Rafter length
Roof pitch
Roof pitch (%)
Roof pitch (x:12)

This roof pitch calculator is a handy tool that will help you quickly assess what is the pitch of your roof of what length of rafters you need for your construction works. We will explain to you how to determine the roof pitch step by step. You can also use this roof slope calculator to recalculate the roof pitch from degrees to percents.

What is the roof pitch?

roof pitch

Roof pitch is simply the slope created by the rafter. It can be assessed in two ways - either as the angle the rafter makes with the horizontal or the proportion between the rise and the run of the roof.

Roof pitch is often expressed as a ratio between rise and run in the form of x:12. For example, a pitch 1:12 means that per every twelve yards of building length the rise will be equal to one yard.

We can divide roofs into the following categories:

  • Flat roofs are not perfectly flat in reality - they need a small slope for water runoff. Generally, these roofs have a pitch from 1/2:12 to 2:12 (from 4.2% to 16.7%).
  • Low pitched roofs have the pitch below 4:12 (33.3%). These are generally difficult in maintenance, as they require special materials to avoid leaks.
  • Conventional roofs have the pitch ranging from 4:12 to 9:12 (33.3% to 75%). They are the easiest ones to construct and they are safe to walk on.
  • High-pitched roofs often require additional fasteners. They hava a pitch that can be as high as 21:12 (175%).

How to calculate roof pitch?

Generally, roof pitch is calculated from the same formula that describes a right triangle. You can use the following equations to find the length of the rafter and the slope:

  • rafter² = rise² + run² (from the Pythagorean theorem);
  • rise / run = pitch, where pitch is expressed in percent;
  • pitch = tan (angle), where angle is the roof pitch expressed in degrees.

If you want to find the roof pitch in the form of x:12, simply calculate the pitch and substitute 12 for the run length. The value displayed as the rise will be your x.

Roof slope calculator - example of use

  1. Measure the run length - it is the horizontal distance between the roof ridge and the wall of the building. Let's assume it is equal to 6 m.
  2. Measure the rise of your roof. Let's say it is equal to 1.5 m.
  3. Calculate the rafter length, substituting these values into the following formula:

rafter² = rise² + run² = 1.5² + 6² = 2.25 + 36 = 38.25 rafter = √38.25 = 6.18 m

  1. Calculate the roof pitch as the proportion of rise and run:

pitch = rise / run = 1.5 / 6 = 25%

  1. Recalculate this value into an angle:

angle = arctan(pitch) = arctan(0.25) = 14°

  1. Finally, you can find the roof pitch in the form of x:12.

x = pitch * 12 = 0.25 * 12 = 3

  1. The pitch of your roof is 3:12. It can also be written down as 25% or 14°.
Bogna Haponiuk

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