How a roof is measured
From the ground a roof is a footprint and a pitch. The footprint is measured
to the drip edge, overhangs included; if you measured the walls instead, the
eave and rake overhangs have boxes of their own. The pitch is a rise over 12, and
a bare number is read that way, as a roofer says it. Degrees and percent have
to say so — 26.6°, 50% — and a bare 50 draws a
warning, since as a rise over 12 it is nearly a wall.
Roof area is the footprint times the slope factor, √(rise² + 144) ÷ 12. A 40 by 30 foot gable at 6:12 has a factor of 1.118 and 1,341.6 square feet of roof: 13.42 squares, 45 bundles with 10% for cutting. An area off a measurement report is already up the slope, and the squares page takes it as it stands.
Bundles are divided by the coverage on the wrapper, not counted at three to the square, and rounded last. A 1,265 square foot roof rounded to 13 squares and tripled is 39 bundles; worked unrounded, with the waste, it is 42.
Most of the list goes by edge
Shingles, underlayment and nails go by area. Starter, ridge cap, drip edge and ice barrier go by edge, so the pages work the edges out of the shape. The same gable has 80 feet of eave, 40 of ridge and 67 of rake, because a rake runs up the slope. Drip edge covers eaves and rakes, 147 feet, in 10 foot sticks lapped 2 inches, so it is divided by 9 feet 10 inches: 15 sticks. An area typed in has no edges, and the page says so.
A hip roof has the same area as a gable on the same footprint and pitch; a "hip factor" of 1.1 on top counts the slope twice. A hip costs more in cutting and cap: 10 feet of ridge and four hips of 22 feet 6 inches make 100 feet to cap, four bundles where the gable takes two, and the hip page opens at 15% waste.
A metal panel is cut to the roof
A corrugated sheet sold as 26 inches covers 21½ on a roof, because a roof laps two corrugations where siding laps one; the maker's detail manual prints both, and the metal roofing page counts with the roof's. Each panel is then cut to its plane. A 24 by 20 foot gable at 4:12 with 12 inch overhangs climbs 11 feet 7⅛ inches from eave to ridge, so the page buys 12 foot panels, 30 to cut and 3 spare.
It takes the shortest length that reaches the ridge in one piece, even where a shorter panel costs less: every end lap is a seam on a water surface, and the cheaper row stays on the list. There the four shingle clauses read "Not checked", and the panel's 3:12 minimum slope is a warning carrying the makers' figures, not a code check.
Where the numbers come from
Two clauses were read in the text of IRC 2021 and IRC 2024, which number them alike: R905.1.2, the ice barrier — from the eave to at least 24 inches inside the exterior wall, and at least 36 inches up the slope at 8:12 and steeper — and R905.2.8.5, drip edge at eaves and rakes. The third, R905.2.2, has not yet been read in the code text: its 2:12 floor for shingles and doubled underlayment below 4:12 come from a shingle maker's data sheet that gives them as the code's. Both are boxes, and the reading is owed.
Coverage per bundle and the membrane laps are makers' listings and data sheets; the one coverage no sheet prints is labeled an estimate. The 10% and 15% waste, from a shingle maker's applicator manual, are conventions, as are the felt roll sizes, the 2 inch drip edge lap and the 72 inches of membrane up a valley. What laps take off an underlayment roll is an estimate, and so is every reference price on a shingle roof.
The ice barrier check never passes on its own. The clause applies only where your jurisdiction's Table R301.2 says so, and these pages hold no table by town: until you switch the barrier on, the check is marked "Not checked" and names what to look up. Drip edge is the reverse — unticking its row is a fail you chose.
What these pages do not do
They do not look up whether your town requires an ice barrier, by ZIP code or any other way.
They do not join the planes of a complicated roof. Where two planes meet, the edge between them is added as if it were an eave or a rake, so starter, cap and drip edge read high, and the page says so. The valley box runs membrane up the valley and nothing else: valley lining, the extra cutting along a valley and a panel roof's valley metal are not counted.
They do not count flashing at a chimney, a skylight or a wall, or price labor or a tear-off. The metal page does not lay out standing seam, which is sold by the clip; it counts no closures, butyl tape, pipe boots or stitch screws; and rib panels, ordered rather than carried out of a store, have no reference price.