Roof Pitch Calculator
Determine your roof pitch in multiple formats — degrees, percent, and the standard rise-per-12-inch-run ratio. Enter the span for an estimated rafter length including overhang.
Vertical rise of the roof over the measured run
Horizontal distance (standard is 12 inches)
Total width of the building for rafter length calculation
Roof overhang beyond the wall
What does roof pitch 6/12 mean?
A 6/12 roof pitch means the roof rises 6 inches vertically for every 12 inches of horizontal run. This is the standard way to express roof slope in North America. The higher the first number, the steeper the roof. Common pitches: 4/12 (low slope), 6/12 (medium), 8/12 (steep), and 12/12 (very steep = 45°). A 6/12 pitch equates to approximately 26.6 degrees.
How do I calculate rafter length from roof pitch?
Rafter length per foot of run = √(rise² + run²) ÷ run. For a 6/12 pitch: √(6² + 12²) ÷ 12 = √180 ÷ 12 = 13.416 ÷ 12 = 1.118. Multiply this by half the total span (plus overhang). For a 24ft span (12ft half) with 1ft overhang: rafter length = 1.118 × 13 = 14.53 feet. This gives the exact rafter length from ridge to birdsmouth.
What is the difference between roof pitch and roof slope?
While these terms are often used interchangeably, roof pitch traditionally refers to the ratio of rise to span (total width), while roof slope refers to rise per 12 inches of run. In modern usage, roof pitch is almost always expressed as rise per 12 inches of run. For example, a roof that rises 6 inches per 12 inches of run is called a 6/12 pitch, and this corresponds to a slope of 50% or 26.6 degrees.
How does roof pitch affect material choices?
Roof pitch determines which roofing materials are suitable. Low slope (2/12-4/12): Requires single-ply membranes, modified bitumen, or standing seam metal. Medium pitch (4/12-9/12): Compatible with asphalt shingles, wood shakes, tile, and most materials. Steep pitch (9/12+): May require special attachment methods, scaffolding, and safety equipment. Pitches below 2/12 require flat roofing systems with positive drainage.