Shingle Estimator

Roofers buy shingles by the square — 100 square feet of roof — and a steeper roof has more surface than its footprint suggests. Enter the footprint or each roof plane, the pitch and the roof style, and this estimator works out squares, bundles and the accessories that go with them.

Roofing squares and bundles

One roofing square covers 100 square feet of roof surface. Most asphalt shingles — three-tab and many architectural (laminated) styles — are packed three bundles to the square, though heavier designer shingles can need four or five. The bundle wrapper states the coverage; always check it before ordering.

Pitch multipliers

Multiply the footprint (including overhangs) by the multiplier to get the sloped roof area. The multiplier is √(1 + (rise ÷ 12)²). Low slopes below about 4/12 usually need special underlayment, and many shingles aren’t allowed below 2/12 — check the manufacturer’s instructions.

Worked example

A 40 × 30 ft house with 1 ft overhangs has a footprint of 42 × 32 = 1,344 sq ft. At a 6/12 pitch the multiplier is 1.118, so the roof area is about 1503 sq ft, or 15.03 squares. For a simple gable roof, adding 10% waste gives 16.53 squares — 50 bundles at three per square. The 42 ft ridge needs 42 ft of ridge cap, and the eaves and rakes need about 152 ft of starter strip and drip edge.

Waste factors

Waste covers cutting at hips, valleys and rakes, starter courses cut from full shingles (if not using starter strip) and damaged pieces. Keep a bundle or two for future repairs.

Measuring a roof safely

• Measure from the ground where possible: the footprint and overhangs plus the pitch give a good estimate.
• Many phone apps and satellite measurement services can estimate roof areas.
• Use a pitch gauge or level and tape from an attic rafter or a ladder at the gable end rather than walking the roof.
• Only go on a roof with proper fall protection — or leave the measuring to a roofer.
• Measure ridges, hips, valleys, eaves and rakes separately for accessories.

Shingle types at a glance

Heavier shingles need more bundles per square and add weight to the roof structure — follow the manufacturer’s specifications.

Measuring pitch

Pitch is rise over a 12-inch run. Hold a 12-inch level horizontally against a rafter or the roof surface, and measure straight up from the 12-inch mark to the underside of the roof — a 6-inch measurement means a 6/12 pitch. Pitch gauge apps on phones and inexpensive mechanical pitch gauges work too. On a gable end, you can also measure the rise and half the span and divide.

Reroofing over existing shingles

Some codes allow one extra layer of shingles over an existing layer, but many roofers recommend a full tear-off so the deck can be inspected and repaired. If you’re tearing off, budget for disposal (often a dumpster) and for replacing damaged decking — a common allowance is a few sheets of plywood or OSB per job. Your estimate of squares also tells you roughly how much old roofing will come off.

Accessories checklist

• Underlayment (synthetic or felt) over the whole deck.
• Ice and water shield along eaves and in valleys where required.
• Drip edge at eaves and rakes.
• Starter strip along eaves (and often rakes).
• Ridge cap shingles for ridges and hips.
• Roofing nails — typically four to six per shingle depending on wind zone and maker.
• Vents, flashing and pipe boots.

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