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

Roof pitch is the steepness of a roof, and getting it right matters whether you are framing a new roof, building a pergola or lean-to, choosing roofing material, or checking a pitch meets a manufacturer's minimum fall for proper drainage. This calculator works out the pitch three ways from two simple measurements. Enter the rise, the vertical height the roof climbs, and the run, the horizontal distance it covers, in the same units such as metres. The calculator instantly returns the pitch angle in degrees, the pitch ratio expressed as a rise in 12 (the format tradies and roofing suppliers use, such as 6 in 12), and the rafter length, the actual sloping distance along the roof line from Pythagoras' theorem. The maths behind it is straightforward trigonometry: the angle is the inverse tangent of rise divided by run, the ratio scales that same relationship to a run of 12, and the rafter length is the square root of the rise squared plus the run squared. A 3 metre rise over a 6 metre run, for example, gives an angle of about 26.6 degrees, a 6 in 12 pitch, and a rafter length of roughly 6.7 metres. Use the results to compare against a roofing product's minimum pitch, order the right length of rafters or trusses, or simply understand how steep a roof design really is before committing to it.

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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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