Roof Pitch Calculator

What Roof Pitch Actually Means

Roof pitch is the steepness of a roof plane, written in the American trades as rise over run in inches per 12 inches. A 6/12 roof rises 6 inches vertically for every 12 inches it travels horizontally. It is spoken as "six in twelve" and written 6/12, 6:12 or 6 in 12.

The run is always 12 in this notation. That is what makes pitches directly comparable: the only variable is the rise. A 4/12 roof is gentle, a 12/12 roof rises at a perfect 45 degrees, and anything past that is unusual outside of steeple and turret work.

Strictly speaking, some older texts use "pitch" to mean total rise over total span expressed as a fraction (a 6/12 roof being a 1/4 pitch) and reserve "slope" for the rise in 12 form. In modern American practice the two words are used interchangeably and everyone means rise in 12.

The Three Ways to Express the Same Slope

• Pitch as x in 12. Used by framers, roofers and material suppliers.
• Degrees, the angle between the roof plane and horizontal. Used by engineers, solar installers and anyone cutting with a miter saw set to an angle.
• Slope percentage, the rise divided by the run times 100. Used on commercial drawings, in drainage calculations and in most of the world outside the United States.

Full Pitch Conversion Table

These values are exact to the rounding shown. The multiplier column is the one that matters for ordering material, because it converts a flat plan area into the real surface a roofer has to cover.

How to Measure Roof Pitch

From Inside the Attic (Safest)

This is the preferred method because nobody has to get on the roof. Take a 2 foot level and a tape measure into the attic. Hold the level horizontally against the bottom edge of a rafter, with one end touching the rafter. Bring the bubble to center. Measure 12 inches along the level from the point of contact and mark it. From that 12 inch mark, measure straight up to the underside of the rafter. That vertical distance in inches is the rise, and the pitch is that number in 12.

If the level is only 12 inches long, use its full length. If it is a 4 foot level, measure 12 inches from the contact end rather than using the whole span, or measure the full 48 inches and divide the rise by 4.

With a Level and Tape on the Roof Edge

The same technique works from a ladder against a gable end, holding the level against the rake board. Never climb onto a roof to measure pitch. A ladder set at the correct 4 to 1 angle, extended 3 feet above the roof edge and tied off, is enough to reach the rake.

With a Speed Square

A rafter or speed square has a pivot point and a common cut scale. Hold the pivot against the underside of the rafter with the square hanging free, level the square's edge, and read the common number that lines up. Digital angle finders and smartphone inclinometer apps work too, though a phone laid on a shingle surface reads the shingle, not the deck, and can be off by a fraction of a degree.

Low Slope Versus Steep Slope

Slope determines what can legally and practically go on the roof. The thresholds below follow the International Residential Code, but jurisdictions amend the code and add local requirements, so confirm the numbers with the local building department before buying material.

• Below 2/12. Asphalt shingles are not permitted. Water moves too slowly and wind driven rain will back up under the laps. These roofs need a continuous membrane: TPO, EPDM, PVC, modified bitumen or a standing seam metal system rated for low slope. Some standing seam profiles are approved down to 1/4 in 12 with sealed seams.
• 2/12 to just under 4/12. Asphalt shingles are permitted but require a doubled underlayment application, with felt courses lapped roughly 19 inches so there are two plies everywhere. Many manufacturers also require a self adhering membrane over the whole deck in this range to keep the warranty intact.
• 4/12 and above. Standard shingle territory. Single layer underlayment, normal installation, and the widest choice of material. Most American houses fall between 4/12 and 9/12.
• 8/12 and above. Considered steep slope for work purposes. Roof jacks and planks, or a full rope and harness system, become necessary. Walking a dry 8/12 in soft soled boots is possible for an experienced roofer, but it is no longer casual, and a 10/12 or 12/12 is not walkable at all without staging.
• Above 21/12 (about 60 degrees). Treated as a wall by some code provisions and by many manufacturers, with different fastening and exposure requirements.

Steeper slopes also affect shingle wind ratings, fastener counts and, for solar, panel racking selection and production modeling.

Rafter Length From Pitch

The pitch multiplier is also the fastest way to size rafters. The run of a common rafter is the horizontal distance from the outside of the wall plate to the center of the ridge, which on a symmetrical gable is half the building width.

Worked example: a house 30 feet wide with a 6/12 gable roof and a 16 inch horizontal overhang. The run is 15 feet. Rafter length over the run is 15 x 1.118 = 16.77 feet, which is 16 feet 9 1/4 inches. The overhang is 16 inches, or 1.333 feet, which becomes 1.333 x 1.118 = 1.49 feet, about 17 7/8 inches. Total rafter length before cuts is roughly 18 feet 3 inches, so 20 foot stock is required. Subtract half the ridge board thickness at the top, which is 3/4 inch for a nominal 2x ridge.

Hip and valley rafters run diagonally and use a different factor, based on a 17 inch run instead of 12, because the diagonal of a 12 by 12 square is 16.97 inches. A 6/12 hip rafter length is the horizontal hip run multiplied by the square root of (1 plus (6/17) squared), which is 1.061 applied to the diagonal run.

Pitch, Snow and Drainage

Steeper roofs shed water faster and hold less snow. Under ASCE 7, the structural standard referenced by most building codes, roof snow load is the ground snow load adjusted by exposure, thermal and slope factors. The slope factor begins reducing the design load above roughly 30 degrees, close to a 7/12 pitch, on ordinary non slippery surfaces. On slippery unobstructed surfaces such as standing seam metal or slate, the reduction begins at a much lower angle, in the 5 to 15 degree range, because snow slides off instead of accumulating.

That reduction disappears if the roof has snow guards, is obstructed by equipment, or has valleys and dormers that let drifts build. Sliding snow off a steep metal roof is its own hazard and is the reason snow guards exist over doorways and driveways.

For drainage, a steeper roof clears water faster and is less prone to leaks at laps and penetrations, but it drives more water into the gutters at a higher velocity, so gutter sizing and downspout count matter more. Low slope roofs need positive drainage designed in, with a minimum of 1/4 inch per foot of fall to the drains on most membrane systems, and they need scuppers or overflow drains so a clogged primary drain does not pond water on the structure.

Ice damming is a slope and insulation problem together. A shallow roof gives melt water more time to refreeze at the cold eave, which is why ice barrier membrane requirements at the eaves are strict in cold climates and why the required coverage extends at least 24 inches inside the exterior wall line measured along the slope.