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.

Re-roofing a gable roof with asphalt shinglesThe front slope of a gable house roof during a re-roof. At the left end the old shingles are being torn off down to the plywood deck and the pieces drop into a roll-off dumpster below. In the middle, ice and water shield runs along the eave and synthetic underlayment is rolled out in horizontal courses that lap the one below, held with cap nails. On the right, architectural shingles go on from the eave up over a starter strip and drip edge, one course at a time, and a coil roofing nailer fastens each course along its nail line. Shingle bundles sit on the ridge, a brick chimney is step-flashed, a ladder leans on the eave and a pallet of bundles and underlayment rolls waits on the driveway.

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Shingle roof reference

126SLOPE FACTOR √(1 + (P/12)²) = 1.1185⅝″ EXPOSUREICE & WATER SHIELD, 24″ INSIDE WALLSTARTER STRIP ¼–¾″ PAST DRIP EDGEDRIP EDGE UNDER ICE & WATER AT EAVERIDGE CAP SHINGLESRIDGE VENT, 1″ SLOT EACH SIDEARCH. SHINGLES, 4–6 NAILS EACHNAILS JUST ABOVE THE CUTOUTSSYNTHETIC UNDERLAYMENTROOF DECK, 7/16″ OSB OR ½″ PLYRAFTER OR TRUSS TOP CHORD
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.

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