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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 v24.06:Durability; Metal-Corrosion

4.2 Metal Corrosion 

Corrosion is the process by which something erodes because of a chemical reaction.

Metal corrosion is a reaction of metal with its environment that causes measurable alteration and is part of metal's inherent tendency to revert to an original, more stable form. The red rusting of iron and steel is a visible example of corrosion and other examples include the weathering of copper and the oxidation of aluminium and zinc.

Corrosion can only happen in the presence of an electrolyte, e.g., water. The occurrence of salt (or other contaminants) in the water increases the conductivity of the electrolyte and therefore greatly increases the reaction rate.

Salt contamination will also affect the time of wetness.  On a clean surface, water vapour will condense at 100% relative humidity; on a salt-covered surface, a wet film can be formed at a relative humidity level of 75% or more.

Corrosion can also be the result of direct contact with another metal or substance, the result of run-off from incompatible surfaces or the fall-out of corrosive particles. Time of wetness, presence or lack of oxygen, and atmospheric contaminants greatly affect the rate of corrosion.

Differences in electrical potential on the surface of corroding metal create microscopic cells comprising cathodes and anodes. In the case of iron, the positively charged electrons in the anode react with the negatively charged hydroxyl ions in the electrolyte to form iron oxide on the anode. Similar reactions occur with other metals. Polarisation changes on the surface cause anodic areas to become cathodic and vice versa, so over time, the rate of corrosion is relatively uniform over the surface.

The build-up of debris on a cladding surface will promote corrosion. The salts in the debris react with the cladding each time they are wetted, and the deposits themselves impede surface drying, increasing the time of wetness.