COP v3.0:penetrations; penetration-design

9.5 Penetration Design 

It is the designer’s responsibility to select the type of penetration flashing appropriate to the design requirements and the client’s expectations. Penetrations can be broadly put into two categories: Sheetmetal flashings and Boot flashings

9.5.1 Sheetmetal Penetration Flashings 

Sheetmetal penetration flashings can be classified as:


9.5.1A Sheetmetal Flashing Classification

Back TrayCurb Design

Over Profile



Under Profile



The first decision should be the back flashing, should it be over the profile (watershed) or under the profile (soaker). 

9.5.2 Watershed Back Flashings 

Watershed back flashings are easy to install and to weatherproof, particularly if the roof is already in place. The drawbacks are their limits in width and, sometimes, noise or condensation issues. Long lengths of watershed flashings may require multiple end laps which are vulnerable to leakage. Where there are end laps or foot traffic is expected on the watershed flashing, the flashing must be supported in the pan or the profile by rigid closed cell foam or similar.

In many residential cases where the flashing is visible, the aesthetic values of watershed flashings may render them inappropriate for this application, unless the penetration is situated close to the apex.

The maximum width of a watershed flashing is controlled by the coil width of 1.2 m The practice of making wider watershed flashings by running flashings horizontally with laps at 1.1 m is not acceptable, as the numerous joins are prone to leakage. Wider watershed flashings can be fabricated using longitudinal standing-seam techniques on suitable support.

9.5.3 Soaker Back Flashings 

Soaker back flashings are visually attractive and are less prone to noise or condensation issues. They are relatively easy and economical to install at the time of roof laying, but more difficult and costlier if post installation is required.

9.5.4 Curb Design Level Back Curbs 

Level back curbs are the most common solution for flashing penetrations and are the easiest to fabricate and install.

They may tend to collect debris as they have little or no transverse fall, which can limit durability. However, with normal maintenance when manufactured from the same material as the roof they should achieve the durability requirements of the NZBC.

For penetrations wider than 600 mm, or those in aggressive environments or in situations where maintenance is difficult, a freer draining design such as an arrowhead or cricket is preferable. Arrowhead Back Curbs 

Arrowhead back curbs have a diverter that provides transverse fall for diverting rainwater, enabling them to accommodate bigger catchment areas and self-cleanse. They have a small flat area at the base of the arrowhead that may require maintenance. Cricket Back Curbs 

Cricket back curbs divert water with less turbulence than either arrowhead or flat back curbs and have no flat areas to catch debris. They may be fabricated from the same material as the roof or welded from 1.6 mm aluminium and powder-coated to match the roof colour, to give a durable and matching solution. They offer the most durable and weathertight solution to penetration back kerbs.