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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; Swarf-Staining-And-Cut-Edge-Corrosion

4.14 Swarf Staining and Cut Edge Corrosion 

Swarf is the term given to steel debris resulting from cutting or piercing a metal sheet or adjacent metal surfaces.

When cutting steel, any swarf remaining on the sheet starts corroding quickly and causes stains.  These stains are often mistaken for early deterioration of the cladding.

To some degree, swarf will normally be evident at the completion of any roof cladding job. The acceptability of swarf depends on how it got there, whether techniques have been applied to minimize it, and the visual exposure of the cladding.


Light, scattered swarf is acceptable in most situations.


Swarf created by acceptable means of cutting — i.e., power drills, self-drilling screws and shears — will be either loose or lightly adhered to the surface film of painted or unpainted sheets.  Most swarf can be removed by daily hosing, sweeping, or blowing which should be done at the end of each day and at the completion of the job.  Avoid blowing loose swarf under adjacent cover flashings.

Any remaining swarf will not be in contact with the metallic substrate and will not cause deterioration of the roof, its effect is aesthetic only.  Overly aggressive efforts to remove such swarf is likely to damage the appearance of the cladding without enhancing its durability

On highly visible surfaces, a soft rag and plastic spatula can be used to remove more tenacious swarf adhesions or a diluted mild household cleaner might work on painted surfaces. Wire brushing, steel wool, or pot scouring cloths must not be used as they will damage the organic or metallic coating.

Swarf created by unacceptable practices, such as the use of grinders and friction power blades on, or adjacent to the cladding, is often hotter in contact with the cladding. The heat may cause it to embed deeply in the organic film and be in contact with the protective metallic substrate.


Friction cutting that creates swarf can also cause heat damage to metallic and organic protective surfaces.

This can severely affect the substrate; removal is difficult or impossible to achieve without mechanically damaging the decorative and/or protective coatings.

Swarf is not the only problem that cutting with friction blades can create. Such blades will often produce excessive heat at the cutting edge, which will degrade the organic and metallic coatings.


Often roof damage is caused by sub-trades accessing the roof after installation.  Roofers and other trades must be aware of how they treat the material they are working on and the effect it may have on adjacent surfaces.

Where work is done above or adjacent to an installed roof surface, or where the roof is used as a work platform for subsequent work, the main contractor must ensure the existing roof remains undamaged.



This roof shows evidence of mechanical damage to the coating, rib traffic damage to adjacent ribs within a purlin span, and excessive swarf by unacceptable cutting practices. In this case, the only logical remedy was replacement.