How to Calculate Roof Pitch

What does a 6/12 roof actually mean? It means the roof climbs 6 inches for every 12 inches it runs sideways. That single ratio sets the roof angle, the rafter length, and even which shingles your local code allows. You can measure it in five minutes with a level and a tape measure. This guide shows the method, then converts the result into degrees, percent grade, and rafter length.

What Is Roof Pitch?

Roof pitch is how many inches a roof rises for every 12 inches of horizontal run. Builders write it as a fraction over 12, such as 4/12 or 8/12. You say it aloud as “4 in 12” or “8 in 12.”

The run is always measured level, not along the sloped surface. The rise is always measured straight up. Together they form a right triangle, and the roof surface is its long side.

People often use “pitch” and “slope” as the same word today. Older carpentry books used pitch differently, as rise divided by the full span of the building. In that older system, a “1/4 pitch” roof rises a quarter of its span. That works out to exactly 6/12 in modern terms, so check which system a plan uses.

The same rise-over-run idea shows up inside your house on every staircase. Our guide on how stair rise and run work uses the identical triangle at a much steeper angle.

How Do You Measure Roof Pitch With a Level and Tape?

Hold a level perfectly flat with one end touching the roof, then measure straight down from the 12-inch mark. That vertical gap in inches is your pitch over 12. A 6-inch gap means a 6/12 roof.

Measuring on the Roof Surface

Use a 24-inch level for a steadier reading. Mark 12 inches from one end with tape. Rest that end on the shingles, center the bubble, and measure down at the mark. Take readings in two or three spots, because older roofs sag slightly between rafters.

A longer reading works too. With a 24-inch run and a 9-inch drop, divide by two to get 4.5/12. That matches about 20.56 degrees.

Measuring From the Attic

The attic is the safer choice for most homeowners. Hold the level against the underside of a rafter, keep it flat, and measure at the 12-inch mark. The rafter follows the roof plane, so the reading matches the pitch outside.

How Can You Find Pitch From Plans or Photos?

Divide the rise by the run and multiply by 12. Plans usually list the total rise and the span, and a straight-on photo of a gable end gives both. The ratio stays the same at any scale.

Start with plans. The run is half the span on a simple gable roof. A 24-foot span with a 6-foot rise has a 12-foot run. Then 6 divided by 12, times 12, gives a 6/12 pitch.

Photos work the same way. Stand well back from the gable end, zoom in, and shoot straight on. Measure on the screen from the eave line to the peak, then half the gable width. A 3-centimeter rise over a 6-centimeter half-width gives 3/6 times 12, which is 6/12.

Photo readings carry some error from lens distortion and camera tilt. Treat a photo as a planning estimate, then confirm with a level before ordering materials. A photo reading such as 5.8/12 points to a likely 6/12 roof that the level can confirm.

How Do You Convert Pitch to Degrees, Percent, and Ratio?

Divide the pitch by 12 to get the slope. Take the arctangent of that slope for degrees, multiply it by 100 for percent grade, or reduce it for a ratio. A 6/12 roof is 26.57 degrees, 50 percent, and 1:2.

The degrees step uses the tangent ratio, which is rise over run in a right triangle. Our guide to sine, cosine, and tangent explains why the inverse tangent returns the angle.

Going the other way is just as quick. Multiply the tangent of the angle by 12. A 30-degree roof is 6.93/12, a 35-degree roof is 8.40/12, and a 40-degree roof is 10.07/12.

Notice that degrees do not scale evenly. Doubling 6/12 to 12/12 raises the angle from 26.57 to 45 degrees, not to 53 degrees.

How Do You Use Pitch to Find Rise and Rafter Length?

Multiply the run by the pitch over 12 to get the rise. Multiply the run by the pitch multiplier to get the rafter length. The multiplier is the square root of 1 plus the slope squared.

Work a full example. A 24-foot span at 6/12 has a 12-foot run. The rise is 12 times 6/12, or 6 feet. The rafter length is 12 times 1.1180, or 13.42 feet, before any overhang.

Overhangs follow the same rule. Add 1 foot of level overhang, and the run becomes 13 feet. The rafter then grows to 14.53 feet from ridge to fascia.

The multiplier also scales roof area from the flat footprint. A 6/12 roof has 11.8 percent more surface than the floor below it. Shingle counts and roofing squares build on that number, and they deserve their own guide.

To skip the math, the roof pitch calculator turns rise and run into pitch, angle, grade, and multiplier. It also converts a span and pitch into rise and rafter length.

What Does Pitch Mean for Walking, Materials, and Code?

Pitch decides how safe a roof is to walk on and which coverings are allowed. Federal safety rules call anything above 4/12 a steep roof. Under the International Residential Code, asphalt shingles need at least 2/12.

Walkability and Safety

OSHA rules for construction work define a low-slope roof as 4/12 or less. Above 4/12 is a steep roof, and workers need guardrails, safety nets, or personal fall arrest near edges 6 feet or higher. Homeowners should take the same hint and measure from the attic instead.

Material Minimums

The IRC sets a minimum pitch for each covering. Asphalt shingles need 2/12, and pitches from 2/12 up to 4/12 require double underlayment. Clay and concrete tile need 2.5/12, metal roof shingles need 3/12, and slate needs 4/12. Local codes vary, so confirm with your building department.

FAQs About Roof Pitch

Sources

This article is general geometry and home-improvement education, not a code ruling or safety plan. Local building codes and manufacturer instructions take priority. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.