How to Calculate the Length of a Roof Rafter?

Rafter Of Roof: Rafter is the gap from the roof's edge or hip to the outside wall's roof plate.

Span Of Roof: Span is the visible gap among the arch, beam, or roof truss supports.

Rise Of Roof: This is the vertical gap from the top of the ridges to the wall.

Pitch Of Roof: A roof's pitch refers to its vertical elevation across its horizontal span. The angle, or pitch of a roof, is measured by the number of inches for each 12 inches horizontal expansion. As for instance, there is a 6/12 pitch on a roof that goes up 6 inches per 12 inches of horizontal run.

There is not any existence of standard universal roof pitch. It differs with culture, environment, style, and existing materials. A pitch ranging from 4/12 to 9/12 is the most common roof pitch.

Pitch = Rise/Run

Run Of Roof: The run belongs to the distance from the exterior of the top plate of the wall to a point directly underneath the center of the ridge in roof framing. Simply put, the run is the span's half size.

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. Simply stated, the run is the distance half of span.

Run = Span/2

The length of roof rafter is measured as follow based on the data provided below :-

Given,
Span = 30 ft.

Also Read: Different types of Roof Trusses construction for diverse lengths

Pitch = 5/12

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

Rafter length = Unknown

In accordance with Pythagoras theorem, a² + b² = c²

Run² + Rise² = Rafter²

Rafter = √(Run² + rise²)

Pitch = 5/12

Rise/Run = 5/12

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

Rafter² = Run² + Rise² = 15² + 6.25² =264.0625

Rafter = √264.0625 = 16.25 ft.