What is a roof pitch ratio, and how does a ratio chart work?

Ratio chart

• 3:12 = 1:4
• 4:12 = 1:3
• 6:12 = 1:2
• 8:12 = 2:3
• 9:12 = 3:4
• 12:12 = 1:1

Roofers almost always keep the run at 12, because shingle instructions, building codes and estimating software all speak in rise per 12.

The old fraction pitch that trips people up

Older carpentry books define pitch differently: rise divided by the whole span of the building, not the run. Under that system a 6:12 slope is called a quarter pitch, an 8:12 slope is a one third pitch, and a 12:12 slope is a half pitch. If an old house plan or a relative says a roof is a quarter pitch, it means 6 in 12, not 3 in 12.

• Quarter pitch = 6:12 slope
• One third pitch = 8:12 slope
• One sixth pitch = 4:12 slope
• Half pitch = 12:12 slope

Pitch, slope and ratio in everyday use

Strictly, slope is rise over run and pitch is rise over span, but today the two words are used interchangeably, and a modern quote that says 6/12 pitch means 6 inches of rise per foot of run.

Putting the ratio to work

The ratio tells you which materials are allowed, how much extra surface the roof has, and whether the crew will need steep slope safety gear. Read how roofers measure a roof and roof anatomy explained for more.