BuildWellHow roofs are measured
Roofing is measured in squares. One square is 100 square feet of roof surface, so a roof with 2,400 square feet of surface is 24 squares. Shingles, underlayment and labor are all priced and ordered by the square.
The surface of a sloped roof is bigger than the ground it covers. A roof that rises steeply has more surface over the same footprint than a nearly flat one. That’s why you can’t take the house’s square footage and call it the roof.
You don’t need to climb up to measure. Measure the footprint of the roof from the ground: the length along the ridge and the width from eave to eave, including the overhangs. Then apply the pitch factor, which converts flat footprint into sloped surface. The calculator does both.
Pitch is how many inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof rises 6 inches per foot. You can read it at a gable end or from the attic: hold a level 12 inches long against the underside of a rafter and measure straight up from the end of the level to the rafter. That measurement is the rise.
The roofing formula
For a gable roof with a footprint L feet along the ridge and W feet from eave to eave:
- Pitch factor = √(rise² + 144) ÷ 12. At 6/12, √180 ÷ 12 = 1.118.
- Roof area = L × W × pitch factor.
- Squares = roof area ÷ 100.
- Squares to order = squares × (1 + waste).
- Bundles = squares to order × bundles per square, rounded up.
The calculator also works out the edges, because they’re ordered by the linear foot:
- Eaves: the two long bottom edges, 2 × L.
- Rakes: the four sloped gable edges, each (W ÷ 2) × pitch factor.
- Drip edge: eaves plus rakes.
- Ridge: L.
When every plane of a roof has the same pitch, the area formula works for any shape. A hip roof over a 40 × 30 footprint at 6/12 has exactly the same surface as a gable roof over it. Only the edges change.
Pitch factor table
Multiply the footprint by the pitch factor for roof area. Multiply the horizontal run of a common rafter by the hip and valley factor for the length of a hip or valley.
| Pitch | Angle | Pitch factor | Hip / valley factor | Roof area per 1,000 sq ft of footprint |
|---|---|---|---|---|
| 1/12 | 4.8° | 1.0035 | 1.4167 | 1,004 sq ft |
| 2/12 | 9.5° | 1.0138 | 1.424 | 1,014 sq ft |
| 3/12 | 14° | 1.0308 | 1.4361 | 1,031 sq ft |
| 4/12 | 18.4° | 1.0541 | 1.453 | 1,054 sq ft |
| 5/12 | 22.6° | 1.0833 | 1.4743 | 1,083 sq ft |
| 6/12 | 26.6° | 1.118 | 1.5 | 1,118 sq ft |
| 7/12 | 30.3° | 1.1577 | 1.5298 | 1,158 sq ft |
| 8/12 | 33.7° | 1.2019 | 1.5635 | 1,202 sq ft |
| 9/12 | 36.9° | 1.25 | 1.6008 | 1,250 sq ft |
| 10/12 | 39.8° | 1.3017 | 1.6415 | 1,302 sq ft |
| 11/12 | 42.5° | 1.3566 | 1.6853 | 1,357 sq ft |
| 12/12 | 45° | 1.4142 | 1.7321 | 1,414 sq ft |
| 14/12 | 49.4° | 1.5366 | 1.8333 | 1,537 sq ft |
| 16/12 | 53.1° | 1.6667 | 1.9437 | 1,667 sq ft |
| 18/12 | 56.3° | 1.8028 | 2.0616 | 1,803 sq ft |
Between 4/12 and 8/12, the pitch adds 5% to 20% to the footprint. Past 12/12, it adds more than 40%, and the roof needs safety equipment and more crew time on top of more material.
Squares and bundles by roof size
Squares to order and bundles for common footprints, at 10% waste and 3 bundles a square.
| Footprint | 4/12 | 6/12 | 8/12 | 10/12 |
|---|---|---|---|---|
| 1,000 sq ft | 11.6 sq · 35 bundles | 12.3 sq · 37 bundles | 13.2 sq · 40 bundles | 14.3 sq · 43 bundles |
| 1,500 sq ft | 17.4 sq · 53 bundles | 18.5 sq · 56 bundles | 19.8 sq · 60 bundles | 21.5 sq · 65 bundles |
| 2,000 sq ft | 23.2 sq · 70 bundles | 24.6 sq · 74 bundles | 26.4 sq · 80 bundles | 28.6 sq · 86 bundles |
| 2,500 sq ft | 29 sq · 87 bundles | 30.8 sq · 93 bundles | 33.1 sq · 100 bundles | 35.8 sq · 108 bundles |
| 3,000 sq ft | 34.8 sq · 105 bundles | 36.9 sq · 111 bundles | 39.7 sq · 119 bundles | 43 sq · 129 bundles |
Worked examples
A 40 × 30 gable roof at 6/12
The footprint is 1,200 square feet. Times the 1.118 pitch factor, the roof is 1,341.64 square feet, or 13.42 squares. With 10% waste, order 14.76 squares. At 3 bundles a square, that’s 44.27, so 45 bundles.
The edges: 80 feet of eave, four rakes of 16.77 feet each for 67.08 feet, so 147 feet of drip edge. The ridge is 40 feet.
A steep 30 × 20 roof at 12/12
The footprint is 600 square feet. At 12/12 the pitch factor is 1.4142, so the roof is 848.53 square feet, or 8.49 squares. Steep roofs waste more in cuts, so use 15%: 9.76 squares, and at 3 bundles a square, 30 bundles. A steep roof also means slower work, roof jacks and fall protection.
The same footprint as a hip roof
Make the first example a hip roof: 40 × 30 at 6/12. The area is unchanged at 1,341.64 square feet. The edges are different. Eaves run all the way around: 2 × (40 + 30) = 140 feet. The ridge is 40 − 30 = 10 feet. Each of the four hips covers a common run of 15 feet, and at 6/12 the hip factor is 1.5, so each hip is 22.5 feet and the four total 90 feet. Hips and ridge together take 100 feet of ridge cap.
Bundles, underlayment and edges
Shingle bundles. Three bundles per square is typical for 3-tab and many architectural shingles. Some heavier laminated shingles take four or more. The coverage is printed on the wrapper; enter it in the calculator.
Starter. A starter course goes along the eaves, and many roofers also run it up the rakes. Starter strips are sold by the linear foot of coverage per bundle.
Ridge cap. Hip and ridge shingles are sold by the linear foot of coverage per bundle. Count the ridge and all hips.
Drip edge. Sold in pieces of a set length. Divide the drip edge length by the piece length and add a few for overlaps and corners.
Underlayment. It covers the whole roof area, plus overlaps. Roll coverage varies widely by product, from a couple of squares for heavy felt to ten squares or more for many synthetics, so check the label.
Ice and water shield. Where ice dams are a risk, codes based on the International Residential Code require an ice barrier from the eave to at least 24 inches inside the exterior wall. Many roofers also run it down the valleys. It’s sold in rolls by area.
Nails. Typically four nails per shingle, more in high-wind areas, as the shingle maker specifies.
How much waste to add
| Roof | A common starting point |
|---|---|
| Simple gable, few penetrations | About 10% |
| Hip roof | About 15% |
| Several valleys, dormers or skylights | 15% or more |
Waste covers cut-offs at rakes, hips and valleys, and the shingles that get damaged. Starter and ridge cap are separate products, so count them separately.
Common mistakes
Measuring the house instead of the roof. The house’s square footage leaves out the overhangs and the slope. Measure the roof’s footprint and apply the pitch.
Using one waste number for every roof. A simple gable and a cut-up hip roof with dormers waste very different amounts.
Forgetting the second layer. If there are two layers of old shingles, tear-off takes longer and makes more debris.
Leaving out the edges. Drip edge, starter and ridge cap are ordered separately from the field shingles, and running short stops the job.
Pricing a steep roof like a walkable one. Above about 8/12, crews slow down and need more equipment. The material count goes up and so does the time.
From quantities to an estimate
Squares, bundles and edge lengths are the start of a roofing estimate, not the end. The estimate also needs tear-off and disposal, sheathing repair you may find, underlayment, ice barrier, flashing at walls, chimneys and vents, and the vents themselves.
When you estimate a roof in BuildWell, describe it by voice, text or photos and it plans the work line by line: tear-off, underlayment, ice barrier, drip edge, starter, shingles, ridge cap, flashing and cleanup, each with its own labor hours. Materials are priced at stores near the job, checked within 30 days, and labor at the median roofer wage for your metro plus your overhead and markup. You can change every quantity, hour and price before you send it.
