Roof Pitch Calculation

Roof pitch (or slope) describes how steep a roof is. It controls which roof coverings may be used, how much underlayment is required, how water and snow drain and how safe the roof is to walk on. In North America it is usually given as inches of rise per 12 inches of horizontal run, written 6:12 or "6 in 12". Elsewhere it is more often given in degrees, and low-slope roofs are frequently described as a ratio or percentage (for example 1:50 or 2 percent).

Pitch versus slope

Strictly, slope is rise over run, while pitch is rise over the full span (twice the run). A roof with a 6:12 slope therefore has a pitch of 1/4. In everyday trade use the two words are used interchangeably, and "pitch" usually means the rise-per-12 figure.

How to measure

• With a level. Place a 12 in (or 1 m) level horizontally against the roof or rafter, and measure vertically from the end of the level to the roof surface. That measurement is the rise per 12 in (or per metre).
• From rafter dimensions. Divide the rise from wall plate to ridge by the run from wall plate to centre.
• With an angle finder or app. Read the angle directly and convert.

Conversions

• Angle = arctan(rise / run).
• Percentage = rise / run x 100.

x:12	Degrees	Percent
2:12	9.5	16.7
3:12	14.0	25.0
4:12	18.4	33.3
6:12	26.6	50.0
8:12	33.7	66.7
12:12	45.0	100.0

Why it matters

• Covering limits. In the IRC, asphalt shingles need at least 2:12, with doubled underlayment up to 4:12. Many metal systems are designed for 3:12 (1:4) and steeper.
• Low slope definitions. The National Building Code of Canada treats slopes below 1 in 3 (4:12) as low slope. OSHA treats roofs of 4:12 or less as low-slope for fall protection.
• Safety. Australian guidance treats roofs over 35 degrees as unsuitable to stand on even with edge protection.
• Estimating. Pitch determines the multiplier used to convert plan area into roof area.