1,342 square feet, or 13.42 squares. Adding 10 percent waste gives 14.76 squares, and at 3 bundles per square that is 45 bundles. The eaves and rakes total 147 feet, which takes 2 bundles of starter strip and 15 pieces of drip edge.
What goes on a shingle roof
- Underlayment covers the whole deck under the shingles. Felt and synthetic products differ a lot in coverage per roll.
- Drip edge is metal flashing along the eaves and rakes that guides water into the gutter and protects the deck edge.
- Starter strip is the first course along the eaves and rakes. It seals the edge of the first row of shingles.
- Hip and ridge cap shingles cover the peaks. They are sold by the linear foot of ridge they cover.
- Not included: valley flashing, step flashing at walls, pipe boots, vents, ice barrier and nails. Check whether your area requires an ice barrier along the eaves, and follow the nailing pattern on the shingle wrapper.
Roof pitch and shingles
Asphalt shingles need a slope of at least 2/12 under the International Residential Code, and slopes from 2/12 up to 4/12 need a double layer of underlayment. Below 2/12, use a low-slope roofing system instead.
Frequently asked questions
How many bundles of shingles are in a square?
Most 3-tab and laminated shingles come 3 bundles to a square, which covers 100 square feet. Some heavier designer shingles take 4 bundles per square, so check the wrapper and change the bundles-per-square input to match.
How much waste should I add for shingles?
The calculator starts at 10 percent, which suits a simple gable roof. Every hip, valley, dormer and skylight adds cut pieces, so raise the allowance for complex roofs. Leftover full bundles are worth keeping for repairs.
Can I measure my roof from the ground?
Yes. Measure the outside of the house and add the overhang on each side, or measure the footprint on an aerial photo. Then find the pitch with the roof pitch calculator. Footprint times pitch factor gives the true roof area for any roof whose planes all share one pitch.
What if my roof has more than one pitch?
Split it into sections that each have a single pitch, such as the main roof and a porch roof. Run the calculator for each section and add the results together.
How many rolls of underlayment do I need?
It depends on the product. A roll of #15 felt covers about 4 squares, #30 felt about 2 squares, and many synthetic underlayments about 10 squares. Enter the coverage printed on your roll. On slopes from 2/12 to 4/12 the code requires two layers, so double the result.
Do I need starter strip on the rakes?
Manufacturers recommend starter strip along the eaves and rakes, and the calculator includes both. Hip roofs have no rakes, so only the eaves are counted there.