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 roof pitch determines the rate of flow; steep slopes shed water faster than shallow slopes. The rib height and spacing of trapezoidal profiles also affects its shedding ability.

    For example: At a rainfall intensity of 200 mm/hour, a five-rib trapezoidal profile at 3°, with a rib height of 27 mm can have a run of 90 m.

The Maximum Run Calculator can calculate capacities for any known profile.

Revision Category: 
3 - Substantial change to recommendation
Revision Detail: 

Clause completely revised.

Draft Clause: