Offcut WorksRoof pitch calculator
Type a value in any box and the others update. Add a horizontal run to get the rise and the rafter length.
How the numbers are worked out
All the pitch values come from the same angle. Gradient = tan(angle) × 100. Rise per 12 in = tan(angle) × 12. Rise per metre = tan(angle) × 1000 mm.
Rise = run × tan(angle). Rafter length = run ÷ cos(angle). A 30° roof with a 3 m run has a rise of 1.73 m and a rafter length of 3.46 m, before any overhang.
Measuring the pitch of an existing roof
Hold a spirit level horizontally against a rafter, measure 300 mm along it, then measure straight down from the level to the rafter. Divide the drop by 300 and multiply by 1000, then enter the answer in the "Rise per 1 m of run" box. A digital angle finder on the rafter also works.
Minimum pitch depends on the covering
Tiles, slates, metal sheets and membranes each have a minimum pitch set by the manufacturer. Check their guidance before you choose a pitch, because a pitch that is too low can let water back up under the covering.
Rafter length and overhang
Rafter length here runs along the slope from the wall line to the ridge. Add the overhang separately, measured along the rafter, and allow for the ridge board thickness if you use one.
Questions
What is roof pitch?
Roof pitch is how steep a roof slopes. In the UK it is usually given as an angle in degrees. In the US it is given as the rise in inches for every 12 inches of run, such as 6/12.
How do I measure the pitch of an existing roof?
Hold a spirit level horizontally against a rafter, measure 300 mm along it, then measure straight down. A digital angle finder on the rafter also works.
What is the minimum pitch for my roof?
It depends on the covering. Check the manufacturer's guidance for the tiles, slates, sheets or membrane you plan to use.
Does rafter length include the overhang?
No. It measures along the slope from the wall line to the ridge. Add the overhang separately.
- This calculator uses geometry only. It does not give a minimum pitch for any roof covering.