Rafter length calculator

Estimate outputs: Rafter length, Rise to ridge. Adjust Building span, Horizontal overhang, Roof angle to match your project.

Inputs and units

Use the units shown beside each field. Replace the sample values with your measurements and the figures on the selected product packaging.

Parameter	Example input
Building span	8 m
Horizontal overhang	0.5 m
Roof angle	30 °

Formula and assumptions

Rafter length
(Building span / 2 + Horizontal overhang) / cos(Roof angle × π / 180)

Rise to ridge
Building span / 2 × tan(Roof angle × π / 180)

ceil = round up; floor = round down; max = the greater value. Trigonometric angles are converted from degrees to radians.

Allowance is applied before rounding to whole packages. Whole units are rounded up; displayed decimals are rounded only for readability.

Roof calculations assume a simple rectangular surface or equal pitch slopes. Include overhangs in the entered plan dimensions; check overlaps and effective coverage.

Worked example

Sample values are examples, not product specifications. Run the calculation again with your own measurements.

Result	Calculation
Rafter length	5.1962 m
Rise to ridge	2.3094 m

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