Roofing Calculator

Two calculators sit above this. The first turns the area of ground your house covers into the area of the roof over it, allowing for the pitch and the overhang at the eaves. The second takes a roof area you already know and works out how much material it takes to cover.

How the area calculator works

A roof is always larger than the ground under it, for two reasons. It tilts, so every square foot of ground sits under more than a square foot of roof, and it overhangs the walls at the eaves.

The calculator handles both in three steps. It treats the house base area as a square and takes its side, adds the eaves overhang to all four sides to get the footprint the roof actually covers, then multiplies by a slope factor worked out from the pitch or the angle you gave it.

Worked through on a 2,000 square foot house with a 6/12 pitch and an 18 inch overhang: the square side is about 44.7 feet, adding 1.5 feet at each end gives 47.7 feet and a footprint of 2,277 square feet, and multiplying by 1.118 gives roughly 2,546 square feet of roof. The tilt and the overhang together add about a quarter to the ground area, which is a useful thing to know before anyone quotes you.

The square assumption is what lets a single area figure be enough. A long narrow house has more perimeter for the same area, so its eaves add slightly more, and if the shape matters to you, measure each roof plane instead and enter the total in the second calculator.

Roof pitch

Pitch is the vertical rise of a roof divided by its horizontal run. It is close to slope but written differently. In the United States the run is fixed at 12 inches, so pitch is quoted as the rise over 12: a 7/12 roof climbs 7 inches for every 12 inches across. Outside the US the same thing is usually given as an angle in degrees, which is why the calculator above has both tabs.

Pitch drives more than the area. It decides how fast water and snow leave the roof, which materials are allowed, whether anyone can walk on it safely, and a good part of the labour cost. Roofs in places with heavy rain or snow tend to be steep. Roofs meant to be walked for maintenance tend to stop around 6/12, since past roughly 7/12 the work needs staging and harnesses and the price goes up with it.

Material choice is tied to it as well. Asphalt shingles are not rated below 2/12, and between 2/12 and 4/12 they need a doubled underlayment to pass inspection. Anything flatter belongs to membrane systems, which shed water by being sealed rather than lapped.

Slope correction factors

If you have a horizontal area, taken from a plan, a survey or a satellite image, multiply it by the factor for your pitch to get the sloping area. The factor is the square root of 1 plus the pitch squared, which is the hypotenuse of the rise and run triangle.

Pitch	Angle	Multiply by	Pitch	Angle	Multiply by
1/12	4.8°	1.003	2/12	9.5°	1.014
3/12	14.0°	1.031	4/12	18.4°	1.054
5/12	22.6°	1.083	6/12	26.6°	1.118
7/12	30.3°	1.158	8/12	33.7°	1.202
9/12	36.9°	1.250	10/12	39.8°	1.302
11/12	42.5°	1.357	12/12	45.0°	1.414
13/12	47.3°	1.474	14/12	49.4°	1.537
15/12	51.3°	1.601	16/12	53.1°	1.667
17/12	54.8°	1.734	18/12	56.3°	1.803
19/12	57.7°	1.873	20/12	59.0°	1.944
21/12	60.3°	2.016	22/12	61.4°	2.088
23/12	62.4°	2.162	24/12	63.4°	2.236

The range matters more than people expect. A shallow 3/12 roof adds only 3% to the plan area, while a steep 24/12 roof adds 124%, so the same footprint can need more than twice the material. Where a roof has sections at different pitches, measuring and correcting each section separately is far more accurate than applying one average factor to the whole thing.

Parts of a roof

Quotes and material lists use the same handful of words, and knowing them makes it much easier to check what you are being charged for.

The ridge is the horizontal line at the top where two slopes meet. A hip is the sloping outside corner where two planes meet and shed water away from each other, and a valley is the inside corner where they meet and channel water together, which is why valleys leak more often than anything else on a roof. The eave is the low edge where water leaves, the rake is the sloping edge at a gable, and the gable itself is the triangle of wall the roof closes off. The soffit is the underside of the overhang, and it usually carries the vents that let air into the roof space.

Ridges, hips and valleys all cost extra. Ridge and hip caps are cut or bought separately and are priced by the linear foot, valleys need flashing or a woven course, and every one of these lines adds cutting waste. A simple gable roof and a cut-up roof of the same area are not the same job.

Materials and how long they last

Material	Typical life	Usual slope
Asphalt or composition shingles	15 to 30 years	4/12 and steeper
Single-ply membrane	10 to 20 years	flat and low slope
Standing seam metal	40 to 70 years	almost any slope
Clay or concrete tile	50 to 100 years or more	4/12 and steeper
Slate	often past 100 years	steep only

Asphalt shingles cover most American houses because they are cheap to buy, quick to lay and easy to repair. Metal costs more up front and tends to win over a long enough horizon. Clay and slate outlast everything else and are heavy enough that the structure has to be built or checked for them, which is the part that catches people retrofitting.

How much material to order

Roofing is sold by the square, and one square is 100 square feet. The calculators above report squares with a 10% buffer already added, which is the usual allowance for cuts, starter courses and the odd damaged piece.

Material	Unit size	Covers
Composition shingles	bundle	about 33 ft², 3 bundles to a square
Roll roofing	roll, 36 in × 36 ft	100 ft² after the laps
#15 felt underlayment	roll, 36 in × 144 ft	400 ft²
#30 felt underlayment	roll, 36 in × 72 ft	200 ft²
Ceramic tile	no standard size	ask the supplier

The material counts are worked out from the roof area itself rather than from the buffered figure, so treat them as the bare minimum and order the extra you have decided on. Note also that the covered area of a roll is smaller than the roll: a 36 inch by 36 foot roll is 108 square feet of material but only covers 100 once the courses lap.

Three things the counts do not include. Ridge and hip caps are usually a separate product sold by the linear foot. Drip edge, valley flashing and step flashing at walls and chimneys are counted in feet, not squares. And an ice barrier along the eaves is required by code in cold regions, typically running from the edge to a line 24 inches inside the exterior wall.

Why this is an estimate

The area calculator works from one number and one pitch, which describes a simple gable or hip roof well and a complicated roof only roughly. Dormers, wings at different heights, turrets and sections at different pitches all move the answer.

For anything complicated, break the roof into planes, work out each one, and add them up. Measuring on the ground and correcting by pitch is safer than climbing, and for irregular outlines the Area Calculator will handle the individual shapes. The Square Footage Calculator covers the same job in square feet with a price per unit area attached.

Reading your result

Take the roof area for material quantities and the roof squares figure for ordering, since suppliers count in squares. The price line applies to the roof area rather than the buffered area, so it is the cost of the material that ends up on the roof and not the cost of what you carry home.

If your number looks far off a contractor's, check the pitch first. Guessing 6/12 when the roof is really 9/12 understates the area by about 12%, which on a 2,500 square foot roof is three squares of material. The angle tab is the easier route when you can measure the slope directly with a level and a tape rather than reading it off a drawing.