Roofing calculator: roof area and shingle bundles

What this calculator does

A sloping roof has more surface than the area it covers on the ground. This calculator converts the footprint of the roof into its real area, using the pitch, and then into bundles of roofing material.

It works for roofs whose surfaces all have the same pitch, such as simple gable and hip roofs. It does not calculate underlayment, ridge caps, flashing, nails or the structure of the roof.

How to use it

1. Choose metric or US units.
2. Enter the length and width of the roof as seen from above, including the overhang at the eaves and gables.
3. Enter the pitch. In metric units it is an angle in degrees. In US units it is the rise in inches for every 12 inches of horizontal run, so a 6 in 12 roof is entered as 6.
4. Enter the area that one bundle covers. It is printed on the wrapper.
5. Set the allowance for waste.
6. Select Calculate.

Do not measure on the roof unless you have the training and equipment to work at height. The footprint can be measured from the ground, and the pitch from inside the attic.

How the result is calculated

1. Area seen from above = length × width.
2. Pitch multiplier = 1 ÷ cosine of the pitch angle. For a pitch given as rise in 12, this is the square root of (1 + (rise ÷ 12)²).
3. Roof area = area seen from above × pitch multiplier.
4. Roof area with allowance = roof area × (1 + allowance ÷ 100).
5. Bundles = roof area with allowance ÷ area one bundle covers, rounded up to a whole bundle.

In US units the result also shows roofing squares. One square is 100 square feet of roof.

Worked example

A roof measures 12 m by 8 m from above, which is 96 m², and has a pitch of 30 degrees. The pitch multiplier is 1 ÷ cos 30° = 1.155. Roof area: 96 × 1.155 = 110.9 m².

With 10 percent allowance: 110.9 × 1.1 = 121.9 m², shown as 122 m². With bundles that cover 3.1 m² each: 121.9 ÷ 3.1 = 39.3, rounded up to 40 bundles.

In US units: a roof of 40 ft by 26 ft has 1040 sq ft from above. A pitch of 6 in 12 gives a multiplier of the square root of (1 + 0.5²) = 1.118. Roof area: 1040 × 1.118 = 1163 sq ft. With 10 percent allowance that is 1279 sq ft, shown as 1,280 sq ft, or 12.8 squares. With bundles of 33.3 sq ft: 1279 ÷ 33.3 = 38.4, rounded up to 39 bundles.

Pitch multipliers

The values follow from the formula above and are rounded.

About the example values

The shingle manufacturers GAF and IKO explain that most asphalt shingles are packed so that three bundles cover one square of 100 sq ft, which is 33.3 sq ft per bundle. Some product lines need four or five bundles per square. IKO lists products from 20 to 33.3 sq ft per bundle. The metric example of 3.1 m² is 33.3 sq ft converted. Use the coverage printed on your bundle.

Both manufacturers give 10 to 15 percent as the usual range for waste. The calculator starts with 10 percent. Roofs with hips, valleys and dormers need more than a simple gable roof.

Common mistakes

• Using the floor area of the house. The roof is larger because of the overhang and the pitch.
• Entering the pitch in the wrong form. 6 in 12 is 26.6 degrees, not 6 degrees.
• Forgetting that starter strips and ridge caps are bought in addition to the field shingles, unless the allowance is meant to cover them.
• Assuming three bundles per square for every product. Check the wrapper.
• Using one pitch for a roof whose parts have different pitches. Calculate each part on its own and add the results.