Runoff is the ability of the roof cladding to discharge maximum rainfall without water penetrating through side laps, end laps or flashings and depends on rainfall, the catchment area, the roof pitch, and the profile geometry.
The pitch determines the rate of flow. Steep slopes shed water faster than shallow slopes. However, at minimum falls, with a maximum rainfall intensity of 100 mm/hour, trapezoidal rib profiles with a height greater than 26 mm can have a maximum run of at least 40 m, and trapezoidal rib profiles with a rib height 20-25 mm can have a maximum run of at least 30 m.
The roof gutter capacity calculator in 5 Roof Drainage can calculate capacities for any known profile, rainfall and pitch, by treating the pan width and rib height as a gutter and the rib to rib dimension plus any discharge from adjacent surfaces as the catchment. When the sheeting length exceeds the above limitations, the capacities should be checked.