A roof or wall penetration such as a dormer or a vent acts as an obstruction to the rainwater flowing off the roof.

The back and side curbs of a penetration flashing act as a gutter draining the catchment behind the obstruction which often discharges onto the roof into one corrugation or pan of a profiled sheet.

The position, orientation, penetration design, and the roof pitch will determine how much the penetration obstructs the free flow of rainwater. A cricket penetration design will shed rainwater more efficiently than a flat back curb because it will reduce turbulence. See Cricket Penetration Patterns.

The water carrying capacity of the roof cladding profile becomes critical when the penetration is wide or is distanced from the ridge. For this reason, rectangular penetrations should be placed lengthwise or at 45° down the roof, and a second penetration should not be placed in tandem down the roof slope as this can cause flooding of the profile at the lower penetration.

Where the use of metal cladding is unsuitable for multi-penetrations alternative materials should be considered. See Alternative Materials.

Designers and roofing contractors often assume that one corrugation is sufficient to drain the catchment without making the calculations necessary to know if this assumption is correct. The discharge capacity of the profile should be calculated as shown in Discharge Capacity.
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