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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:Maintenance; Washing

16.7 Washing 

Regular washing of pre-painted and metallic-coated roofing products increases their durability by reducing attacks from airborne salts and pollutants.

Washing may be carried out with a hose and a soft bristle brush, using fresh water or a 10% solution of household detergent and water followed by a thorough rinse with clean water. Alternatively, low-pressure water blasting can be used at pressures up to 20 MPa, with the jet directed away from openings and sheet laps.

Stronger concentrations of cleaners than those recommended can damage coating surfaces, and avoid using organic solvents and abrasive cleaners. When cleaning coated surfaces, tar and similar substances may be removed with mineral turpentine, but the surfaces should then be washed thoroughly with detergent and water.

Always clean coated surfaces from top to bottom, and rinse immediately and thoroughly with fresh, clean water avoiding over-cleaning or scrubbing, which can damage painted surfaces.

Hard scrubbing of un-weathered bare AZ-coated steel cladding can remove the thin factory-applied clear acrylic film and cause differential weathering, affecting appearance.

If water runoff is used for drinking water, roof outlets must be disconnected before washing any roof or wall cladding using detergents. Care must also be taken not to contaminate waterways.

16.7.1 Lichen And Mould 

Lichen is a naturally occurring phenomenon with its spores being dispersed by the wind. Lichen will grow even on inert materials such as G.R.P., glass, and painted or unpainted metal roofs.

Time of wetness of a surface affects lichen and mould growth. Sheltered and shady environments are particularly conducive to its growth and although light-coloured roofs may stay wet for longer than dark roofs, it can equally proliferate in open areas on dark coloured roofs.

Lichen and mould retain moisture, are acidic, and have tiny roots that can penetrate a paint coating. Removal is necessary to prevent damage to the organic coating, but recolonisation is very likely. Where lichen has been treated, regular inspection should be undertaken to curtail spread of re-growth.



Lichen growth can be removed by washing down the roof or wall cladding, and applying a 2% solution of sodium hypochlorite to all surfaces by low-pressure spray, broom or brush. The surface should be left for 5 minutes but should then be rinsed and thoroughly washed down with cold water. Household bleach contains various concentrations of sodium hypochlorite; therefore, it may be necessary to dilute it.

For example:

  • One brand has 30 g/L solution (3%) — to obtain a 2% solution, 2 parts of bleach should be diluted with 1 part of water. (3 - 2 = 1).
  • Another brand has 40 grams/L solution (4%) — to obtain a 2% solution, 2 parts of bleach should be diluted with 2 parts of water. (4 - 2 = 2).
  • Another brand has 50 grams/L solution (5%) — to obtain a 2% solution, 2 parts of bleach should be diluted with 3 parts of water. (5 - 2 = 3).

Another option is using benzalkonium chloride products which are less corrosive, although slower acting.

16.7.2 Graffiti 

Metal wall cladding, like most vertical surfaces, is subject to being defaced by graffiti. Graffiti removal is likely to affect the pre-painted finishes on metal roof and wall cladding, and before removal is attempted a small area should be cleaned as a trial. Graffiti removers may soften the paint, remove the gloss or cause permanent damage.

Do not use MEK (methyl ethyl ketone), toluene, acetone or thinners. Overpainting or replacement are the alternative options.

There are clear anti-graffiti coatings available, but their compatibility with the pre-painted cladding must be checked with their supplier.