Building Underlays

10 July 2006

A new Standard is being developed to describe the properties that building underlay needs to have, in order to perform as it is intended to. Use of the Standard to specify the correct underlays will also support weather-tightness in buildings for dry and durable structures. NZS 2295:2007 Pliable, Permeable Building Underlays, is expected to be published in early 2007.

“The currently-used building underlays joint Standard, AS/NZS 4200 Parts 1 and 2:1994, does not cater sufficiently for specific NZ conditions and the use of breather-type building underlay, nor does it cover synthetic permeable underlays,” says Stuart Hayman, Technical Development Manager at AHI Roofing and Chair of the Standards Committee.

“The new Standard will incorporate synthetic products for wall underlay and cater for technological advances in building design, such as drained and vented cavities.” The new Standard NZS 2295 will supersede the existing Standard only in part and will be used for breather-type and synthetic underlays, while AS/NZS 4200 will continue to be used for other types of underlays, such as foils and vapour barriers.

“The building underlays Standard will outline property requirements and test methods for various types of underlay material for roofs and walls,” says Stuart Ng, Project Manager at Standards New Zealand. “It will also specify the correct underlay for each application and reflect up-to-date test methods and products currently in use within the NZ building industry.”

The Standard provides clear guidelines for the requirements to be met by manufacturers and importers of underlay material. It will ensure users of building underlay will have a clear description of the performance and application requirements.

Designers, architects, underlay manufacturers and importers, building inspectors, builders and roofers will use the Standard, to help:

  1. Specifiers to select and specify correct underlay type on plans;
  2. Manufacturers and importers meet market demand for suitable materials with the required properties;
  3. Installers to recognise the correct products that have been specified for installation; and
  4. Territorial authorities to know what to look for and test when  inspecting.

Material properties and test methods are given for pliable and  permeable underlay.  Where the underlay requires specific  properties for an application, this is provided.  A classification table and summary properties table in the appendices provide additional guidance.

Further research into moisture management within roofs will support a future amendment to the Standard that incorporates further advances in roofing technology and materials.

For more information please contact Stuart Ng, e-mail or phone 04 495 0921.