Roof shingle calculator

Pitch is the number everyone drops

A roof is a slope, and a slope is longer than the ground under it. For a roof rising r inches per 12 in of run, the surface is

multiplier = √(1 + (r ÷ 12)²)

which gives 1.031 at 3/12, 1.054 at 4/12, 1.118 at 6/12, 1.202 at 8/12 and 1.414 at 12/12. A 6/12 roof — the most common pitch on an American house — therefore has 11.8% more surface than its footprint. On a 1,000 sq ft footprint that is 118 sq ft, or between three and four extra bundles. A calculator that takes your footprint and treats it as roof area will send you back to the supplier mid-tear-off, matching a dye lot you may no longer be able to get.

To measure pitch without climbing: hold a 2 ft level against a rafter or the gable trim, mark 12 in along it from the point of contact, and measure straight down from that mark to the roof. That drop in inches is the rise.

Squares, and why we do not round to them

One square = 100 sq ft, and standard 3-tab and architectural shingles pack three bundles to the square — CertainTeed's Landmark data sheet lists 3 bundles and 65 shingles per square, at a 5-5/8 in exposure. Heavier laminates go four or five bundles to the square because a bundle has to stay liftable.

Bundles are sold one at a time, so the arithmetic is:

bundles = ⌈ footprint × pitch multiplier × (1 + waste) ÷ 100 × bundles per square ⌉

A 1,000 sq ft footprint at 6/12 with 10% waste is 1,229.8 sq ft, which is 12.30 squares, which is 37 bundles. Round up to 13 whole squares first and you get 39 — two bundles of dead money and an effective waste factor of 15% rather than the 10% you asked for. Whole-square buying is a real yard habit, but it is a purchasing decision, not arithmetic, and you should make it knowingly.

One footnote: three bundles of Landmark cover 98.4 sq ft, not a full 100. That 1.6% shortfall sits comfortably inside a 10% waste factor, which is part of why the waste factor exists.

Starter and cap are separate purchases

Field shingles do not include the two courses that actually keep the roof on.

• Starter strip runs the eaves, and the rakes if you want sealed gable edges. It is priced by lineal foot; CertainTeed SwiftStart covers 116-1/4 lineal ft per bundle.
• Hip and ridge cap runs every ridge and every hip. Shadow Ridge covers 30 lineal ft per bundle, so caps go quickly on a hip roof.

Cutting three-tabs into your own caps is still done, but it voids most manufacturer system warranties, and it does not work at all with architectural shingles.

What the pitch multiplier does, and what it does not

The multiplier is a plane-to-slope conversion and nothing more. It takes the area the roof covers on the ground and returns the area of the sloping surface above it. It applies one pitch to the whole roof, and it knows nothing whatever about the roof's plan.

So a hip roof, a cross gable, a valley, a dormer or a turret leaves this page with the same answer as a plain rectangle of identical footprint and pitch. The material those shapes eat — every shingle cut on the diagonal into a valley, the courses that die into a dormer cheek — is carried entirely by the waste percentage. That is an allowance, not a calculation, and it is why the field says 15% the moment the roof stops being simple. Where planes have different pitches, run the page once for each and add the answers rather than averaging the pitch.

Two things about the accessory bundles. Neither carries the waste allowance: starter and cap are your measured run divided by that product's published coverage, with nothing on top, so add a bundle of each unless both runs are straight and simple. And both lengths are measurements you supply — nothing here can derive eaves, ridges and hips from an area, and a hip roof holds far more cap than its footprint hints at.

Nothing here checks that shingles belong on the roof at all. IRC R905.2.2 allows asphalt shingles only on slopes of 2:12 and steeper, and from 2:12 up to 4:12 it requires a doubled underlayment — 19 in laps instead of 2 in — beneath them. This page sizes the shingles, not the layer under them, so it does not raise either point: the pitch list starts at 3/12 and the "already the sloped area" option accepts any surface you hand it. A low-slope porch or an addition will get a bundle count whether or not it may be shingled. The underlayment calculator handles the double-coverage case.

The rest of a roof is a separate order and none of it is on this page: nails, ice-and-water membrane at the eaves, valley metal, step and counter flashing, pipe boots, ridge vent, and the tear-off and skip for what is coming off. Rotten sheathing you will only discover once the old roof is down.