Rafter Length Calculator

How to Calculate Rafter Length

Calculating the length of rafters is crucial for accurate roof construction. This process involves determining the span, rise, and pitch of the roof to find the length of the rafter.

Rafter Length Calculation Formula

The formula for calculating rafter length is:

Rafter Length = √(Span² + Rise²)

Where:

• Span = (Building Width / 2) + Overhang - (Beam Thickness / 2)
• Rise = Span × Pitch
• Pitch = Rise / Run (typically expressed as x/12)

Calculation Steps

1. Calculate the span using the building width, overhang, and beam thickness
2. Determine the rise using the span and roof pitch
3. Use the Pythagorean theorem to calculate the rafter length
4. Convert the result to feet and inches if necessary

Example Calculation

Let's calculate the rafter length for a roof with the following dimensions:

• Building Width: 24 feet
• Overhang: 12 inches
• Beam Thickness: 6 inches
• Roof Pitch: 6/12

1. Calculate the span: \[Span = (24 ft × 12 in/ft / 2) + 12 in - (6 in / 2) = 147 in\]
2. Calculate the rise: \[Rise = 147 in × (6/12) = 73.5 in\]
3. Calculate the rafter length: \[Rafter Length = \sqrt{147^2 + 73.5^2} = 164.3 in\]
4. Convert to feet and inches: \[164.3 in = 13 ft 8.3 in\]

For this roof, the rafter length would be approximately 13 feet 8 inches.

Rafter Length Visualization

This diagram visually represents the relationship between the building width and the calculated rafter length, showing how the rafter length is longer due to the roof pitch.