Roofing Calculator

Roof area in squares, shingle bundles, starter, ridge cap, underlayment, ice and water shield, drip edge, nails, flashing and a dumpster for the tear-off, from the house footprint, roof shape and pitch. Change any input and the roof plan and pitch triangle redraw.

Shingle roof reference

Typical eave-to-ridge section at 6/12, not to scale. Exposure, nail count and lap values vary by product: follow the wrapper, it is what the warranty is written against.

Layers, eave to ridge

Deck: 7/16″ OSB or ½″ ply
Eave drip edge: under ice and water
Rake drip edge: over underlayment
Ice and water: 24″ inside the wall
Valleys: 36″ ice and water
Underlayment laps: 4″ side, 6″ end
Starter strip: eaves and rakes

Shingles

Square: 100 ft² of roof
Bundles per square: 3, architectural
Exposure: 5⅝″, check wrapper
Overhang at eave: ¼–¾″ past drip edge
Nails, standard: 4 per shingle
Nails, high wind: 6 per shingle
Nail length: 1¼″, ¾″ into deck

Pitch and slope factor

Minimum, shingles: 2/12 (2–4 needs 2 layers)
4/12: × 1.054
6/12: × 1.118
8/12: × 1.202
10/12: × 1.302
12/12: × 1.414
Walkable: up to about 6/12

Ventilation and flashing

Net free vent area: 1 ft² per 300 ft²
Split: about 50% ridge, 50% soffit
Ridge slot: 1″ each side of ridge
Step flashing: 1 piece per course
Counter flashing: let into the mortar
Pipe boots: 1 per vent stack
Layers allowed: 2 max, tear off is best

Safety Falls from roofs are the leading cause of death in construction. Above 6/12, or on any wet, frosty or dusty roof, work tied off to a ridge anchor with a full-body harness, and use roof jacks with toe boards. Set the ladder at 4:1, tie it off and run it 3 ft past the eave. Keep clear of overhead power lines.

Full notes

Measure the footprint at the walls and let the calculator add the overhangs and the slope. Measuring on the roof is more accurate if you can do it safely: add up each plane and skip the slope factor.
A square is 100 sq ft of roof surface. Architectural shingles come 3 bundles to the square; ridge cap and starter are sold separately and are not in the bundle count.
Drip edge goes on the deck under the ice and water shield at the eaves, and on top of the underlayment along the rakes, so water always laps over the metal below it.
Nail in the nail line printed on the shingle, 4 nails per shingle (6 in high-wind areas and where the code asks for it). High nails let shingles slide and void the warranty.
Replace step flashing and pipe boots when you re-roof. The shingles rarely fail first; the flashing around chimneys, walls and vents does.
Most codes allow at most two layers of shingles. A full tear-off lets you check and replace rotten decking, and it is the only way to get the full manufacturer warranty on many systems.

Planning a bigger project?

Drywall CalculatorSheets of 4x8, 4x10 or 4x12 drywall for walls and ceilings, plus screws, tape and joint compound.Paint CalculatorHow many gallons of paint and primer a room needs from its size, ceiling height, doors, windows and coats, rounded to what you actually buy.Concrete CalculatorHow many cubic yards of concrete, or how many 40, 60 and 80 lb bags, you need for a slab, patio, walkway, footing pad or round pier.