How to Use This Tool
Measure the footprint your roof covers — the outline including overhangs, seen from above — then set the pitch. Everything else follows from those two numbers.
The pitch multiplier, and where it comes from
Pitch is written as rise over a run of 12. A sloped surface is the hypotenuse of that triangle, so
the multiplier is sqrt(rise² + 144) / 12. Nothing is looked up in a table here; it is
computed from the pitch you enter.
The practical consequence is that the error from ignoring pitch is small on a shallow roof and large on a steep one. At 4/12 you would be under by 5.4 per cent. At 12/12 you would be under by 41.4 per cent, which is the difference between finishing the job and stopping halfway.
multiplier sqrt(36 + 144) / 12 = 1.1180 roof area 1600 x 1.1180 = 1,788.9 sq ft with waste 1788.9 x 1.15 = 2,057.2 sq ft per plane 2057.2 / 2 = 1,028.6 sq ft squares ceil(10.29) per plane = 11, x 2 = 22 bundles 22 x 3 = 66 ignoring pitch you would order = 20 squares
Squares, bundles, and why rounding happens twice
Roofing is sold by the square, which is 100 sq ft, and most architectural shingles are three bundles to the square. Three-tab is also three; some heavyweight and designer products are four or five, so it is an input.
Rounding up once at the very end under-orders on a roof with several planes, because an off-cut from one plane usually cannot be used on another. Ordering per plane and then totalling is the conservative approach, and it is what a roofer does on paper.
Waste is about cuts, not carelessness
A simple gable wastes about 10 per cent: the ends of courses, and the starter row. Add hips and valleys and it rises to 15 per cent or more, because every valley cut discards part of a shingle and every hip needs shingles cut at an angle on both sides.
Skipping the waste allowance is the single most common way to end up one bundle short on the last afternoon, which is also the worst time to discover a dye-lot difference.
What this does not cover
Underlayment, starter strip, ridge cap, drip edge, ice and water shield and nails are all separate line items, and ridge cap in particular is sold by the linear foot rather than the square. This calculator sizes the field shingles only.
