Underlay Requirements

The NZBC requires underlays under profiled metal roofing and direct-fixed metal wall cladding on lined residential buildings to be both permeable and absorbent. 

Underlays fixed to the dry side of a lined drained cavity may be permeable and non-absorbent. 

Underlays are not required in unlined structures, but in such cases non-permeable, non-absorbent underlays such as foil are typically used to increase reflectivity and to minimise condensation.

In some wall and roof applications, the underlay is required to be Fire Retardant with Flammability Index ≤5 when tested to AS1530.2. Refer NZBC C/AS3-AS6, clause 4.17.8 

Underlays designated as self-supporting can be laid without support at spans up to 1.2 m.

Other requirements for underlays vary for different cladding systems, but the important features of absorbency, permeability, water resistance, tensile strength, edge tear resistance, PH, and durability are important to comply with the requirements of the NZBC.

The Code of Practice recommends synthetic self-supporting fire-retardant underlays for residential roofing applications.