Two shingle makers add 10% to 15% for waste to a roof’s squares, and the asphalt roofing trade group puts trim waste at 2% to 10%

A roofer orders shingles by the square, 100 square feet of roof surface, and then adds extra for the pieces cut away at valleys, vents and edges. The Asphalt Roofing Manufacturers Association, the trade group for asphalt shingle makers, puts that trim waste at 2% to 10% in its Residential Asphalt Roofing Manual. GAF and IKO, two shingle makers, tell buyers on their own websites to plan on 10% to 15%.

On an example twin worked through below, the full range from 2% to 15% changes the order by 3 bundles, or one square. The roof’s pitch, or steepness, adds 1.6 squares to the area a level plan of the same roof shows.

1Question 7 on the borough’s form asks for squares installed, each 10 feet by 10 feet

Conshohocken’s Roof Permit Application Addendum, revised January 2, 2025, goes with every re-roofing permit in the borough. Question 6 asks for the approximate pitch of the roof. Question 7 asks how many squares will be installed and defines a square as 10 feet by 10 feet. The site’s walk through a roofing quote line by line covers the rest of the form.

The form asks for squares installed. Waste is cut off and thrown away, so it is not installed.

A quote may give squares installed, squares ordered, or both, and the gap between them is the waste allowance.

2An 8 in 12 roof has 20.2% more surface than its level plan

The manual describes measuring a roof by splitting it into rectangles and triangles, adding up their areas and dividing by 100 to get squares. It recommends that a roofing professional take the measurements and advises homeowners against climbing up to do it.

3The trade group puts trim waste at 2% to 10%, and GAF and IKO say 10% to 15%

The manual says the extra shingles depend on how many valleys, dormers, hips and roof penetrations the roof has. “In most cases it should be between 2% and 10%,” it says. GAF’s blog post on bundles says contractors usually include 10% to 15% for trim and waste.

IKO’s page calls 10% to 15% a general range, says smaller roofs tend to have a higher waste factor than larger ones, and suggests keeping a leftover bundle in a dry place for later repairs.

4Most shingles come 3 bundles to a square, and heavier ones take up to 5

GAF and IKO both say most standard shingles, three-tab and architectural, come three bundles to a square, and that heavier specialty shingles can take up to five. GAF lists its Timberline HDZ at three bundles and 64 shingles per square, and its Woodland line at four bundles and 56 shingles per square.

IKO says the coverage per bundle must be printed on the wrapper, and that the number of shingles in a bundle tells you little because shingle sizes differ.

5The twin’s order runs from 30 bundles at 2% waste to 33 at 15%

The two houses here are made up, with round numbers. The twin is one half of a pair of attached houses. Its gable roof measures 22 by 36 feet on a plan, including a 1-foot overhang at the eaves, and it is pitched 8 in 12. The rowhome’s main roof is 16 by 30 feet on a plan, pitched 6 in 12, with a 1-foot overhang at the eaves and a party wall on each side.

The twin’s plan area is 792 square feet. Multiplied by 1.202 for its pitch, the surface is about 952 square feet, or 9.52 squares installed. With 2% for waste the order comes to 9.71 squares and 30 bundles of a three-bundle shingle. With 15% it is 10.95 squares and 33 bundles. Measured on the plan alone, without the pitch factor, the same roof would show 7.92 squares.

The rowhome’s 480 square feet of plan becomes about 537 square feet of surface at 6 in 12, or 5.37 squares. It takes 17 bundles at 2% waste and 19 at 15%.

6The twin’s eaves take two courses of membrane to reach 24 inches inside the wall

IKO notes that most accessories are ordered by length, so a roofer also measures the eaves, rakes, valleys, ridges and hips. The rakes are the sloped edges at a gable end. Drip edge, the metal strip at the roof’s edges, runs along the eaves and rakes, and IKO uses the same total for starter strip, the first row of material at the eave. IKO lists a bundle of its starter shingles at 123 linear feet, a roll of its synthetic underlayment at 1,000 square feet and a roll of its ice and water protector at 65 feet long.

The borough’s addendum requires ice and water shield, a self-adhering membrane that keeps water backed up behind ice from leaking through, at least 24 inches inside the exterior wall line. CertainTeed sells its Vycor membrane in rolls 36 inches wide.

On the twin, the membrane has to run 1 foot of overhang plus 2 feet inside the wall, 3 feet measured level and about 3.61 feet along an 8 in 12 slope. One 36-inch course falls short, so the roofer adds a second course, lapped over the first; the manual gives a 2-inch lap or whatever the maker specifies. Two courses along 44 feet of eave come to 88 linear feet, or two 65-foot rolls. The rowhome needs 3.35 feet along its 6 in 12 slope and also takes two courses.