Rafter Length Calculator -

What is a Rafter Length Calculator?

A Rafter Length Calculator determines the true length of a common rafter, which is the diagonal structural member extending from the top wall plate to the ridge board. The calculation is based on the Pythagorean theorem (a² + b² = c²), where the rafter is the hypotenuse of a right triangle formed by the "run" (half the span) and the "rise".

This tool simplifies framing by providing the precise measurement needed for cutting rafters, ensuring a proper fit and a structurally sound roof. It accounts for the roof pitch and overhang to give a "line length" measurement.

Common Rafter vs. Hip vs. Valley Rafter

Rafter Type	Function	Calculation Complexity
Common Rafter	Spans from wall plate to ridge board; the most numerous type.	Standard
Hip Rafter	Forms the outside corner of a hip roof, running diagonally to the ridge.	More complex (different pitch angle)
Valley Rafter	Forms the inside corner where two roof planes meet.	More complex (different pitch angle)

How to Use the Rafter Length Calculator

Roof Span: Enter the total width of the building from one wall plate to the other.
Roof Pitch: Enter the roof's pitch (e.g., 4 in 12). This represents the amount of vertical rise for every 12 units of horizontal run.
Overhang: Input the horizontal length of the eave, from the building wall outwards.
Unit: Choose your preferred unit of measurement.
Calculate: Get the exact length of the common rafter, including the portion needed for the overhang.

Rafter Length vs. Roof Pitch

Comparison based on a 24-foot span

Frequently Asked Questions (FAQ)

What is the "run" of a rafter?

The "run" is the total horizontal distance the rafter covers. For a standard gable roof, the run is exactly half of the total roof span.

Does this calculator account for the thickness of the ridge board?

For simplicity, this calculator provides the theoretical length to the center of the ridge. In practice, carpenters typically subtract half the thickness of the ridge board from the rafter's run before making the final cut to ensure a flush fit.