All penetration flashings should have all-around clearance so that vibration, wind or expansion does not induce any stress or noise at the juncture with the roof. If the penetration flashings are fixed positively to the roof, they should be considered a part of it and expansion allowance should be provided. A 10 mm minimum clearance allowance should be made for movement between the flashing upstand and the penetration structure.
A minimum of 100 mm clearance must be provided between the end of the sheeting upside of the penetration and the back-curb upstand to provide for any temporary obstruction or abnormal runoff. The back-curb must be deep enough to provide 150 mm roofing cover and roofing fasteners must not penetrate this flashing.
All back curbs should pass the 'tennis ball test', which requires the free passage of a tennis ball down the roof, past the penetration. N.B. Back curbs without fall will fail this test.
Roof underlay should lap the back curb by 50 mm.
When several penetrations are in tandem or close proximity and an alternative design is considered appropriate, another material such as membrane roofing can be used. Such materials should be compatible with the metal cladding and be designed with the same parameters as metal penetrations as described in this COP. See
Alternative Materials. It is usual with low pitch roofs to fold the back and side curbs and the front apron at 90° to the roof, which means a transition is needed if the unit within the penetration is required to be vertical, or the front and back curbs should be folded to suit the roof pitch. See
Penetration Upstand.
There are two conditions under which a different design of penetration flashing may have to be considered.
It is accepted practice when roofing large commercial or industrial buildings that the roof cladding will be laid over the openings for any intended penetrations, which means that timber or other support should not extend above the purlin line at the time of laying.
The penetration design must be determined before the roof cladding is installed temporarily over the opening, and the extra purlin support for the cladding must be in position before roof cladding work commences.
This technique has the advantage that it closes the building in quickly and, also, provides safety for workers since penetration openings are regarded as a safety hazard. The preferred method is to order longer sheets so that they may be lapped at the penetration.
There are two positional variations that can affect the design of the flashings used to weather any penetration.
When the opening falls between the ridge purlin and the penultimate or next purlin down the roof, a back flashing, known as a watershed, can be run up to the ridge for weathering, as shown in
Watershed (a).When the opening falls between the gutter purlin and the penultimate or next purlin up the roof, a flat tray under-flashing known as a soaker can be used to drain directly into the gutter.
All other positions on the roof require soaker flashings Type A. See
Type A Soaker.
