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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:Structure; Modes-Failure

3.7 Modes of Failure 

Serviceability limit is a state when a building, or any part of it, becomes unfit for its intended use due to deformation or deflection.

Ultimate limit is a state associated with collapse or failure, or when a building or any part of it becomes unstable or unsafe.


3.7.1 Serviceability and Ultimate Failure 

Initial failure under wind uplift for pierce-fastened cladding is usually by local buckling of the rib crest adjacent to the fastener. While the cladding can still resist a load, this permanent deformation is liable to cause leakage at that point; therefore, it is a serviceability failure and is often the limit state for trapezoidal profiles. Medium and high rib trapezoidals may have ultimate loads as the determining factor.

Failure under wind load for clip-fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.

If a standard corrugate or low rib trapezoidal product passes UDL for serviceability it will comfortably exceed ultimate design load requirements. For medium and high ribbed trapezoidal, both serviceability and strength loads should be checked. Clip-fastened products are typically restricted by strength load only.

Point load resistance should also be checked against the expected roof traffic load of the finished structure.  (See 3.7.4 Roof Traffic)

3.7.2 Deflection 

Deflection is the temporary bending of roof or wall cladding that occurs under load when the material is subjected to forces within its elastic limits.

Deflection under distributed load is a voluntary limit of wall cladding, deformation causing residual deflection after application of point load is a serviceability failure of roofing.

3.7.3 Deformation 

Deformation is a permanent alteration to the profile's shape that occurs when the material is taken beyond its elastic limits. This is most often seen as deformation under the fastener head after high outwards loads, or buckling of the rib after the imposition of a point load


With high-strength claddings buckling failure is often quite sudden and severe once the point of yield is reached, while progressive deflection under repeated loads within the material’s elastic limit is minimal.

Low-strength (G300) steel or non-ferrous metals can progressively yield under repeated heavy foot traffic, and are more prone to point load damage. This should be considered at the design stage. Machine roll-curved, crimp-curved cladding, and metal roof tiles are usually made from low-strength steels. 

It is not acceptable for G300 steel to be substituted for G550 steel in products that are traditionally run from high-tensile strength material.

3.7.4 Roof Traffic 

The designer must consider the degree and type of foot traffic that may be expected on a roof. The following requirements are subjective standards and must be considered in line with customer expectations, and building use and type.

More robust design than specified below (reducing purlin spacing or adding protection from mechanical action), is required for roofs that are regularly accessed, roofs used as staging by subsequent trades, or areas that are adjacent to access points, particularly step down access.

Type A – Unrestricted Access

Type A are roofs:
  • that need to be regularly traversed by the roofer for access during installation;
  • that will be accessed regularly by sub-trades;
  • that butt on to walls or windows that may require maintenance;
  • that have plant, chimneys, or solar installations requiring regular maintenance; or
  • that require regular access for clearing gutters or spouting of debris.
For Type A roofs, the cladding must resist the load of 1.1 kN applied to the pan or a single rib, and a strength load of 2 kPa applied to the weakest point.

Type B – Restricted Access

Type B are roofs:
  • that are simple in design and do not have to be regularly traversed by the installer;
  • which are infrequently accessed by qualified trades people for maintenance; or
  • with a pitch of more than 35°.
For Type B, roofs the cladding must resist the load of 1.1 kN applied to the pan or over two ribs and a 2 kPa load applied to the weakest point.


Type C – Non-Trafficable

Type C are roofs:
  • where supports are required to be laid to support roof traffic, or traffic should take place within 300 mm of purlin lines,
  • which have a pitch of 60° or greater; and
  • including non-trafficable translucent roof sheeting.
For Type C roofs, the cladding must resist the minimum load of 0.5 kN applied to the pan or over two ribs.

3.7.5 Dead Loads 

Any permanent load added to the roof cladding or structure is termed imposed; that includes air-conditioning equipment, solar installations, television aerials, anchor points, and walkways.

All loads must be supported on the rib of the profile or through the profile to the primary structure. Any attachment to the roof cladding must be compatible with the cladding, or isolated from it.


3.7.5A Correctly Installed Permanent Load

An air conditioning unit correctly installed on a roof, using durable and compatible materials.
No additional equipment must be directly connected to the cladding without considering the effect of increased dead and live loads.
When designing installations for placement on the roof, the roof traffic implications of installing and servicing such must be considered when determining point load resistance requirements.

3.7.6 Construction Loads 

Construction loads on a building include the wind load on a partially clad or braced roof or building. Depending on the method and sequence of construction, it can be greater than the load on a completed building. Construction loads also include temporary localised loads from material storage, scaffolding, and tradespeople.

Other forces contributing to construction load include:

  • The intensity of internal wind pressures due to a temporary absence of ceilings, walls and glazing.
  • Storage of roof cladding on the structure. Bundles of roof cladding should be placed so they do not cause overstress in purlins.
  • Any scaffolding above an existing roof must be designed to avoid damage to the roof structure or coatings, and be designed or sequenced to avoid wet storage corrosion. It is far better practice to stage jobs so that lower roofs are not used as a working platform for work on higher levels


3.7.7 Fatigue and Brittle Failure 

All metals may be subjected to fatigue under repeated heavy load conditions.

While high-strength steel is susceptible to fatigue, this seldom happens in practice. Aluminium brackets and clips have been known to fail by fatigue and this should be considered in designing such items.

Other metals, such as lead and copper, are restricted in length or overall panel size to avoid cracking by fatigue. Sharply folded corners should be avoided on these materials and the minimum radii requirements should be followed.

Hydrogen embrittlement can occur when folded items such as external gutter brackets are subjected to post-forming hot dip galvanizing. This can be avoided by forming to radii appropriate to the material grade and thickness being formed.