Roof Pitch Calculator
Calculate roof pitch from the vertical rise and horizontal run, with angle, slope percentage, pitch factor and rafter length.
How roof pitch is calculated
Roof pitch is the angle created by the vertical rise of the roof compared with its horizontal run. Enter the rise and run measured over the same roof slope and BuildCalcs returns the pitch in degrees, the slope percentage, a pitch factor and the straight slope length.
Roof pitch formula
Pitch angle = arctan(rise ÷ run)
The slope or rafter-line length is calculated using Pythagoras: √(rise² + run²). This is the straight geometric length before any allowance for overhangs, ridge details, birdsmouths or construction-specific cuts.
Worked example: 1200 mm rise and 2400 mm run
A 1200 mm rise over a 2400 mm horizontal run gives a roof pitch of approximately 26.6°. The slope is 50%, the pitch factor is approximately 1.118, and the straight slope length is about 2.683 m.
If the building is 6 m long and has two equal roof slopes, the simple geometric roof area is about 32.2 m² before overhangs, laps, wastage and other roof details are added.
What is the horizontal run?
The run is the horizontal distance from the outside wall or reference point to the ridge centreline for one roof slope. On a symmetrical duo-pitch roof it is often half the building span, but this is not true for every roof form.
Pitch factor and roof area
The pitch factor shows how much longer the slope is than its horizontal run. It is useful when converting a plan dimension into an approximate sloping dimension. Roof covering quantities should still be checked against the actual roof geometry and the manufacturer's coverage information.
Frequently asked questions
What is a 1 in 2 roof slope in degrees?
A rise of 1 for every 2 of horizontal run is approximately 26.6°. Enter any proportional rise and run, such as 1000 mm and 2000 mm, to confirm it.
Is roof pitch the same as roof slope percentage?
No. They describe the same geometry in different ways. The calculator shows both the angle in degrees and the rise divided by run expressed as a percentage.
Does the rafter length include the eaves overhang?
No. The displayed slope length runs only over the rise and run you entered. Add any required overhang or construction detail separately.
Can I use the roof area result to order tiles?
Use it as an early area estimate, then use the Roof Tile Calculator with the actual product coverage and an appropriate allowance.