Roof Pitch Calculator

What roof pitch means (and why it is written as 6:12)

Roof pitch is the steepness of a roof expressed as how many inches it rises for every 12 inches it runs horizontally. A 6:12 pitch, said "six in twelve", climbs 6 inches for every 12 inches you move sideways, which is an angle of about 26.57° above horizontal. Roofers write it in the X:12 form because it is the easiest version to check on a finished roof: hold a level out horizontally, mark 12 inches along it, and measure straight down to the roof surface. That measurement in inches is the pitch.

It pays to keep three related numbers straight, because people mix them up constantly:

• Pitch / slope ratio, rise over run written as X:12, for example 6:12.
• Angle, the same steepness measured in degrees, for example 26.57°.
• Slope percentage, rise ÷ run × 100, for example 50%. This is the form used on site plans and by engineers, and it is not the same number as the X:12 value.

A 6:12 roof is a 50% slope but only 26.57°, so "50% steep" and "50 degrees" describe completely different roofs. The calculator above converts between all three so you never have to guess, and it also reports the slope category and the roofing materials approved at that pitch.

Roof pitch conversion chart: X:12 to degrees, slope percent and multiplier

This is the table to bookmark. It converts the common pitches into the four numbers roofers, suppliers and inspectors work with, the angle in degrees, the slope as a percentage, and the roof multiplier you use for rafter and area math.

The multiplier (also called the rafter factor or pitch multiplier) is the diagonal distance along the roof for every 1 foot of horizontal run. It equals the square root of 1 + (rise ÷ 12)². Multiply your building's footprint by it to get the true sloped roof area, which is how you work out squares and bundles of shingles.

How to measure the pitch of an existing roof

You do not need trigonometry on the roof, you need a level and a tape. There are two reliable methods.

Level and tape (most accurate on a slope):

• Hold a 12-inch level so one end touches the roof surface and the bubble is centred.
• Measure the gap between the 12-inch mark on the level and the roof surface, straight down.
• That gap in inches is the pitch. A 5-inch gap is a 5:12 roof.

Speed square (fast, works at the gable end): lay the square against the gable rafter with the pivot at the top edge, read the rise where the 12-inch mark of the common scale meets the edge, and you have the X:12 pitch directly. For example, a reading of 7 on the common scale is a 7:12 roof.

A worked example: you level off, mark 12 inches, and measure a 5½-inch drop. That is a 5.5:12 pitch, 24.62°, a 45.8% slope, and a multiplier of about 1.0885. Confirm the measurement on two or three rafters before you order material, because a single bowed rafter will read a little high.

How to calculate rafter length (two worked examples)

The rafter length formula is simple once you have the multiplier: rafter = horizontal run × multiplier. For a gable roof with the ridge centred, the run is half the building width. Always add the eave overhang to the run before multiplying, and add a little for the ridge cut.

Example A, 24 ft wide gable at 6:12. Run = 12 ft. Multiplier at 6:12 = 1.1180. Rafter = 12 × 1.1180 = 13.42 ft, or about 13 ft 5 in. Add a 12-inch eave overhang and the run becomes 13 ft, giving 13 × 1.1180 = 14.53 ft ≈ 14 ft 6 in of lumber per rafter before the ridge cut.

Example B, 30 ft wide gable at 4:12. Run = 15 ft. Multiplier at 4:12 = 1.0541. Rafter = 15 × 1.0541 = 15.81 ft ≈ 15 ft 10 in.

Now the roof area. A 24 ft × 40 ft footprint is 960 sq ft. Multiply by the 6:12 multiplier: 960 × 1.1180 = 1,073 sq ft of actual roof, which is about 10.7 "squares" (a square is 100 sq ft). Asphalt shingles come three bundles to the square, so 10.7 squares needs roughly 32 bundles, buy 34 to allow for cuts, hips and waste. Notice that if you forgot the multiplier and ordered on footprint alone you would be 113 sq ft short.

Which roofing material works at which slope (IRC R905)

Slope is not a preference, it decides which coverings are legally allowed. The International Residential Code (IRC) sets the minimums, and manufacturers add their own warranty floors on top.

IRC section R905.2.2 states that asphalt shingles may be used only on slopes of 2:12 or greater, and from 2:12 up to 4:12 a double layer of underlayment is required. Clay and concrete tile generally need at least 2½:12. Anything flatter than 2:12 is a low-slope roof and should use a membrane or metal, not shingles. You can read the code text yourself in IRC Chapter 9, Roof Assemblies, and the ICC's wind-mitigation summary explains how R905 handles wind resistance.

Why slope decides drainage, warranty and ice dams

Gravity is what moves water off a roof, and slope is how much of it you give gravity to work with. On a steep roof, water leaves quickly and rarely sits. On a low-slope roof, water moves slowly, and below about 2:12 it can wick sideways under shingle tabs and pond near the eaves. Ponding water is the number one cause of premature membrane and shingle failure.

That is why warranties track slope. Most asphalt-shingle manufacturers require a minimum of 4:12 for their full standard warranty; between 2:12 and 4:12 you can still shingle, but the manufacturer may reduce the warranty period or exclude leaks entirely. Below 2:12 a shingle roof is outside the product's scope, full stop.

Slope also drives ice dams. On a low-slope roof in a cold climate, meltwater refreezes at the cold eave and backs up under the covering. Steeper roofs shed that water before it can refreeze. If your roof is between 2:12 and 4:12 in a snow state, plan on an ice-and-water shield membrane along the eaves and in the valleys regardless of what the code minimum says.

Steep roofs, safety and labour cost (OSHA)

The US Occupational Safety and Health Administration draws a hard line at a 4:12 slope: anything steeper is classified as a steep roof, and workers must be protected from falls by guardrails, safety nets or a personal fall-arrest system whenever the edge is 6 feet or more above a lower level. Low-slope roofs may use warning lines in some cases; steep roofs may not.

That classification is why roofing prices jump with pitch. On a 4:12 to 6:12 roof a crew can walk the surface. Above 8:12 they need roof jacks or a harness anchored to the structure, and above 12:12 they are often working from staging or a boom lift. Expect steeper roofs to add roughly 20-50% to the labour portion of a quote, and more on steep or cut-up roofs, because the same square footage takes longer and carries more risk. The material list barely changes with pitch; the labour and safety plan change a lot. The rule is spelled out in OSHA 1926.501, and the agency's Fall Protection in Construction booklet explains what counts as a steep roof.

Common mistakes that throw off your pitch and rafter numbers

Most wrong answers on a roofing job come from one of five mix-ups:

• Using building width as the run. For a centred-ridge gable the run is half the width, not the full width. Using 24 ft instead of 12 ft doubles every rafter.
• Forgetting the overhang. Add the eave overhang to the run before you multiply, or every rafter comes up short.
• Confusing slope percent with degrees. A 6:12 roof is 50% slope but 26.57°. Feeding "50" into a degrees box gives nonsense.
• Reading the wrong plane. Measuring the fascia or a fascia-mounted gutter gives a different number than measuring the roof surface.
• Assuming every "pitch" is X:12. Some tools and plans report pitch as a plain ratio (rise:run) or as a percent grade, so check the units before you trust a number.

Get the pitch right once and the degrees, slope, multiplier, rafter length and material choice all fall out of it.