How do you calculate common rafter length?

Common rafter length is the hypotenuse of a right triangle where the base is half the roof span and the height is the rise. Using Pythagoras: Rafter = sqrt((Span/2)² + Rise²). For example, with a 6m span and 2m rise: sqrt(3² + 2²) = sqrt(13) = 3.606m. Add the eave overhang (measured along the slope) for the total cutting length.

How do you calculate hip rafter length?

A hip rafter runs diagonally from the ridge to the corner of the building. Its length is: sqrt(Common rafter length² + (Span/2)²). Alternatively, it can be calculated as sqrt((Span/2)² + (Span/2)² + Rise²). Hip rafters are always longer than common rafters and are typically one size deeper to account for the compound angle.

What is a birdsmouth cut on a rafter?

A birdsmouth is a notch cut into the rafter where it sits on the wall plate. It consists of a plumb cut (vertical) and a seat cut (horizontal). The seat cut should be no more than one-third of the rafter depth to maintain structural integrity. Typical birdsmouth depth is 38mm, leaving maximum timber above for strength.

How much eave overhang should I allow?

Standard eave overhang in UK construction is 300-450mm. This protects the wall below from rain and provides a fixing point for fascia and soffit boards. The overhang is measured horizontally from the wall face. Along the rafter slope, it will be slightly longer due to the angle. Building Regulations don't specify a minimum overhang.

How do I calculate roof pitch from span and rise?

Pitch angle = arctan(Rise ÷ (Span/2)). For example, with 6m span and 2m rise: arctan(2/3) = 33.7°. Common UK roof pitches are: 17.5° (low pitch tiles), 22.5° (concrete interlocking tiles), 30-35° (plain tiles), 40-45° (steep/traditional). The pitch determines which roofing materials can be used.

TradeCalculator

Rafter Length Calculator

Calculate common, hip and valley rafter lengths from span and rise using Pythagoras. Includes birdsmouth depth and eave overhang.

Rafter Type

Common, hip or valley

Roof Span (wall to wall)

Full span from wall plate to wall plate

Rise (wall plate to ridge)

Vertical height from wall plate to ridge

Eave Overhang

Horizontal overhang beyond wall (300-450mm typical)

Price Per Metre of Rafter (optional)

Cost per metre of rafter timber

How We Calculate This

This calculator determines rafter lengths using Pythagorean geometry. Enter the roof span and rise, and it calculates the hypotenuse length for common, hip or valley rafters, including eave overhang.

The formula

Common rafter = √((Span/2)² + Rise²)
Hip rafter = √(Common² + (Span/2)²)
Pitch = arctan(Rise ÷ (Span/2))
Total = Rafter + Overhang (on slope)

For a 7m span, 2.5m rise roof: common rafter = √(3.5² + 2.5²) = 4.30m. Hip rafter = √(4.30² + 3.5²) = 5.55m. Pitch = 35.5°.

Birdsmouth rules

Maximum birdsmouth depth: one-third of rafter depth
Seat cut must be fully supported on wall plate
Plumb cut angle matches the roof pitch
Level cut angle = 90° minus pitch

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.