COP v3.0:flashings; flashing-design

8.2 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 4.10.6 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 7.3.2 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 4.9 Compatibility.) When notched flashings are used, the cut edge must not touch the pan, as that can cause corrosion from abrasion.



8.2.1 Flashing Edges 

The dressed soft edging or the downturn of a notched flashing acts as a baffle to wind and rain at the outside edge of a transverse flashing, as does the stop-end of the cladding at the upper end. The void in between these two barriers acts as a pressure equalisation gap.  Longitudinal flashings should also have a small gap between the downturn and  the adjacent rib to allow for pressure equalisation and avoid capillary action.

Soft edging can be supplied in material compatible with the roofing material.  This may be a naturally malleable metal, or metal perforated to enable it to be shaped to the profile, see 17.3 MRM Soft Edge Standard.

Stop ends must be provided for all pitches in all wind zones

In extreme circumstances, a profiled or notched closure can be used between the stop end and the downturn or soft edge.  When closures or filler blocks are used, they must be adhered to the profile or secured by the cladding fasteners to ensure they remain in position.

Additional ventilation requirements must be considered when using profiled closures, see 10.10 Ventilation Pathways.

8.2.1G Flashing Hook

Anti-capillary hems must be flattened parallel but not completely crushed.