Roofing calculator

How to measure a roof without climbing on it

You don't need a ladder to get close. Measure the house from the ground, add the overhangs, then let the pitch do the rest. Here is the math for the calculator's default: a 40 x 30 ft footprint with a 6/12 gable roof and architectural shingles.

1. 1

   Find the footprint

   Multiply length by width, overhangs included: 40 × 30 = 1,200 sq ft. For an L-shaped house, split it into rectangles and add them up.

2. 2

   Multiply by the pitch factor

   A sloped roof has more surface than the ground under it. The factor is √(1 + (rise ÷ 12)²). For 6/12 that's 1.118, so 1,200 × 1.118 = 1,342 sq ft of roof.

3. 3

   Add waste and convert to squares

   A gable roof wastes about 10 percent. 1,342 × 1.10 = 1,476 sq ft, which is 14.8 squares.

4. 4

   Count bundles

   At three bundles per square, 14.8 × 3 = 44.3, so buy 45 bundles. Starter strip and ridge cap are separate products, so they don't come out of this count.

5. 5

   Measure the edges

   Two 40 ft eaves make 80 ft. Each of the four rakes runs 15 ft across and 16.8 ft along the slope, so 67 ft of rake. That's 147 ft of drip edge and starter, and a 40 ft ridge for ridge cap.

Pitch factor table

Multiply the footprint by the factor to get the roof surface. Pitch is inches of rise per 12 inches of run. You can read it with a level and a tape at the gable end, or in the attic on a rafter.

A hip roof with the same pitch has the same surface area as a gable roof over the same footprint. It just has more edges to cut, which is why its waste factor is higher.

Waste by roof shape

What a new roof costs

Shingles are only part of the bill. In the default example, 45 bundles at $43 come to about $1,900, and the starter, ridge cap, underlayment, drip edge and nails add about $700. A roofer charges far more than materials because the job is fast, dangerous and needs a crew, a dumpster and insurance.

Figures are HomeGuide's national averages from late 2025. Steep roofs (8/12 and up), two-story eaves, many penetrations and rotten decking all push quotes to the top of the range. Get at least three written quotes that list the underlayment, ice barrier, drip edge, ventilation and how many nails per shingle.

Architectural, 3-tab or metal?

Architectural (laminated) shingles are now the default on most US homes. They cost a little more per bundle than 3-tab, hide an uneven deck better, carry higher wind ratings and last about 20 to 30 years against 15 to 20 for 3-tab. Many suppliers stock 3-tab mainly for repairs and starter courses. Metal costs two to four times as much up front but can last 40 years or more, sheds snow, and suits low-slope porches and outbuildings where shingles struggle. Exposed-fastener panels are the budget metal option; standing seam hides the screws and costs more. Whatever you choose, match the underlayment and fasteners to what the maker's warranty asks for.

Safety, and when to hire a roofer

Falls from roofs are one of the leading causes of death in construction, and a two-story or steep roof is no place to learn. If you do any of the work yourself, use a roof harness tied to a proper anchor, and stay off wet, frosty or very hot shingles. Most towns require a permit for a full reroof, and the code allows no more than two layers of shingles. Many roofers won't warranty a layover, so tear-off is the safer bet.

Code basics worth knowing

• Minimum slope. Asphalt shingles need a pitch of at least 2/12. From 2/12 to 4/12 the underlayment must be doubled, so the calculator doubles it there.
• Drip edge. Required at eaves and rakes on shingle roofs, lapped at least 2 in at joints.
• Ice barrier. Where ice dams are common, a self-adhering membrane must run from the eave to at least 24 in inside the exterior wall. With a typical overhang that takes two 3 ft courses.
• Nails. Galvanized roofing nails, four per shingle at minimum, or more where the maker or the wind zone calls for it.

Sources and assumptions

Area is plain geometry from the footprint and pitch. Edge lengths assume a rectangular house, and a hip roof with four equal slopes. Prices are US retail prices checked in October 2026, and the main material price can be edited.

• Waste factors follow common estimating practice, as described in Fine Homebuilding's guide to estimating shingles.
• Bundles and nails: three bundles per square and four nails per shingle are the standard for laminated shingles such as Owens Corning's nailing guidance. The calculator uses about 320 nails per square, starter and ridge included.
• Code items (minimum slope, double underlayment below 4/12, drip edge, ice barrier) are from the 2021 International Residential Code, Chapter 9, Sections R905.1 and R905.2. Your town may have amended them.
• Material prices are October 2026 list prices at Menards: architectural shingles $37.99 to $48.99 a bundle (default $43), 3-tab $35.99, synthetic underlayment $89.99 per 1,000 sq ft roll, ice barrier $119 per 200 sq ft, starter strip $62.48 per 105 ft, hip and ridge $83.48 per 33 ft, 10 ft drip edge $7.98, 7,200 coil nails $41.98, pipe flashing $16.99.
• Metal roofing defaults to $350 per square for exposed-fastener steel panels, inside HomeGuide's $100 to $800 range, with $37.99 ridge cap and $24.37 gable trim per 10 ft and about 80 screws per square.
• Installed prices are from HomeGuide's roof replacement cost guide (November 2025).

Common questions