Roofing Calculator

How to use the roofing calculator

The calculator above opens with a real example already filled in: a 40 by 30 foot house with a gable roof, a 6:12 pitch and 1-foot overhangs. Change any number and every result updates as you type. Here is what each part does.

1. Pick your units. The Units switch at the top of the tool chooses feet and inches or Metric (meters). In feet and inches you can type lengths the way a tape measure reads them, such as 32' 6" or 12 ft 6 in.
2. Describe the roof. On the Roof & materials tab, From the footprint works for most houses: choose gable, hip, shed or flat, enter the building's length and width at the walls, the pitch, and how far the eaves and gable ends overhang. If your roof is an L, a T or has several pitches, switch to Measure each section and add each plane as a rectangle, triangle or trapezoid, measured either flat (from a plan or aerial photo) or along the slope.
3. Choose the material. Asphalt shingles, metal panels or tile. The waste percentage fills in from the roof shape until you type your own.
4. Check the coverage. Open Coverage & options to match the products you will actually buy: bundles per square, underlayment roll size, ice and water shield width, starter and ridge cap coverage, drip edge length, nails per square, metal panel width and length, or tiles per square.
5. Read the results. You get the roof area, the number of squares with and without waste, a materials list that shows how each quantity was worked out, the roof's edge lengths, and any warnings, such as a pitch too low for shingles. Copy the list, download it as a CSV file or print it.
6. Find the pitch or a rafter length. The Pitch & rafters tab converts between rise and run, degrees, x:12 pitch and span with total rise, and works out common rafter length from span, overhang and ridge board thickness.
7. Estimate the cost. The Cost estimate tab takes the materials list and lets you enter unit prices, labor and tear-off per square, disposal, permit, tax and a contingency. It opens with example prices that are only there to show how the math works. Replace them with your local quotes.

How to calculate roof area

Roofers measure the sloped surface, not the floor plan. A pitched roof is longer from eave to ridge than the flat distance it covers, so its surface is always bigger than the footprint below it. The extra is a fixed multiplier for each pitch, called the pitch factor (sometimes "slope factor" or "roof multiplier"):

pitch factor  = √(1 + (rise ÷ 12)²)
roof area     = plan area × pitch factor
plan area     = (length + 2 × gable overhang) × (width + 2 × eave overhang)

The plan area is the roof's outline seen from directly above, overhangs included. That is why a roof square footage calculator asks for the overhangs as well as the wall dimensions: on a typical house, 1-foot overhangs add more than a tenth to the area.

Worked example: a 40 × 30 ft gable roof at 6:12

A house measures 40 feet along the ridge and 30 feet across, with a 6:12 pitch and 1-foot overhangs at the eaves and at both gable ends. These are the calculator's default values, so you can follow along above.

1. Outer length: 40 + 2 × 1 = 42 ft. Outer width: 30 + 2 × 1 = 32 ft.
2. Plan area: 42 × 32 = 1,344 sq ft.
3. Pitch factor for 6:12: √(1 + 0.5²) = √1.25 = 1.118.
4. Roof area: 1,344 × 1.118034 = 1,502.6 sq ft, or 15.03 squares.
5. Edges: 84 ft of eaves (two sides of 42 ft), 71.6 ft of rakes (four sloped edges of 17.9 ft) and a 42 ft ridge.
6. With 10% waste for a gable roof: 15.03 × 1.10 = 16.53 squares, and at 3 bundles per square, 49.6, so you buy 50 bundles of shingles.

With the default coverage settings, the full asphalt list for this roof is 50 bundles of shingles, 2 rolls of synthetic underlayment, 2 rolls of ice and water shield (a 3-foot strip along the 84 feet of eaves), 2 bundles of starter strip, 2 bundles of ridge cap, 16 10-foot pieces of drip edge and 5,300 roofing nails. The results card shows how each line was worked out.

The same footprint with a hip roof (1-foot overhang on all four sides) has exactly the same plan area and roof area, because every plane of a hip roof shares one pitch, and the pitch factor applies to all of them alike. What changes are the edges: 148 ft of eaves all the way around, a ridge of only 42 − 32 = 10 ft, and four hips totaling 96 ft, which need ridge cap. Hips use more cap and cause more cut-off waste, so the hip version needs more material even though its area is the same.

For a roof you cannot describe as one shape, split it into planes. The hip roof above is two trapezoids (42 and 10 ft parallel sides, 16 ft deep in plan) plus two triangles (32 ft base, 16 ft deep): 832 + 512 = 1,344 sq ft in plan, the same answer. Set Measured to Plan (flat) for planes taken from a plan and the calculator applies the pitch factor; set it to On the slope for planes you measured on the roof surface and it uses them as they are.

Roof pitch explained

In the United States, pitch is written as inches of rise per 12 inches of horizontal run. A 6:12 roof (also written 6/12, or "6 in 12") climbs 6 inches for every foot it travels inward. The same slope can be described three other ways, and a roof pitch calculator should convert between all of them:

• Degrees: the angle between the roof and level, the inverse tangent of rise ÷ 12. A 6:12 roof is 26.6°. To go back, rise = 12 × tan(angle), so 30° is 6.93:12.
• Slope percent: rise ÷ run × 100. A 6:12 roof is a 50% slope; 12:12 is 100%, or 45°.
• Ratio: rise to run in lowest terms, so 6:12 is 1:2 and 4:12 is 1:3. Older carpentry books also use "fractional pitch," rise over the full span, so 6:12 is a "quarter pitch." It is easy to confuse with the others, which is why roofers stick to x:12.

The pitch factor turns flat measurements into sloped ones: multiply a plan area or a horizontal run by it. The hip/valley factor, √(2 + (rise ÷ 12)²), does the same for hips and valleys on a roof whose planes share one pitch: multiply the common run (half the building's width, overhang included) by it to get the length of each hip or valley. In the example above, each hip is 16 × 1.5 = 24 ft.

Roof pitch table: 1:12 to 18:12

Roofers group pitches roughly like this. Below 2:12 is treated as a flat or low-slope roof and needs a membrane system rather than shingles. From 2:12 to under 4:12 is low slope: shingles are allowed with extra underlayment. From 4:12 to under 8:12 is the conventional range most houses fall in, and most people can walk it with care. From 8:12 up is steep: crews need roof jacks or harnesses, and many contractors add a steep-slope charge.

How to measure a roof safely from the ground

Falls are the leading cause of death in construction, and you do not need to climb onto a roof to get numbers good enough for an estimate. Work from the ground, a ladder at the gable end, or the attic.

• Measure the walls, not the roof. Run a tape or a laser measure along each outside wall at ground level. Sketch the outline, label every side, and check that opposite sides add up.
• Add the overhangs. Standing under the eave, measure how far the fascia sticks out from the wall, then do the same under the rake at a gable end. Enter these as the eave and rake overhangs. They range from a few inches to 2 feet or more, and they add real area, so do not skip them.
• Find the pitch from the attic. Hold a level against the underside of a rafter, mark a point 12 inches along the level, and measure straight up from that mark to the rafter. That distance in inches is the rise per 12. Rafters are usually the same pitch as the roof surface.
• Or from a photo. Photograph a gable end from straight in front, as square to the wall as you can, and measure the angle of the roof edge on screen with a protractor or angle app. Enter it in degrees on the Pitch & rafters tab. Phone inclinometer apps held against a rake board from a ladder work too.
• Or from the gable wall. If you can measure the height of the ridge above the top of the wall and the building's width, use the Span & total rise option: rise per 12 is total rise ÷ half the span × 12.
• Use aerial imagery for the outline. Satellite and aerial views show the roof from above, overhangs included, which is exactly the plan area. Multiply by the pitch factor, or enter each plane as Plan (flat) under Measure each section.

If a number is uncertain, round the dimensions up to the next half foot. A slightly generous estimate costs a bundle or two; a short one costs a second delivery and a day of delay.

What is a roofing square?

A square is 100 square feet of roof surface, the unit shingles, underlayment, labor and most contractor quotes are priced in. To convert, divide the roof area by 100: the example's 1,502.6 sq ft is 15.03 squares. In metric terms, one square is 9.29 m², so squares = m² ÷ 9.29 (or m² × 10.7639 ÷ 100).

Quotes sometimes give two numbers. The measured squares are the actual roof. The ordered squares add the waste allowance and round up to whole bundles. Labor and tear-off are usually priced on the measured squares, materials on the ordered ones, and this calculator keeps the two apart the same way.

How many bundles of shingles per square?

Most asphalt shingles, both three-tab and architectural (laminated), come 3 bundles to a square, so the shingle calculator multiplies squares by 3 by default. Some heavy designer and impact-resistant shingles are packed 4 or even 5 bundles to the square to keep each bundle liftable. The coverage is printed on every wrapper; change the bundles per square under Coverage & options if yours differs.

Where the numbers come from: a standard three-tab shingle is 12 by 36 inches laid with a 5-inch exposure, so each one covers 36 × 5 = 180 square inches, 1.25 sq ft, and a square takes 80 shingles. A metric-size laminated shingle, about 13¼ by 39⅜ inches with a 5⅝-inch exposure, covers about 1.54 sq ft, so a square takes about 65. At four nails per shingle that is 320 and 260 nails per square; high-wind installations that call for six nails per shingle need half as many again.

So how many bundles of shingles do you need? Roof area ÷ 100 × (1 + waste) × bundles per square, rounded up. For the example: 1,502.6 ÷ 100 × 1.10 × 3 = 49.6, so 50 bundles. Ridge cap and starter strip are separate products, and the calculator counts them on their own lines.

Waste factor by roof type

Every roof loses material to cuts at rakes, hips, valleys and around penetrations, plus the occasional damaged piece. The more edges and angles, the more waste. These are common starting points, and the calculator fills them in from the shape you choose:

Add a few points for many skylights, chimneys or vents, for a steep roof where offcuts are harder to reuse, or for a shingle with a large repeating pattern. Experienced crews on a simple roof often need less. Round the final count up to whole bundles, and remember that unopened bundles can often be returned to the supplier. Ask before you order.

Underlayment and ice & water shield

Underlayment is the water-resistant layer between the roof deck and the shingles. It is the backup when wind drives rain under the shingles, and it protects the deck during installation. The three common types differ mostly in how much a roll covers:

• Synthetic underlayment: light, tear-resistant, and commonly about 10 squares per roll, though roll sizes vary by brand.
• No. 15 asphalt felt: about 4 squares per roll.
• No. 30 asphalt felt: heavier, about 2 squares per roll.

Recent editions of the International Residential Code (IRC) set underlayment rules in section R905.1.1. For asphalt shingles on slopes from 2:12 up to 4:12, the underlayment must be applied in two layers, which roughly doubles the rolls. Clay and concrete tile has a similar rule from 2½:12 up to 4:12 (section R905.3.3). The calculator doubles the underlayment automatically in those ranges and says so in the warnings. It does not double it for metal panels, which have no such rule.

Ice and water shield is a self-adhering, polymer-modified bitumen membrane that seals around nails. IRC section R905.1.2 requires an ice barrier in areas with a history of ice forming along the eaves and backing water up under the roofing, as designated by your local jurisdiction. Where it is required, it must run from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. On a 6:12 roof with a 1-foot overhang, that point is 3 feet in from the eave measured flat, which is about 3.35 feet up the slope, a bit more than one 36-inch-wide course, so enter a wider coverage or plan on two courses. The calculator multiplies eave length by the width you enter, adds a 3-foot strip along every valley, and divides by the roll coverage (200 sq ft for a typical 36-inch by 66.7-foot roll). Many roofers also run it around chimneys, skylights and walls whether or not code requires it.

Codes are adopted and amended locally, and your building department has the final word. Check the edition and any local changes before you buy.

Minimum roof slope for shingles

IRC section R905.2.2 allows asphalt shingles only on roof slopes of 2:12 (about a 17% slope) or steeper, and requires double underlayment from 2:12 up to 4:12. Below 2:12, water drains too slowly for overlapping shingles to keep it out, so the calculator warns you and you should look at a low-slope membrane system instead. Shingle manufacturers often set their own limits too, and following their installation instructions is usually a condition of the warranty.

At 4:12 and above, a single layer of underlayment meets the code minimum for shingles. Shingle roofs between 2:12 and 4:12 are more exposed to wind-driven rain and ice dams, so the details at the eaves, valleys and walls matter more there.

Metal roofing and tile

Metal panels

As a metal roof calculator, the tool counts panels instead of bundles: area with waste ÷ (panel coverage width × panel length). Exposed-fastener panels commonly cover 36 inches; standing-seam panels are often 12 to 24 inches wide. Panels are usually cut to length for each job, so by default the panel length is the rafter length from eave to ridge. Set your own under Coverage & options. The materials list switches ridge cap to linear feet of ridge trim and drops nails and starter strip, because metal systems use screws or clips and their own trim, which come from the panel supplier's takeoff.

Minimum slopes for metal panels depend on the seam. IRC section R905.10.2 sets 3:12 for lapped, non-soldered seams without lap sealant, ½:12 when the laps are sealed, and ¼:12 for standing-seam systems. The calculator cannot tell which seam you will use, so it warns below 3:12, the limit for ordinary lapped exposed-fastener panels. Standing seam can go lower; check the manufacturer's minimum, which may be stricter than the code's.

Clay and concrete tile

Tile is counted per square. Standard concrete tiles run around 90 per square, which is the default, but clay and specialty profiles vary widely, so take the number from the manufacturer's data sheet. IRC section R905.3.2 requires a slope of at least 2½:12 for clay and concrete tile, and section R905.3.3 calls for double underlayment from 2½:12 up to 4:12. The calculator warns below 2½:12 and doubles the underlayment in that range. Tile is several times heavier than asphalt shingles, so have the framing checked before switching an existing house to tile.

Rafter length

The Pitch & rafters tab doubles as a rafter length calculator. A common rafter runs from the wall plate to the ridge, so its horizontal run is half the span, less half the ridge board's thickness. Its length is that run, plus the overhang, times the pitch factor:

run            = (span − ridge board thickness) ÷ 2
total rise     = run × rise ÷ 12
rafter length  = (run + overhang) × pitch factor

Example: a 24-foot span at 6:12 with a 1-foot overhang and a 1½-inch ridge board. The run is (24 − 0.125) ÷ 2 = 11.94 ft (11' 11 1/4"), the total rise is 5.97 ft (5' 11 5/8"), and the rafter is 12.94 × 1.118034 = 14.46 ft (14' 5 5/8"), or 13.35 ft (13' 4 1/8") without the tail. That is the line length along the top edge of the rafter. Add stock for the plumb cut at the ridge and the tail cut, and buy the next standard board length, here a 16-footer.

What drives roof replacement cost

A roof replacement cost estimate depends far more on local labor rates and the condition of your roof than on any national figure, which is why this page does not quote one. The Cost estimate tab starts from example prices so you can see how the pieces add up; they are illustrations, not a quote. Replace them with prices from local suppliers and contractors. These are the things that move the number:

• Size and material. Squares times the price per square for the material you choose. Architectural shingles cost more than three-tab; metal and tile cost more again.
• Tear-off. Removing old roofing is priced per square per layer. Two layers cost about twice as much to strip as one, and the IRC does not allow a new roof covering over two or more existing layers.
• Pitch and height. Steep roofs and second or third stories slow the crew and need more safety gear, so many contractors charge more per square above about 8:12.
• Complexity. Valleys, hips, dormers, skylights and chimneys mean more cutting, flashing and labor per square.
• Decking repairs. Rotted sheathing only shows up once the old roof is off. Contracts often price it per sheet; ask for that number up front.
• Disposal, permits and access. A dumpster or haul-away fee, the building permit, and anything that makes the site hard to reach.
• Extras. New flashing, ventilation, gutters and drip edge, and a higher warranty tier, all add to the total.

Get at least two written quotes that list the same scope: squares, layers removed, underlayment type, ice barrier, flashing, ventilation and how decking repairs are charged. The itemized estimate here makes a useful checklist for comparing them.

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