Roof Pitch & Rafter Calculator
Calculate rafter length, rise and pitch for a pitched roof. Enter the run (half the span) and either the pitch angle or the rise.
Rafter length is the “line length” (run ÷ cos of the pitch) plus your overhang, measured along the top of the rafter. It does not include the plumb/seat cuts or ridge thickness deduction, allow for those when marking out.
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How to use the roof pitch calculator
Enter the roof's rise and run (half the span) and the calculator returns the pitch as an angle and as a ratio, along with the rafter length. Pitch is simply how steep the roof is, the vertical rise over the horizontal run, and it drives everything from rafter lengths to which coverings you can use.
Frequently asked questions
How do I calculate roof pitch?
Divide the vertical rise by the horizontal run and take the inverse tangent for the angle, or express it as a rise:run ratio. The calculator works out both from your measurements.
What is a standard roof pitch?
Most UK pitched roofs fall between about 30° and 45°. Below roughly 20° is a low pitch roof needing specific coverings; above about 45° is steep.
Does roof pitch affect which tiles I can use?
Yes, every tile and slate has a minimum pitch it is rated for. Always check the manufacturer's minimum pitch before choosing a covering.