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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:Fitness-Purpose; Purlin-Creasing

12.2 Purlin Creasing 

Due to improvements in colour coating technology, the level of reflection and retention of gloss is higher. It will become much less obvious over time as gloss levels diminish and dirt accumulates on the roof.

Overdriven nails or screws can produce visible distortion on the purlin line in the pan of trapezoidal profiles that cannot be easily remedied.



Trapezoidal profiles with a wide pan manufactured from 0.4 mm steel and 0.7 mm aluminium are particularly susceptible to purlin creasing, and although it does not affect performance, their appearance can be aesthetically unacceptable

It is the responsibility of the roofing contractor to ensure that nails are not overdriven. A nail or screw should only be driven into the purlin to produce a 50% compression of the sealing washer or until the roof is firm.

Before fixing the roof cladding, the contractor should check the alignment of the purlins or girts. Purlins should be aligned within 5 mm tolerance of each other to avoid purlin creasing.

Purlins should be accurately positioned with their top face parallel to the rafter and should be fixed to a straight line.

When appearance is important or where wide pan trapezoidal cladding is close to eye level, heavier gauge cladding should be specified because light gauges such as 0.4 mm steel and 0.7 mm aluminium are likely to show distortion. Purlin creasing will happen on both concave and convex curved roofs if the recommended purlin spacings are exceeded, and great care should be taken to align purlins on such roofs.

Purlin creasing can be exacerbated by roof traffic. 14.6 Walking On Roofs

All trapezoidal and secret fix profiles will exhibit purlin creasing to some degree, the extent to which it is noticeable depends greatly on the line of sight and light conditions, which can change it from being immediately obvious, to almost invisible.  Purlin creasing can be minimised by design by specifying 0.55 mm, rather than 0.40 mm material, or selecting a profile with a narrower pan. Minimising roof traffic on G300 tray roofing will also help, but the only way to ensure that purlin creasing will not be an issue is to lay a roof on solid sarking, or by using a corrugate profile.