Footprint and pitch in — true roof area, squares, bundles, underlayment and nails out, with the pitch multiplier applied so you are not ordering for a flat roof.
Added to all four sides of the footprint before the pitch multiplier is applied.
%
10% for a simple gable · 15% for hips and dormers · 20% for a complex cut-up roof with valleys.
Ridge, hips and valleys
Materials
Shingle bundles
—
Footprint area—
Pitch multiplier—
Actual roof area—
Squares (with waste)—
Underlayment rolls—
Ridge cap bundles—
Starter strip—
Roofing nails—
Drip edge—
Estimated material cost—
—
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Footprint is not roof area
The ground a house covers and the surface a roof presents are different numbers, and the gap grows fast with pitch. Multiply the footprint by the pitch multiplier to convert:
multiplier = √(rise² + 12²) ÷ 12
Pitch
Multiplier
Angle
1,200 ft² footprint becomes
4/12
1.054
18.4°
1,265 ft²
6/12
1.118
26.6°
1,342 ft²
8/12
1.202
33.7°
1,442 ft²
10/12
1.302
39.8°
1,562 ft²
12/12
1.414
45°
1,697 ft²
Ordering a 12/12 roof off the footprint leaves you 41% short. Remember to add the overhang to the footprint first — a 12 in eave on all four sides of a 40 × 28 building adds roughly 140 ft² before the multiplier is even applied.
Squares and bundles
Roofing is sold by the square, which is 100 ft² of finished roof. Almost all asphalt shingles — three-tab and architectural alike — come three bundles to the square, so bundles = squares × 3. Heavier designer and luxury shingles run four or five bundles per square, so check the wrapper before ordering.
Everything else
Underlayment — #15 felt covers 400 ft² per roll, #30 covers 200, synthetic typically 1,000.
Ridge cap — roughly 20 to 35 linear feet per bundle of hip-and-ridge shingles.
Starter strip — runs the full eave length, and up the rakes on most manufacturers' instructions.
Nails — four per shingle in normal conditions, six in high wind zones; roughly 320 nails, about 2¼ lb, per square at four nails.
Drip edge — every eave and rake, sold in 10 ft lengths.
From inside the attic, hold a level horizontally against a rafter, mark 12 inches along it, then measure straight down to the rafter. That vertical drop is the rise per 12 — a 6 inch drop is a 6/12 pitch. A smartphone level app against a gable end works too. If the roof already has shingles you can count courses: standard 5 inch exposure means the pitch is whatever the geometry says, so the level method is still the reliable one.
Waste, and when to raise it
Ten percent covers a straightforward gable roof where full shingles run wall to wall. Hips and valleys force diagonal cuts and the offcuts are mostly scrap, so a hip roof wants 15% and a heavily cut-up roof with dormers, valleys and multiple planes needs 20% or more. Architectural shingles with a random pattern also waste more than three-tab, because you cannot always use the leftover half.
Two safety notes worth the paragraph
Roofing is the most dangerous common DIY job there is, and anything above about 6/12 needs roof jacks or a harness rather than confidence. Local codes also limit how many shingle layers a roof may carry — usually two — and require a full tear-off beyond that, which changes the disposal cost substantially. Check both before you order.
How many bundles of shingles do I need for a 1000 square foot roof?
A 1,000 ft² roof is 10 squares, or 30 bundles at three bundles per square. With 10% waste, order 33 bundles. Note that this is 1,000 ft² of actual roof surface, not house footprint.
How do I convert roof pitch to a multiplier?
Divide the square root of (rise² + 144) by 12. A 6/12 pitch gives √(36 + 144) ÷ 12 = 1.118, so the roof surface is 11.8% larger than the footprint it covers.
What is a roofing square?
One hundred square feet of finished roof surface. It is the standard unit for pricing and ordering roofing materials, and almost all asphalt shingles come three bundles to the square.
How much waste should I add for roofing?
Ten percent for a simple gable, 15% for a hip roof, and 20% or more for a complex roof with valleys and dormers. Diagonal cuts along hips and valleys generate offcuts that mostly cannot be reused.
Can I put new shingles over old ones?
Most codes allow a maximum of two layers, and some require a full tear-off regardless. A second layer saves labour but hides deck problems, shortens the new roof's life through trapped heat, and adds weight. Check your local code before deciding.