There are two main types of penetration laps described.

Type A:

Type A  is the normal type of roof cladding flashing overlap, which is not necessarily mating or sealed and has a minimum cover of 150 mm. It can be an under or overlap. An under-lap on a flat sheet, however, requires a flattened hem or hook. Sealing is required if the lap is exposed to the wind, if the pitch is below 10°, or in a very high design wind zone.

Level back-curbs without fall can be sealed if snow or ice is to be expected, but 45° diverter or cricket flashings do not require sealing. Snow and hail do not cause leaks, as it is only when they melt and water discharge is blocked that leakage can occur.

Type B:

Type B laps are completely filled with sealant, are a minimum of 25 mm wide, used with mating surfaces, and is mechanically fastened using 4 mm sealed rivets at a minimum of 50 mm centres. As all laps are required to be self-cleaning, they should be made in the horizontal plane, i.e., across the roof. If sheets or flashings require being lapped vertically ( i.e., down the roof) they should also be angled to be self-cleaning. See Flashing Laps.

Because of the many permutations of roof pitch and penetration width, it is not possible to provide a universal design or pattern for penetrations. It is, however, possible to provide a pattern for cricket flashings and acceptable angles for diverter flashings using the same method described in Cricket Penetration Patterns.


Clause Number: 
Revision Category: 
0 - Clause Removed
Revision Detail: 

Clause removed, see End Laps.

Draft Clause: