Roof pitch calculator | Known rise and run

Roof Pitch Calculator

Calculate roof pitches with this free and simple online tool. Enter the rise and run for the pitch, length, and angle of the rafter.

A roof pitch that is based on a fixed rise and run and does not end up with a whole number is known as a bastard pitch. This is rarely is a whole number, it is not a rare event, and it is not a nightmare! It just means the top of the roof has to end out at a specific place.


The rise is basically the distance from the top of the roof to the bottom. It is the vertical distance a rafter travels. Although the top of the roof may be clearly defined, the bottom is a little harder to determine. It is very important to know that the rise for a bastard pitch rafter is just like the rise for a standard rafter. It starts at the top of the HAP on the outside edge of the wall. You must be familiar with the HAP to make this work.

bastard roof pitch rafter calculator


The run is the horizontal distance a rafter travels. It is simple. The run is the distance from the outside of the wall to the inside of the ridge. This holds true for standard two rafters as well as situations requiring a single rafter (lean to’s).