How to calculate roofing materials
Shingles are sold by the square, which is 100 sq ft of roof surface. Most shingles come 3 bundles to the square. The tricky part is the roof area: a sloped roof has more surface than the flat footprint of the house, so you measure the footprint (including overhangs) and multiply by a pitch factor.
Pitch multiplier = √(1 + (rise ÷ 12)²)
Roof area (sq ft) = footprint × pitch multiplier
Squares = roof area ÷ 100
Bundles = squares × (1 + waste) × bundles per square, rounded up
This works for any roof where every plane has the same pitch, including hip roofs. A hip roof and a gable roof over the same footprint at the same pitch have the same roof area; the hip roof just needs more cutting, more waste allowance and more ridge cap.
Worked example: 40 × 30 ft house, 6:12 gable roof
- Add the 12 in. overhang to every side: 42 × 32 ft = 1,344 sq ft footprint.
- Pitch multiplier for 6:12: √(1 + 0.5²) = √1.25 = 1.118.
- Roof area: 1,344 × 1.118 = 1,503 sq ft, or 15.03 squares.
- Add 10% waste: 15.03 × 1.10 = 16.53 squares.
- Bundles at 3 per square: 16.53 × 3 = 49.6, so buy 50 bundles.
- Underlayment: 16.53 ÷ 10 = 1.65, so 2 rolls of synthetic, or 16.53 ÷ 4 = 4.1, so 5 rolls of #15 felt.
- Edges: the ridge is 42 ft, the two eaves total 84 ft, and the four rakes total 2 × 32 × 1.118 = 71.6 ft.
Roof pitch multiplier table
Pitch is the rise in inches for every 12 in. of horizontal run. Multiply your footprint by the factor for your pitch to get the actual roof surface.
| Pitch | Angle | Multiplier | 1,000 sq ft footprint becomes |
|---|---|---|---|
| 2:12 | 9.5° | 1.014 | 1,014 sq ft |
| 3:12 | 14.0° | 1.031 | 1,031 sq ft |
| 4:12 | 18.4° | 1.054 | 1,054 sq ft |
| 5:12 | 22.6° | 1.083 | 1,083 sq ft |
| 6:12 | 26.6° | 1.118 | 1,118 sq ft |
| 7:12 | 30.3° | 1.158 | 1,158 sq ft |
| 8:12 | 33.7° | 1.202 | 1,202 sq ft |
| 9:12 | 36.9° | 1.250 | 1,250 sq ft |
| 10:12 | 39.8° | 1.302 | 1,302 sq ft |
| 12:12 | 45.0° | 1.414 | 1,414 sq ft |
| 14:12 | 49.4° | 1.537 | 1,537 sq ft |
| 16:12 | 53.1° | 1.667 | 1,667 sq ft |
| 18:12 | 56.3° | 1.803 | 1,803 sq ft |
How to find your roof pitch
- From the attic: hold a level horizontally against the bottom of a rafter, mark 12 in. along the level, and measure straight up from that mark to the rafter. That measurement is the rise.
- From a ladder at the edge: do the same on the roof surface at the rake, without climbing onto the roof.
- From the ground: many smartphone level apps can read the angle of a gable from a photo. Use the angle column in the table to convert.
Waste factor by roof type
| Roof | Typical waste |
|---|---|
| Simple gable, few penetrations | about 10% |
| Hip roof, or gable with valleys or dormers | about 15% |
| Many valleys, dormers and short runs | 20% or more |
Waste comes from cutting shingles at rakes, hips and valleys. Starter strip and hip-and-ridge cap are separate products, so buy them on top of the field shingles. If your area comes from a professional roof measurement report, it may already suggest a waste figure; set the waste to match and don't double-count.
Underlayment and other materials
| Material | How to estimate |
|---|---|
| Synthetic underlayment | Rolls typically cover about 10 squares (roll sizes vary, so check the label) |
| #15 felt | Typically about 4 squares per roll |
| #30 felt | Typically about 2 squares per roll |
| Ice and water shield | Eaves in cold climates, plus valleys and around penetrations |
| Starter strip | Length of the eaves, and rakes if the shingle maker calls for it |
| Hip and ridge cap | Total length of ridges and hips |
| Drip edge | Eaves plus rakes, sold in 10 ft pieces that overlap |
| Roofing nails | 4 per shingle, or 6 in high-wind areas, per the wrapper |
Building codes based on the International Residential Code (IRC) require an ice barrier in areas where ice dams are common, running from the eave edge to at least 24 in. inside the exterior wall. The same codes require a double layer of underlayment under shingles on roofs from 2:12 to 4:12 and don't allow asphalt shingles below 2:12.
Tips before you order
- Measure the footprint, not the living area. A two-story house with 2,400 sq ft of living space may have only a 1,200 sq ft footprint. Use the outside wall dimensions and add the overhang. The square footage calculator helps with L-shaped homes.
- Add each section separately. Garages, porches and additions often have different pitches. Calculate each one and add them together.
- Tear-off or overlay? The IRC generally doesn't allow a new layer over a roof that already has two layers, and many roofers tear off regardless. Count the existing layers at the edge of the roof.
- Follow the shingle manufacturer's instructions. Nail count, nail placement and underlayment type can affect the warranty. Most areas also require a permit for a reroof.
- Stay safe. Roof work is one of the most common causes of serious falls at home. On steep roofs or two-story houses, use fall protection or hire a pro.
Planning other exterior work? See the deck calculator and the fence calculator.
Frequently asked questions
How many bundles of shingles do I need for a 2,000 sq ft roof?
A 2,000 sq ft roof is 20 squares. At 3 bundles per square that is 60 bundles, or 66 bundles with a 10% waste allowance. Make sure the 2,000 sq ft is the roof surface, not the house's living area.
How many square feet does a bundle of shingles cover?
About 33.3 sq ft for shingles packed 3 bundles per square, which covers most 3-tab and architectural shingles. Heavier shingles packed 4 bundles per square cover about 25 sq ft per bundle.
What is a square in roofing?
A roofing square is 100 sq ft of roof surface. Shingles, underlayment and roofing labor are usually priced by the square.
How do I calculate roof area from house square footage?
Use the footprint, not the living area. Multiply the footprint (including overhangs) by the pitch multiplier. For example, a 1,500 sq ft footprint with a 6:12 pitch has about 1,500 × 1.118 = 1,677 sq ft of roof.
How many nails do I need per square of shingles?
With 4 nails per shingle, a square of 3-tab shingles (about 80 pieces) takes roughly 320 nails, or 480 at 6 nails per shingle. Architectural shingles have fewer pieces per square (often around 65), so they take fewer nails.
Does a hip roof need more shingles than a gable roof?
Not for the area itself. At the same pitch over the same footprint, both roofs have the same surface area. A hip roof does need more waste allowance (about 15%) and more hip-and-ridge cap.