The roofer talks in degrees, the development plan in percent, and on site you measure rise and run. The Roof Pitch Calculator translates between all three – this tutorial shows the maths and the building practice behind it.
The conversion
Pitch in % = tan(angle) × 100. A 35° roof thus rises 70 cm per metre (70%), a 45° roof exactly 100%. Important: percent and degrees are not proportional – 100% is not 90° but 45°. Measured from rise and run: angle = arctan(rise ÷ run).
Step by step
- Choose the given quantity: angle in degrees, pitch in percent, or rise + run in metres
- Enter the run even in degree/percent mode: then the calculator delivers the rafter length as well
- Read the result: angle, percent, pitch ratio, rafter length and roof type classification
Worked example
Measured at the roof truss: 3.50 m rise over 5.00 m run. Result: angle = arctan(0.7) = 35°, pitch 70%, rafter length via Pythagoras √(3.5² + 5²) = 6.10 m – excluding overhangs at eaves and ridge, which need an extra 30 to 50 cm depending on the construction. This figure is the basis for ordering rafters, battens and insulation.
Minimum pitch: the regulatory core
Every roofing material needs a minimum slope to be rainproof without extra measures: large-format concrete tiles around 22°, plain tiles around 30°, slate 12–25° depending on the laying method, trapezoidal sheet with sealed overlaps from about 7°. If the pitch falls below the minimum, German ZVDH trade rules demand a watertight sub-roof – a significant cost factor to know before choosing the material. And for solar roofs: 30–35° tilt is yield-optimal in Central Europe, and from about 15° modules largely self-clean in the rain.
