Roofing Calculator: How Many Bundles of Shingles Do I Need?

Slope factors by roof pitch

GAF says almost all of its shingles can be installed on slopes as low as 2:12, and that slopes from 2:12 up to 4:12 need a double layer of underlayment. Code text based on the IRC says the same: asphalt shingles only on slopes of 2:12 or more, with double underlayment below 4:12 (section R905.2.2 in Seattle's 2021 residential code, for example).

Roof square footage by footprint and pitch

Here is the roof surface in sq ft that the calculator's formula gives for common footprints, overhangs included, with the squares (100 sq ft each) in parentheses, before waste:

A 2,000 sq ft footprint at 6/12, for example, has 2,236 sq ft of roof: 22.4 squares, or 24.6 with 10 percent waste, which is 74 bundles. For a footprint between the rows, multiply it by the slope factor in the table above.

Porch and shed roof area

165 sq ft of roof for a 20 × 8 ft shed-style porch roof at 3/12. A porch roof that slopes one way is a single plane, so its area is its length times its depth times the slope factor. Measure the roof, not the porch floor: the length along the house and the depth out from the wall to the front edge, both level and with the overhangs included.

In the calculator, enter the porch roof's length and depth and set the eave overhang to 0: it adds the overhang to all four sides, and a porch roof has none at the wall. For the 20 × 8 ft roof at 3/12 it shows 1.8 squares (6 bundles) with 10 percent waste. IRC-based codes allow asphalt shingles only from 2/12 up, with a double layer of underlayment below 4/12, as noted above. A gable or hip porch roof is figured like the main roof, footprint times slope factor, and so is a shed roof over an addition or a deck.

Bundles per square by shingle type

The calculator assumes 3 bundles per square. GAF says that's the count for typical asphalt shingles, Owens Corning says the same for standard 3-tab shingles, and GAF's Timberline HDZ architectural shingle is packed 3 to the square. GAF also notes that some specialty shingles need up to 5 bundles per square because they're heavier. Here's what the example roof (16.53 squares with waste) would need:

The spec sheets show the difference. Timberline HDZ lists 64 pieces per square at a 5 5/8 in exposure, while the larger Grand Sequoia lists 72 pieces per square at a 5 in exposure. For a 4 or 5 bundle shingle, multiply the calculator's squares by that number and round up. Always go by the bundles per square printed on the wrapper or spec sheet.

Squares and bundles by roof area

A square of shingles covers 100 sq ft of roof, measured along the slope. With 3 bundles per square, here is what the calculator's formula gives for common roof areas:

To check another roof area in the calculator, enter a length and width that multiply to it (50 × 30 for 1,500 sq ft) and set the overhang and pitch to 0. Starter strip and ridge cap come on top; see the next section.

Starter strip, ridge cap, and the rest

The calculator counts field shingles only. You'll also need:

• Starter strip along the eaves and the rakes. One bundle of GAF Pro-Start covers about 120 linear feet.
• Hip and ridge cap. GAF Seal-A-Ridge covers 100 linear feet per 4 bundles, or 25 ft per bundle.
• Underlayment, drip edge, flashing, and nails, each figured by the area or the length of edge it covers.

Say the example house has a simple gable roof with the ridge running the 42 ft length. The eaves are 2 × 42 = 84 ft. Each rake runs 16 ft horizontally at a 1.118 slope factor, or about 17.9 ft, so four rakes are 71.6 ft. That's 155.6 ft of starter, or 2 bundles of Pro-Start. The 42 ft ridge takes 2 bundles of Seal-A-Ridge (42 ÷ 25 = 1.7). If the roof deck needs new sheathing too, the plywood calculator turns the same 1,503 sq ft into 4 x 8 sheets.

Gable vs. hip roof: same area, different edges

1,503 sq ft either way: a hip roof on the example house, with all four sides at 6/12, has the same roof area as the gable, since both are the 1,344 sq ft footprint times the 1.118 slope factor. The edges are what change. A hip roof has eaves on all four sides and no rakes; its ridge is the length minus the width (42 − 32 = 10 ft), and each of its 4 hips runs 24 ft: the 16 ft run × 1.5, the hip factor for a 6/12 roof (√(6² + 12² + 12²) ÷ 12).

The calculator gives the same 16.5 squares for both, so enter the footprint and pitch the same way and pick the waste: GAF's 10 to 15 percent range puts a simple gable at the low end and a hip roof, with cut shingles along every hip, at the high end. A roof whose planes don't share one pitch needs the plane-by-plane method above.

Starter strip and ridge cap per bundle: GAF, Owens Corning, CertainTeed

105 to 120 linear ft of starter and 25 to 33 linear ft of ridge cap per bundle, for each maker's standard product. Divide the eaves and rakes (starter) or the ridge and hips (cap) by the linear feet on the bundle and round up:

The example gable has 155.6 ft of eaves and rakes and 42 ft of ridge; the hip version has 148 ft of eaves and 106 ft of ridge and hips. Adding Owens Corning's 10 percent trim allowance changes none of these counts. Coverage varies within each maker's lineup, so go by the label on the bundle you buy.

Underlayment rolls, drip edge, and nails per square

2 rolls of synthetic underlayment, 18 pieces of drip edge, and about 4,230 nails cover the example gable roof, figured from its 16.53 squares (the roof area plus 10 percent, which Owens Corning's guide adds to every material total) and its 155.6 ft of eaves and rakes:

GAF's Deck-Armor roll covers 10 squares, not counting laps, and Owens Corning's guide says underlayment usually comes in rolls of 4 squares. Drip edge goes along the eaves and the rakes. IRC-based code text (section R905.2.8.5 in Seattle's 2021 residential code) has adjacent pieces overlap at least 2 in, with the underlayment over the drip edge at the eaves and under it at the rakes, and Gibraltar Building Products sells most of its drip edge in 10 ft lengths. The same code asks for at least 4 nails per strip shingle (R905.2.6), long enough to reach 3/4 in into the sheathing (R905.2.5). GAF calls 4 nails per shingle the industry standard, though some shingles need 6 for their top wind rating. Starter strip and ridge cap take nails of their own.

Tips and common mistakes

• Using the footprint without the pitch. For the example, 1,344 sq ft instead of 1,503 sq ft would leave you more than 10 percent short.
• Forgetting the overhangs. With no overhang, the same house shows 14.8 squares and 45 bundles, 5 fewer than it really needs.
• Assuming every shingle is 3 bundles per square. Designer shingles can take 4 or 5.
• Using too little waste on a complex roof. At 15 percent, the example becomes 17.3 squares and 52 bundles.
• Mixing pitches in one calculation. A low-slope porch roof figured at the main roof's pitch will be overcounted, and a steep dormer undercounted.
• Going over old shingles without checking. IRC-based codes don't allow a new layer over a roof that already has two or more layers of roof covering (section R908.3.1.1 in Seattle's 2021 residential code, for example), and local rules can be stricter. Ask your building department whether a tear-off is required, since that changes the scope of the job. If solar panels are in your plans, the U.S. Department of Energy suggests replacing a roof that will need it within a few years before installing solar; the solar panel calculator estimates how much roof the panels cover.