The profile capacity, the roof pitch, the catchment area and the Design Rainfall Intensity (DRI) should all be considered to ensure that the catchment area behind the penetration can be drained without flooding.

The penetration side flashing may only restrict water flow by up to 50% of the normal pan width, and may only do so if:
  • the design rainfall is no greater than 100 mm/hr;
  • the sheeting length is less than 40 m;
  • the catchment is less than 20 m²;
  • the profile is asymmetrical; and
  • the pan is at least 100 mm wide.
Symmetrical sinusoidal and symmetrical trapezoidal profiles must have full pan width for free discharge. See Roof Cladding Capacity.
When the total catchment area exceeds 40 m², the discharge must be spread over two or more pans to provide for additional drainage. When the catchment area includes the walls of adjacent buildings, it must be calculated as shown in Catchment Area.
In snow zones more than 500 m above sea level, only full pan width discharge must be used.

A diverter or cricket penetration design should be used when:

  • the width of the penetration exceeds 600 mm;
  • the catchment is greater than 40 m²; and
  • there is a likelihood of snow.

As rain flows faster on steeper pitches and will discharge at a greater speed, the capacity at the discharge area is determined by the roof pitch as well as the capacity of the pans, the height of the profile, and the catchment area.

Penetrations should ideally line up with the ribs of the sheeting. Because this does not happen often, the module of the roof cladding determines the width of the penetration.

The symmetrical profiles normally used on draped curved buildings to avoid panning, do not have sufficient rain carrying capacity to include large penetrations; consequently, the discharge should be shared over a number of corrugations or pans on a curved roof.

All metal roofs in N.Z. are required to have a minimum fall of 3° and this is not possible when using a drape curve design.

No penetration must be made in the pitch area between 0° and 3° for trapezoidal or 8° for corrugate profiles on a curved roof.
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