Roof Area Calculator

A sloped roof always has more surface than the ground it covers, in the same way that a ramp is longer than the floor beneath it. The good news is that you don't need to climb up with a tape. Measure the building at ground level, find the slope, and multiply the two together. The multiplier is called the slope factor, and the roofing calculator on this site calls the same number the slope multiplier.

Formula

Roof area = (l + 2o) × (w + 2o) × √(1 + (rise ÷ 12)²)

l, w

the length and width of the building, measured level

o

the overhang past each wall, measured level

rise

how far the roof climbs for every 12 units it runs

With an angle θ instead of a pitch, the slope factor is 1 ÷ cos θ, which gives the same number.

Roof area from the footprint and pitch

Pitch is written as rise per 12, so a 6/12 roof climbs 6 units for every 12 it runs. Because it is a ratio, it works the same in inches, centimeters or anything else. The slope factor is √(1 + (rise ÷ 12)²).

Take a house 40 × 30 ft with a 1 ft overhang all round and a 6/12 pitch. The overhang adds to both ends of each side, so the covered footprint is 42 × 32 = 1,344 ft². The slope factor is √1.25 ≈ 1.118034, which makes the roof about 1,502.64 ft². That is about 139.6 m², or 15.03 roofing squares, a square being 100 ft² of roof.

Roof area from the footprint and angle

If you know the slope as an angle, divide by its cosine, in degrees or radians. A 40 × 30 ft building with no overhang and a 30° roof has a slope factor of 1 ÷ cos 30° ≈ 1.154701, so its roof is about 1,385.64 ft², or 13.86 squares.

Metric measurements work the same way. A building 12 × 9 m at 30° has a roof of 108 × 1.154701 ≈ 124.71 m², which is about 1,342.34 ft² or 13.42 squares. To turn an angle into a pitch, multiply 12 by its tangent: 30° is about 6.93/12.

Gable and hip roofs have the same area

A gable roof and a hip roof with the same footprint and the same pitch have the same area. It feels wrong, since the hip roof looks more complicated, but every plane of either roof tilts by the same amount, so each plane's surface is its share of the footprint times the same slope factor.

You can check this with a 40 × 30 ft footprint at 6/12, with no overhang. Each slope runs 15 ft horizontally, which is 15 × 1.118034 ≈ 16.77 ft along the roof. The gable has two rectangles of 40 × 16.77 ft, totalling about 1,341.6 ft². The hip has two triangular ends with a 30 ft base and two trapezoids with edges of 40 ft and 10 ft. Those come to about 503.1 + 838.5 = 1,341.6 ft², the same answer as 1,200 × 1.118034.

A shed roof with one slope at that pitch also comes to the same figure. So you can use this page for a hip roof without any separate hip option. Roofs with two pitches, such as gambrels and mansards, are different, and each pitch needs its own calculation.

Dormers, valleys and mixed pitches

The result assumes a plain roof over one rectangle. Dormers and valleys need a separate allowance, and this calculator doesn't add one. A dormer has its own small roof and side walls to measure. A valley doesn't change the surface of two planes at the same pitch, but it does add cutting waste and flashing, so ask your roofer or check the material maker's guidance on how much extra to allow.

For an L-shaped house or sections at different pitches, split the plan into rectangles that don't overlap, run each one and add them. A 30 × 20 ft main section at 4/12 is about 632.46 ft², and a 20 × 10 ft wing at 8/12 is about 240.37 ft², so together they make about 872.83 ft².

Measuring safely from the ground

Measure the length and width around the outside walls, then measure the overhang level from the wall to the edge of the eave, not along the slope. You can often get the pitch from inside the attic: hold a level against a rafter, mark 12 inches along it, and measure straight up to the underside of the rafter. A reading of 5 inches is a 5/12 pitch, with a slope factor of about 1.0833.

For shingle bundles and cost, carry on to the roofing calculator. It starts from a house's base area and treats it as a square when adding the eaves, so a 40 × 30 ft house with a 1 ft overhang at 6/12 shows about 1,501 ft² there and 1,503 ft² here, because this page uses the two actual side lengths.

Frequently asked questions