How to Calculate Roof Pitch (and Convert It to Degrees)

What roof pitch means

Roof pitch describes steepness as the vertical rise for a given horizontal run. North American practice writes it as rise per 12 units of run: a 6/12 roof rises 6 in for every 12 in horizontally. The UK, Europe and Australia more often express it as an angle in degrees.

How to measure it safely

Avoid walking on a roof to measure it unless you are trained and using appropriate fall protection.

• From inside the attic: hold a level horizontally against a rafter, mark 12 in along the level, and measure vertically from that mark to the rafter.
• At a gable end from a ladder: use a pitch gauge or digital angle finder on the rake board.
• From plans: pitch is usually stated on elevation or section drawings.

Converting between formats

Why pitch matters for materials

The pitch multiplier, √(1 + (rise/run)²), converts horizontal measurements into lengths along the slope. Multiply a footprint area by it to get roof area, or a horizontal rafter run by it to get rafter length. Roofing materials also have minimum pitches set by their manufacturers and local codes, so pitch can determine which products are suitable.

Worked example: rafter length and roof area

• A gable roof with a 6/12 pitch has a multiplier of 1.118.
• Horizontal rafter run of 12 ft × 1.118 = 13.42 ft along the slope, before adding any overhang.
• Building footprint including overhangs 32 × 42 ft = 1,344 ft².
• Roof area = 1,344 × 1.118 = 1,502.6 ft², or 15.0 roofing squares (1 square = 100 ft²).
• Add the waste allowance your roofing manufacturer or contractor recommends for cutting, hips, valleys and starter courses before ordering.

Converting degrees back to X/12

If your plans or an angle finder give the pitch in degrees, multiply 12 by the tangent of the angle. A 30° roof is 12 × tan 30° = 6.93, so it is just under 7/12. A 22.5° roof is 12 × tan 22.5° = 4.97, almost exactly 5/12.

Low, medium and steep slopes

Roofs are often grouped into low-slope and steep-slope roofs, because the two need different materials. Asphalt shingles, for example, are generally used on slopes of 4/12 and above, and the Asphalt Roofing Manufacturers Association describes low-slope application between 2/12 and 4/12 with special underlayment requirements; below that, membranes and other low-slope systems are used. Always follow the specific product's installation instructions and your local code.

Very steep roofs also change how a job is done: above roughly 8/12 to 9/12, working safely usually requires roof jacks, harnesses or scaffolding, which affects the cost of a quote as much as the materials do.