Roofing Calculator

The footprint is not the roof

Order off the footprint and you will come up short. A roof's plan area is its shadow at noon, and the shingles have to cover the sloped surface, which is always larger. The math is Pythagoras: for every 12 units you travel horizontally you also climb the rise, so each shingle course runs along the hypotenuse of that triangle, longer than its base.

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

Two entries in that table are exact, and they make a quick test for any calculator. 9:12 is exactly 1.25, because 9, 12 and 15 are a Pythagorean triple. 12:12 is exactly √2, because at 45° the diagonal of a unit square is √2. If a tool says a 12:12 roof is 20% bigger than its footprint, it is wrong, and it is wrong in the direction that leaves you short.

That error carries into everything after it. Undercount the area and you undercount the bundles, the underlayment, the nails and the labor in one go, all from a single slip at the top of the calculation.

Roofing is sold in squares and bundles, not square feet

A square is 100 ft² of finished roof, or 9.29 m². Suppliers, estimators and crews all quote in it, because shingles have been packaged and priced that way for a century. A 2,000 ft² roof is 20 squares, and a roofer who hears "twenty squares" already knows the crew size and how many days it takes.

You don't carry squares out of the store, though. Shingles come in bundles. Three bundles make one square for standard three-tab and most architectural lines, roughly 33.3 ft² each. Premium laminated and designer shingles weigh more per square foot, so the maker splits them into four or five bundles to keep each one under about 80 lb, which is what a person can carry up a ladder.

So the number you need is a bundle count. Nobody sells a third of a bundle, which means you round up once, at the end, on the finished count. Round the area up first and then multiply, and you quietly buy an extra bundle on every large order.

Roofing waste is a function of geometry

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

• A shed roof, one plane, wastes about 8%. There is nothing to cut into but the edges.
• A gable, two planes, runs about 10%. It is the case the rule of thumb came from.
• A hip roof, four planes, runs about 15%. Every course that runs into a hip gets a diagonal cut, and the offcut from one side rarely fits the other.
• Valleys push it to 15% to 20%. A valley cuts both planes that meet in it, and the offcuts come out at the wrong angle to reuse. On a house roof, valleys drive waste more than anything else.
• Each penetration adds about a point. Chimneys, skylights, dormers and vent stacks each mean a course cut around an obstacle.

The product matters too. A three-tab shingle repeats with a fixed offset, so an offcut often starts the next course. A random-offset architectural shingle gives up more, and so does a laid pattern like slate or shake. The calculator pre-fills the allowance from the shape you pick and lets you overwrite it. The person holding the tape measure knows things about the job that a shape name can't 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 doesn't scale with area at all. Double the footprint of a gable roof and the ridge grows by roughly the square root of two. It does not double.

• Ridge and hip cap is its own product on a modern roof; nobody cuts it from field shingles anymore. A bundle covers roughly 20 to 25 linear feet and costs several times more per unit of covered area than the field.
• Valley flashing is either an open metal W-valley or a woven or closed-cut shingle valley over an ice-and-water membrane. Either way you buy it in linear feet.
• Drip edge runs the entire perimeter, eaves plus rakes, and comes in 10-foot lengths. Most jurisdictions require it by code on both eaves and rakes.
• Starter strip goes along the eaves, and along the rakes on many manufacturer specifications. It is what holds the first row down in wind.

This tool builds the roof as a solid, so all of those lengths come out of the same geometry. Ridge, hips, valleys, eaves and rakes are edges of the model, and each one is reported on its own. Change the pitch and the eave and ridge lengths stay put, while the rakes and hips get longer, because they climb. The tool keeps them apart for that reason: they measure different things.

Underlayment: the roll covers less than it measures

A roll of No. 15 asphalt felt holds 432 ft² of felt and covers 400 ft² of roof. The 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 can't count. Synthetic underlayments print the same distinction, usually as a 10-square roll that nets somewhat less.

Divide the roof area by the gross roll area and you buy one roll too few, and you find out at the end of the day. The calculator takes the lap out before dividing. Both the roll size and the lap percentage stay editable, since a low-slope application needs 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. You measure it along the eave, not over the whole roof, and it costs several times what felt costs.

Minimum roof pitch is a requirement

Go below a material's minimum slope and water gets underneath it. On a shallow roof water moves slowly, and wind drives it sideways and back up under the course above. These are the common manufacturer and code minimums:

The calculator puts a warning about an incompatible combination above the price. A perfect budget for the wrong material still buys a roof that leaks. It tells apart three states: below the absolute minimum; between the minimum and the standard application, which is legal but only with the extra underlayment; and clear of both.

How to measure a roof without getting on it

You need two numbers, the footprint and the pitch, and you can get both without walking a slope.

Start with the 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. That is 1,247 ft², 11% more than the house itself. Overhang is easy to miss, and every inch of it is extra area you have to buy for.

Then get the 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 photo taken square-on to the gable end plus a protractor tool gets you within about a degree. If the roof has planes at different pitches, run the calculator once per pitch and add the results. One average pitch across a shallow porch and a steep main roof is wrong for both.

Labor, tear-off and the half of the roofing budget nobody quotes

Material is the smaller half of a reroof. Here are typical North American numbers, and the calculator exposes every one as an editable field:

• Installed labor runs roughly $150 to $400 per square, driven by pitch, story height and how cut-up the roof is.
• Tear-off costs $50 to $100 per square for each existing layer. Two layers cost more than twice one layer, because the second layer is stuck to the first.
• Disposal means a dumpster at a flat few hundred dollars, and shingles are heavy: 20 squares of stripped three-tab is on the order of two tons.
• Deck repair can't be known until the old roof is off, so put it in a contingency line and keep it out of the quote.
• A permit and inspection is required for reroofs in most jurisdictions, and it is cheap next to the work.

Anything that slows the crew raises labor and leaves material alone. A 12:12 pitch needs roof jacks and staging. A second story needs longer ladders and a taller throw into the dumpster. A roof with six valleys and three dormers is mostly cutting. That is how two quotes for the same 20 squares can differ by thousands and both be honest.

What this roofing calculator does not do

• It doesn't size 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 carry a heavier material is engineering.
• It doesn't size ventilation. Intake at the soffit and exhaust at the ridge keep a deck dry and a shingle warranty valid, and they are sized from attic area, not from roof area.
• It doesn't handle wind and impact ratings. Fastener count, nailing pattern and starter placement change with the wind zone, and in many places the six-nail pattern is required.
• It doesn't detail flashing. Step flashing, counter-flashing, chimney crickets and pipe boots are where roofs leak, and they get drawn rather than calculated.
• It doesn't model complex roofs in one pass. Break dormers, turrets, wings at different pitches and roofs that aren't built on a rectangle into pieces, run each one, then add them up.

Everything stays in your browser

Every figure is arithmetic done on your device. Nothing is uploaded or stored, there is no account and there is no limit. The geometry and the quantities live in a separate module that is checked 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. The plan drawing comes from the same model that produces the numbers, so the picture and the estimate can't disagree.