Convert roof pitch between degrees, percent and rise/run – including rafter length, pitch ratio and classification by roof type.
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Calculating Roof Pitch: Degrees, Percent and Rafter Length
Roof pitch can be stated in three ways, and this tool converts between all of them: the angle in degrees (common among roofers and in construction drawings), the pitch in percent (rise per 100 cm of horizontal length, common for flat roofs and solar installations), and the ratio of rise to run when you measure directly on the roof. The maths behind it: pitch % = tan(angle) × 100. A 35° roof therefore rises by about 70% – 70 cm per metre.
Rafter length calculated along the way
From run and rise, the rafter length follows via Pythagoras: √(rise² + run²). With a 5 m run and 35° pitch you get a 3.5 m rise and a 6.10 m rafter length – excluding overhangs at eaves and ridge, which you add depending on the construction. For ordering material (rafters, counter battens, insulation, underlay) this is the decisive figure.
Minimum pitch: how much slope does each roof need?
Every roofing material has a minimum pitch below which it is not rainproof without extra measures: large-format concrete tiles need about 22°, classic plain tiles 30°, slate 12–25° depending on the laying technique, and trapezoidal sheet metal manages 7° with sealed overlaps. If the planned pitch falls below the minimum, German ZVDH trade rules require a watertight sub-roof. Flat roofs below 5° need full-surface waterproofing with at least a 2% fall for drainage.
Practical examples
Photovoltaics: modules from about 15° pitch largely self-clean in rain; the yield-optimal tilt in Central Europe is 30–35°. Carport permits: many development plans cap the roof pitch – with percent mode you can quickly check whether your planned 15 cm fall over 3 m depth (5%, just under 3°) still counts as a flat roof. And if you inherit an old drawing stating '1:2', simply enter rise 1 and run 2: 26.57°.
