Roofing Calculator

The footprint is not the roof

A roof's plan area is its shadow at noon. The material has to cover the sloped surface, and the sloped surface is always larger. The relationship is plain Pythagoras: for every 12 units you travel horizontally you also climb the rise, so the shingle course travels the hypotenuse of that triangle rather than the base.

The ratio between the two is the pitch factor: √(1 + (rise ÷ run)²). It is the number that turns a footprint into a roof, and it is the number most quick estimates leave out.

Two entries in that table are exact rather than rounded, which is a useful sanity check on any calculator: 9:12 is exactly 1.25, because 9, 12 and 15 are a Pythagorean triple, and 12:12 is exactly √2, because at 45° the hypotenuse of a unit square is √2. If a tool tells you a 12:12 roof is 20% bigger than its footprint, it is wrong, and it is wrong in the direction that leaves you short.

The error compounds with everything downstream. Under-count the area and you under-count the bundles, the underlayment, the nails and the labour, all at once, from a single mistake at the top of the calculation.

Roofing is sold in squares and bundles, not square feet

A square is 100 ft² of finished roof — 9.29 m². It is the unit every supplier, every estimator and every crew quotes in, and it exists because shingles have been packaged and priced by it for a century. A 2,000 ft² roof is 20 squares, and a roofer who hears "twenty squares" immediately knows the crew size and the day count.

Squares are not what you carry out of the store, though. Shingles come in bundles, and three bundles make one square for standard three-tab and most architectural lines — roughly 33.3 ft² each. Premium laminated and designer shingles are heavier per square foot, so they are split into four or five bundles to keep each one under about 80 lb, which is what a person can carry up a ladder.

That is why the useful answer is a bundle count and not an area. Nobody sells a third of a bundle, so the count has to be rounded up — once, at the end, on the finished count. Rounding the area up first and then multiplying quietly adds a bundle to every large order.

Waste is a function of geometry

The standard advice is "add 10%", and it is right for exactly one shape: a simple gable roof with rectangular planes, where the only cuts are the starter course and the last row.

• Shed roof, one plane — about 8%. Nothing to cut into but the edges.
• Gable, two planes — about 10%. The reference case behind the rule of thumb.
• Hip, four planes — about 15%. Every course running into a hip gets a diagonal cut, and the offcut from one side rarely fits the other.
• Valleys — 15% to 20%. A valley cuts both planes that meet in it, and the offcuts have the wrong angle to reuse. Valleys are the single largest driver of waste on a residential roof.
• Penetrations — about a point each. Chimneys, skylights, dormers and vent stacks each mean a course cut around an obstacle.

Waste also depends on the product. A three-tab shingle has a repeating pattern with a fixed offset, so an offcut often starts the next course. A random-offset architectural shingle and a laid pattern like slate or shake give up more. The calculator pre-fills the allowance from the shape you pick, and then lets you overwrite it — because the person holding the tape measure knows things about the job that a shape name does not capture.

Ridge, hip, valley and drip edge are linear-foot items

This is where estimates leak. Field shingles are an area item; almost everything else on a roof is measured in linear feet, priced per linear foot, and does not scale with area at all. Double the footprint of a gable roof and the ridge grows by roughly the square root of two, not by two.

• Ridge and hip cap. A dedicated product, not cut-up field shingles on a modern roof. A bundle covers roughly 20 to 25 linear feet and costs several times more per unit of covered area than the field.
• Valley flashing. Either an open metal W-valley or a woven or closed-cut shingle valley over an ice-and-water membrane. Either way, linear feet.
• Drip edge. Runs the entire perimeter — eaves plus rakes — and comes in 10-foot lengths. Required by code in most jurisdictions on both eaves and rakes.
• Starter strip. The eave course, plus the rakes on many manufacturer specifications, and it is what actually holds the first row down in wind.

Because this tool builds the roof as an actual solid rather than as a single area number, all of those lengths come out of the same geometry: ridge, hips, valleys, eaves and rakes are edges of the model, and they are reported separately. Note that the eave and ridge lengths do not change when you change the pitch, while the rakes and hips do — they climb, so they get longer. That is not a rounding artefact; it is what makes them different quantities.

Underlayment: the roll covers less than it measures

A roll of No. 15 asphalt felt contains 432 ft² of felt and covers 400 ft² of roof, because 2 inches of side lap and 4 inches of end lap disappear into the overlap. That is 7.4% of the roll you paid for and cannot count. Synthetic underlayments print the same distinction, usually as a 10-square roll that nets somewhat less.

Dividing roof area by the gross roll area therefore buys one roll too few, and it always shows up at the end of the day. The calculator discounts the lap before dividing, and both the roll size and the lap percentage stay editable, since a low-slope application requires a doubled lap that halves the effective coverage.

Ice-and-water shield is a separate line. It goes along the eaves, generally to a point 24 inches inside the exterior wall line, which on a shallow pitch can mean two courses; it also lines valleys and wraps penetrations. It is measured along the eave, not over the whole roof, and it costs several times what felt costs.

Minimum pitch is a requirement, not a preference

Every roofing material has a slope below which water gets underneath it. At low slopes water moves slowly, and wind drives it sideways and back up under the course above. The limits below are the common manufacturer and code minimums:

The calculator flags an incompatible combination above the price, not below it, because a perfect budget for the wrong material is still a roof that leaks. It distinguishes three states rather than two: below the absolute minimum, between the minimum and the standard application — legal, but only with the extra underlayment — and clear of both.

How to measure without getting on the roof

You need two numbers: the footprint and the pitch. Neither requires walking a slope.

Footprint. Measure the outside of the building at ground level and add the overhang on every side. A 40 × 26 ft house with an 18-inch overhang has a 43 × 29 ft footprint, which is 1,247 ft² — 11% more than the house itself. Overhang is easy to overlook and it is pure additional area.

Pitch. From inside the attic, hold a level horizontally against a rafter, mark 12 inches along it, and measure straight down from that mark to the rafter: that distance in inches is the rise. From the ground, a photograph taken square-on to the gable end and a protractor tool works to within about a degree. If the roof has planes at different pitches, run the calculator once per pitch and add the results — a single average pitch across a roof with a shallow porch and a steep main body is wrong for both.

Labour, tear-off and the half of the budget nobody quotes

Material is the smaller half of a reroof. Typical North American numbers, all of which the calculator exposes as editable fields:

• Installed labour: roughly $150 to $400 per square, driven by pitch, storey height and how cut-up the roof is.
• Tear-off: $50 to $100 per square per existing layer. Two layers is not twice one layer — it is more, because the second layer is stuck to the first.
• Disposal: a dumpster is a flat few hundred dollars, and shingles are heavy: 20 squares of stripped three-tab is on the order of two tons.
• Deck repair: unknowable until the old roof is off, which is why it belongs in a contingency line rather than in the quote.
• Permit and inspection: required for reroofs in most jurisdictions, and cheap relative to the work.

Anything that slows the crew raises labour without touching material: a 12:12 pitch needs roof jacks and staging, a second storey needs longer ladders and a taller dumpster throw, and a roof with six valleys and three dormers is mostly cutting. That is why two quotes for the same 20 squares can differ by thousands and both be honest.

What this calculator does not do

• Structure. It reports the weight the covering adds — slate and concrete tile run four to five times the weight of asphalt — but sizing rafters and checking whether an existing frame can take a heavier material is engineering.
• Ventilation. Intake at the soffit and exhaust at the ridge are what keep a deck dry and a shingle warranty valid, and they are sized from attic area, not from roof area.
• Wind and impact ratings. Fastener count, nailing pattern and starter placement change with the wind zone; the six-nail pattern is a requirement in many places, not an upgrade.
• Flashing details. Step flashing, counter-flashing, chimney crickets and pipe boots are where roofs actually leak, and they are drawn, not calculated.
• Complex roofs. Dormers, turrets, multiple wings at different pitches and roofs that are not built on a rectangle need to be broken into pieces and run separately, then added.

Everything stays in your browser

Every figure is arithmetic computed locally. Nothing is uploaded, nothing is stored, there is no account and there is no limit. The geometry and the quantities live in a separate module that is verified outside the browser against hand-worked examples — including the exact cases above, where 9:12 must return 1.25 and 12:12 must return √2 — and the plan drawing is generated from the same model that produces the numbers, so the picture and the estimate cannot disagree.