A roof is measured from the ground as a footprint and covered on the slope, and the slope is always larger. Getting that multiplier wrong is the most common way a roofing order arrives short. This roofing calculator applies it for you, so you never have to be on the roof to order.
What the pitch does to the number
Pitch is rise over a 12 inch run, and the multiplier is the hypotenuse of that triangle divided by the run. On a 50 by 30 foot building, a 1,500 square foot footprint:
- 4/12 multiplies by 1.054, so 1,581 square feet. 53 bundles.
- 6/12 multiplies by 1.118, so 1,677 square feet. 57 bundles.
- 9/12 multiplies by 1.25, so 1,875 square feet. 62 bundles.
- 12/12 multiplies by 1.414, so 2,121 square feet. 71 bundles.
At 6/12 the slope adds 177 square feet over the footprint, which is close to two squares of material. At 12/12 it adds 621, more than six. Order off the footprint and a steep roof leaves you six squares short on the last day.
Squares and bundles
Roofing is quoted in squares, meaning 100 square feet, and bought in bundles. Architectural shingles usually run three bundles to the square, but three bundles do not quite cover one. GAF and CertainTeed both publish three bundles as covering 98.4 square feet, so a bundle is 32.8 square feet and that is the default above. GAF’s Timberline HDZ spec sheet lists 64 pieces to the square at a 5 5/8 inch exposure, which is where the 98.4 comes from. The square is the sales unit; 98.4 is the roof.
Check the wrapper rather than assuming. Heavier and premium shingles run four or five bundles to the square, and that changes the count sharply: the same roof takes 57 bundles at three per square, 74 at four and 93 at five. If your supplier quotes by the square, enter 100 as the coverage and the answer comes back in squares instead.
Finding your pitch without climbing
You do not need to be on the roof for any of this, which matters because falls are the leading cause of death in construction and a shingle roof is no place to be taking measurements.
Go into the attic, hold a level horizontally against the underside of a rafter, mark 12 inches along it, and measure straight down from that mark to the rafter. That measurement is your rise. Six inches means a 6/12 pitch.
Take the footprint from the ground, including the overhang. Walk the outside of the building and measure the outline, adding the eaves projection since it gets covered too. A roof with dormers, valleys or several sections is far easier to treat as separate rectangles added together than to measure as one shape.
Where the waste allowance really goes
Ten percent suits a simple gable roof, where waste is mostly starter course and ridge caps. A cut-up roof with hips, valleys and dormers wants 15%, because every valley is a diagonal cut down both sides and the offcuts are unusable.
On this roof that is the difference between 56 and 58 bundles, roughly $76. Running two bundles short at the ridge on a Friday afternoon costs considerably more than that.
What the shingles do not include
Underlayment, ice and water shield, drip edge, starter strip, ridge cap, valley flashing and nails are all separate line items, and on a re-roof so is the disposal of what comes off. Ridge cap in particular is sold by the linear foot rather than the square.
For a brick chimney or gable end going up alongside it, the brick calculator counts that, and once the structure is closed in, the drywall calculator covers the ceiling below.
Roofing calculator FAQ
Multiply the footprint by the pitch multiplier. A 6/12 roof multiplies by 1.118, so a 1,500 square foot footprint is 1,677 square feet of roof. A 12/12 roof multiplies by 1.414, giving 2,121 square feet from the same footprint.
A 50 by 30 building with a 6/12 roof needs about 57 bundles of architectural shingles including a 10% allowance. That assumes three bundles to the square; heavier shingles run four or five, which would need 74 or 93.
About 32.8 square feet. GAF and CertainTeed both state that three bundles cover 98.4 square feet rather than a full 100, so the common 33.3 figure is about 1.5 percent optimistic. Heavier and premium products run four or five bundles to the square, so always check the wrapper before ordering.
From inside the attic. Hold a level against the underside of a rafter, mark 12 inches along it, and measure straight down to the rafter from that mark. That drop is your rise, so a 6 inch drop is a 6/12 pitch.
Ten percent for a simple gable roof, where waste is mostly starter course and ridge caps. Fifteen for a cut-up roof with hips, valleys and dormers, since every valley is a diagonal cut and the offcuts cannot be reused.