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Although the information contained in this Code has been obtained from sources believed to be reliable, New Zealand Metal Roofing Manufacturers Inc. makes no warranties or representations of any kind (express or implied) regarding the accuracy, adequacy, currency or completeness of the information, or that it is suitable for the intended use.

Compliance with this Code does not guarantee immunity from breach of any statutory requirements, the New Zealand Building Code or relevant Standards. The final responsibility for the correct design and specification rests with the designer and for its satisfactory execution with the contractor.

While most data have been compiled from case histories, trade experience and testing, small changes in the environment can produce marked differences in performance. The decision to use a particular material, and in what manner, is made at your own risk. The use of a particular material and method may, therefore, need to be modified to its intended end use and environment.

New Zealand Metal Roofing Manufacturers Inc., its directors, officers or employees shall not be responsible for any direct, indirect or special loss or damage arising from, as a consequence of, use of or reliance upon any information contained in this Code.

New Zealand Metal Roofing Manufacturers Inc. expressly disclaims any liability which is based on or arises out of the information or any errors, omissions or misstatements.

If reprinted, reproduced or used in any form, the New Zealand Metal Roofing Manufacturers Inc. (NZMRM) should be acknowledged as the source of information.

You should always refer to the current online Code of Practicefor the most recent updates on information contained in this Code.


This Code of Practice provides requirements, information and guidelines, to the Building Consent Authorities, the Building Certifier, Specifier, Designer, Licensed Building Practitioner, Trade Trainee, Installer and the end user on the design, installation, performance, and transportation of all metal roof and wall cladding used in New Zealand.

The calculations and the details contained in this Code of Practice provide a means of complying with the performance provisions of the NZBC and the requirements of the Health and Safety at Work Act 2015.

The scope of this document includes all buildings covered by NZS 3604, AS/NZS 1170 and those designed and built under specific engineering design.

It has been written and compiled from proven performance and cites a standard of acceptable practice agreed between manufacturers and roofing contractors.

The drawings and requirements contained in this Code illustrate acceptable trade practice, but recommended or better trade practice is also quoted as being a preferred alternative.

Because the environment and wind categories vary throughout New Zealand, acceptable trade practice must be altered accordingly; in severe environments and high wind design load categories, the requirements of the NZBC will only be met by using specific detailing as described in this Code.

The purpose of this Code of Practice is to present both Acceptable Trade Practice and Recommended Trade Practice, in a user-friendly format to ensure that the roof and wall cladding, flashings, drainage accessories, and fastenings will:

  • comply with the requirements of B1, B2, E1 E2 and E3 of the NZBC;
  • comply with the design loading requirements of AS/NZS 1170 and NZS 3604 and with AS/NZS 1562;
  • have and optimised lifespan; and
  • be weathertight.

COP v24.03:Internal-Moisture; Minimising-Ingress-Water-Vapour-Ceiling-Cavity

10.9 Minimising Ingress of Water Vapour into the Ceiling Cavity 

The first line of defence for managing roof space moisture levels is maintaining low relative humidity in the dwelling areas. The second line is preventing excessive amounts of moisture entering the ceiling cavity. It is recommended that all ceilings are square stopped and all penetrations (cables, pipes, hatches, etc.) are caulked. Only use downlights that are airtight and have a gasket

A gloss painted plasterboard ceiling presents some resistance to the passage of water vapour but is not a complete barrier. Vapour will also find its way through any minor gaps in architectural details, and it is air transport through gaps that is responsible for 95% of the passage of water vapour into the ceiling space.

Ceiling tiles and tongue and groove ceilings are considerably more porous than plasterboard. Unsealed downlights can be a major source of moisture movement into the ceiling cavity and should be avoided where possible.

See BRANZ Facts Roof Ventilation #3 .

In some older New Zealand homes, vapour barriers have been used to limit entry of moist air into the ceiling space, but control of air movement into the cavity and removal of damp air by ventilation is a more practical approach.

Cavity systems prescribed under E2AS1, except those behind masonry veneer, are classified as drained not ventilated. That means they must be closed at the top to restrict air movement between the cavity and the roof space. Closing a cavity off at the top still allows reasonable ventilation of the cavity while preventing excessive amounts of moisture rising to the ceiling space.  Alternatively, wall cavities can be vented externally.