Roof pitch is how steep a roof is. It is described as a rise over a run, such as 3 in 12, meaning the roof rises 3 units for every 12 horizontal. It can also be given as an angle in degrees, which this calculator works out from the rise and run.

How do I calculate rafter length?

The rafter is the sloping side of the right triangle formed by the rise and the run. By Pythagoras, it is the square root of the rise squared plus the run squared. The calculator gives the rafter length for the rise and run you enter.

What is a low pitch roof?

A low or shallow pitch is generally under about 15 degrees, and needs roofing designed for it so water drains properly. Steeper roofs shed water and debris more easily. Always follow the roofing manufacturer's minimum pitch.

Roof Pitch Calculator

Work out a roof pitch from the rise and the run. Enter the vertical rise and the horizontal run, in the same units, and the calculator gives the pitch as an angle in degrees, as a rise in 12 ratio, and the rafter length along the slope. Useful for planning a roof, a pergola, or a lean-to.

Rise

Run (horizontal)

0°

roof pitch angle

Pitch ratio0 in 12

Rafter length0 m

How it works

The pitch angle is the inverse tangent of the rise divided by the run. The rise in 12 ratio scales the rise so the run is 12. The rafter length, the sloping side, is the square root of the rise squared plus the run squared, from Pythagoras.

Worked example

A 3 metre rise over a 6 metre run is an angle of about 26.6 degrees, a 6 in 12 pitch, and a rafter length of about 6.7 metres.

Related calculators

Right Angle Triangle Calculator: sides and angles.
Pythagorean Theorem Calculator: find a side.
Trigonometry Calculator: angles and sides.
Square Metre Calculator: roof area.

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