Underlay Durability

NZBC Clause B2.3.1 requires building elements that are non-structural and are moderately difficult to replace to have a durability of 15 years. It also requires building elements that are part of a building system and are difficult to replace to have the same durability or to be designed so materials with lesser durability can be replaced without removal of more durable elements.

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

For durability reasons, the roof underlay should ideally finish on to an eaves flashing, so that the underlay is not exposed to UV in the long run or be able to flap or vibrate in the wind.