Flashing Design

Transverse flashings run across the roof slope at right angles to the ribs of the roof and longitudinal flashings run down the roof slope.

Ingress of rain into the roof or wall cavity via the flashings can be caused by the pressure differential between the air outside and that inside the roof or wall cavity. The pressure differential caused by wind gusting fluctuates greatly, so a gap should be created behind the outer edge of a flashing to provide a pressure cushion. Longitudinal flashings are best designed with a pressure equalisation gap to balance varying pressures and prevent capillary action. (See Capillary Action.)

The preferred maximum production length of flashing is 6–8 m, depending on profile strength. As any sealed lap secured by rivets or screws effectively becomes one length, provide expansion joints where required. Flashings are similarly restricted in length as roof and wall cladding sheets and are subject to the same requirements and expansion provisions. (See Roof Cladding Expansion Provisions.) Inadequate provision for flashing expansion can also cause roof noise

Avoid wet contact between the edges of flashings and concrete, plaster or butyl rubber (See Compatibility.) When notched flashings are used, the cut edge must not touch the pan, as that can cause corrosion from abrasion.

Notched Flashing

Notched Flashing

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