Ponding happens when water is unable to drain from a roof or gutter surface. Possible causes include lack of fall, poor penetration design, and damage to sheet ribs due to excessive spans or foot traffic. The accumulated water increases the time of wetness and can lead to poultice corrosion.
All paint systems on factory pre-painted materials are permeable to a degree and will delay, but not prevent, the corrosive effects of ponding.
Ponding can occur in gutters and spouting when joints or outlets are higher than the sole of the gutter, or when debris is left to accumulate.
To help prevent ponding, the minimum pitch for all metal roof cladding in New Zealand is set at 3°. At pitches of lesser pitches deflection of the structural members or settlement of the building can compromise drainage.
Low pitch roof spans must be sized according to the type and frequency of roof traffic to prevent ponding caused by rib damage, and penetration flashings must be of a free draining type.