How To Calculate Length Of Roof Rafter

How To Calculate Length Of Roof Rafter

In this article, we will discuss how to calculate the length of roof rafter. But, before we jump on the calculation, we need to know some technical terms related to a pitched roof.

Rafter Of Roof:

Rafter is the distance between the ridge or hip of the roof to the wall plate of the external wall.

Span Of Roof:

Span is the clear distance between the supports of an arch, beam or roof truss.

Rise Of Roof:

It is the vertical distance between the wall and the top of the ridges.


Pitch Of Roof:

The pitch of a roof is its vertical rise over its horizontal span. The angle, or pitch of a roof, is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. For example, a roof that rises 6 inches for every 12 inches of horizontal run has a 6/12 pitch.

There is no standard, universal pitch of the roof. It varies depending on culture, climate, style, and available materials. The most common roof pitch is designed with a pitch ranging from 4/12 to 9/12.

  • Pitch = Rise/Run

Run Of Roof:

In roof framing, the run is the distance from the outside of the wall’s top plate to a point directly below the center of the ridge. In simple words, the run is the half distance of span.

  • Run = Span/2

Exmple 1:


  • Span = 30 ft.
  • Pitch = 5/12

Run = Span/2 = 30/2 = 15 ft.

Rafter length =??

According to Pythagoras theorem, a ² + b ² = c ²

Run ² + Rise ² = Rafter ²

Rafter = √ (Run ² + rise ² )

  • Pitch = 5/12
  • Rise/Run = 5/12

∴ Rise = Run x 5/12 = 15 x 5/12 = 6.25 ft.

Rafter ² = Run ² + Rise ² = 15 ² + 6.25 ² =264.0625

∴ Rafter = 264.0625 = 16.25 ft.

Example 2:

Calculate Rafter Length Using Angle:


  • Length of roof = 30 ft.
  • Rise = 6 ft.
  • Angle = 20°

We know,

Sinθ = Opposite/Hypotenuse, Cosθ = Adjacent/Hypotenuse and

tanθ = Opposite/Adjacent

In the above triangle,

  • a = Opposite = Run
  • b = Adjacent = Rise
  • c = Hypotenuse = Rafter
  • Cosθ = Adjacent/Hypotenuse = b/c

c = b/Cosθ = 6/Cos20°

= 6.38 ft.

∴ Rafter length = 6.38 ft.

So, it was an easy calculation. I hope you now can easily calculate the length of your roof rafter. If you have any questions let me know in the comments.

Also Read

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10 thoughts on “How To Calculate Length Of Roof Rafter”

  1. Pitch is the rise over the span and slope is the rise over run. So a 6/12 slope equals a 6/24 pitch which reduces to 1/4 pitch. The term Pitch is a rarely used corectly in roof framing. A print will give the slope on the elevation drawing.


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