Roof Pitch and Slope

Roof pitch is the measure of a roof's steepness. It controls which roof coverings can be used, how quickly water and snow shed, how much attic space there is, and how a roof looks. Different regions and trades express it in different ways.

Ways of expressing pitch

• Rise per run (North America): the vertical rise in inches for every 12 in of horizontal run, written as 4:12, 4/12 or "4 in 12". A 12:12 roof rises 12 in for every 12 in of run.
• Degrees (UK, Europe, Australia, much of the world): the angle between the roof surface and horizontal, such as 22.5° or 35°. Tile and slate manufacturers in these markets specify minimum pitches in degrees.
• Ratio (Canada and elsewhere): the National Building Code of Canada describes slopes as ratios such as 1 in 3 (equivalent to 4:12).
• Percentage: rise divided by run times 100. A 4:12 roof is about 33%; 12:12 is 100%. Often used for low-slope roofs and drainage falls.
• Mm per metre: rise per metre of run.

Conversions

Degrees can be found from rise and run as the arctangent of rise/run. For example:

• 3:12 is about 14.0°
• 4:12 is about 18.4°
• 6:12 is about 26.6°
• 8:12 is about 33.7°
• 12:12 is 45°

Pitch versus slope

Strictly, carpenters distinguish pitch (total rise divided by total span, so a 6:12 roof is a "1/4 pitch") from slope (rise over run, where run is half the span). In everyday use the two words are used interchangeably, and framing squares are marked in slope even when called pitch.

Why pitch matters

• Low-slope and steep-slope roofs: in North America roofs below about 3:12 are commonly called low-slope and need membrane systems; Canada's NBC treats slopes under 1 in 3 as low slope.
• Covering limits: each material has a minimum pitch set by codes and manufacturers. Asphalt shingles, for example, have a code minimum slope in the US, with extra underlayment required toward the low end of their range.
• Snow, wind and access: steeper roofs shed snow better but are more difficult and dangerous to work on.