Roofing Calculator

Three, for the great majority of shingles sold in the United States. That covers standard 3-tab strip shingles and standard architectural or dimensional shingles alike, so each bundle covers about 33.3 sq ft. The three-bundle convention exists because a bundle has to stay light enough for one person to carry up a ladder, roughly 60 to 80 lb.

The exception is the heavy designer and luxury lines, which are thicker and come 4 bundles per square, and a few of the heaviest are 5. It is printed on the wrapper, so check it before you order rather than after. Use the shingle type selector above to switch this calculator between 3 and 4.

Do it in the attic, against a rafter. Hold a level out horizontally from the underside of a rafter, mark 12 inches along the level from where it touches the rafter, then measure straight up from that 12 inch mark to the rafter. That vertical measurement is your rise. Six inches up over 12 inches out is a 6/12 pitch.

From the ground you can hold a level against a gable end and do the same thing, or use the rafter-tail method on a ladder at the eave. Pitches are almost always whole numbers, so if you measure 5.8 inches you have a 6/12. A reading that lands genuinely between two, like 4.5, does happen on additions and porches, and this calculator accepts decimals for exactly that case.

Barely, for the field shingles. A hip roof and a gable roof over the same footprint at the same pitch have almost exactly the same surface area, because the hip roof simply folds the same planes in a different direction. What changes is everything measured in linear feet, and the waste.

A hip roof has four hips to cap and no rakes, so it needs far more hip and ridge cap and much more starter strip along its four eaves. It also cuts more waste, because every course dies into a hip at an angle, which is why 15% is the sensible floor on a hip roof rather than 10%. A gable roof is the cheapest shape to cover and the quickest to lay.

Ten percent on a plain gable with two clean planes, no valleys, no dormers and square corners. Fifteen percent the moment you have valleys, hips or dormers, because a shingle cut at an angle into a valley leaves an offcut nobody can use. Twenty percent for a genuinely cut-up roof: several dormers, multiple small planes, or a steep roof where handling waste is high.

The number to be careful about is the low one. Running out on a Sunday afternoon costs you a day and possibly a different production batch, and shingle color varies between batches enough to see from the street. Leftovers cost you far less than a second trip does.

Yes. Underlayment goes over the entire deck under the shingles, so this calculator figures it on the same waste-adjusted area as the shingles. The roll coverage figures used here (1,000 sq ft for a 10 square synthetic roll, 400 sq ft for #15 felt, 200 sq ft for #30 felt) are covered area, with the standard head lap already deducted, which is why a "4 square" felt roll is physically longer than 400 sq ft.

Two things this does not count. Ice and water shield along the eaves is a separate product, required by code in cold climates, and it usually runs a course or two up from the eave plus every valley. And a low-slope roof, 3/12 to 4/12, requires a doubled underlayment layer under most shingle warranties, which doubles the roll count.

Starter strip is the first course at the eave. It runs with the adhesive line at the bottom edge so the first full course of shingles is sealed down at its most exposed point, and it fills the gaps under the cutouts of the course above. It is the single most important thing standing between your roof and a windy night.

Cutting up regular shingles for starter is old practice and it is still done, but it puts the adhesive strip in the wrong place and most manufacturers now list purpose-made starter as a wind warranty condition. If the warranty matters to you, buy the starter. This calculator figures the eaves by default, and the rake edges too if you tick the box, which several manufacturers require.

Two reasons stack up. The pitch multiplier is the first: at 6/12 your roof is 11.8% larger than the footprint, at 8/12 it is 20.2% larger, and at 12/12 it is 41.4% larger. The second is the overhang, which nobody remembers to include. A 12 inch overhang on all four sides of a 40 by 30 footprint turns 1,200 sq ft of plan area into 1,344 sq ft, a 12% increase before pitch is even applied.

Put both together on a 40 by 30 house at 8/12 with 12 inch overhangs and you are covering about 1,615 sq ft of roof over a 1,200 sq ft footprint. That is why quotes based on the square footage of the house always come in low.

About 320 nails per square with the standard four-nail pattern, or 480 per square with the six-nail high-wind pattern that most manufacturers require in high wind zones and for steep slopes. Roofing nails run somewhere around 140 to 180 to the pound depending on length and shank, so a square works out at roughly 2 lb of nails with four nails and 3 lb with six.

The ticket above uses the four-nail figure and a nominal 160 nails per pound. Buy by the box and buy long enough: the nail has to go through the shingle, the underlayment and the deck, and penetrate at least three quarters of an inch into the deck or fully through it. Over a typical half inch deck that means a 1-1/4 inch nail.