Underlay Durability

NZBC Clause B2.3.1 requires non-structural building elements and those that are moderately difficult to replace to have a durability of 15 years, or, if difficult to replace, to have the same durability as the more durable elements of a system. Alternatively, the design should allow the replacement of the less durable elements without removing the more durable components.

Compliance with NZBC B2, therefore, requires roofing underlay to have durability equal to that of the roof cladding and no less than 15 years.

When finishing on an eaves flashing the eaves flashing should finish on the horizontal face of the eaves flashing so that is is not exposed to UV or able to flap in the wind.

All underlays, particularly those made from synthetics (including some foils), may be vulnerable to degradation from fumes and reflected UV. Advice should be requested from the supplier when they are to be used in a situation where they may be exposed to fumes or reflected UV.