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Disclaimer

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.

Scope

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 v24.09:Durability; Nzbc-Clause-B2

NZBC Clause B2 (Extract)

4.1 NZBC Clause B2 (Extract) 

Source: New Zealand Building Code, Clause B2 Durability

B2 Objective

4.1.1 B2 Objective 

The objective of this provision is to ensure that a building will continue to satisfy the other objectives of this code throughout its life.

B2 Functional Requirements

4.1.2 B2 Functional Requirements 

Building materials, components and construction methods shall be sufficiently durable to ensure that the building, without reconstruction or major renovation, satisfies the other functional requirements of the NZBC throughout the life of the building

Performance Requirements

4.1.3 Performance Requirements 

Building elements must, with only normal maintenance, continue to satisfy the performance requirements of the NZBC for the lesser of the specified intended life of the building, if stated, or:

  1. The life of the building, being not less than 50 years, if:
    • those building elements (including floors, walls, and fixings) provide structural stability to the building; or
    • those building elements are difficult to access or replace; or
    • failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.
  2. 15 years if:
    • those building elements (including the building envelope, exposed plumbing in the subfloor space, and in-built chimneys and flues) are moderately difficult to access or replace; or
    • failure of those building elements to comply with the building code would go undetected during normal use of the building, but would be easily detected during normal maintenance.
  3. 5 years if:
    • the building elements (including services, linings, renewable protective coatings, and fixtures) are easy to access and replace; and
    • failure of those building elements to comply with the building code would be easily detected during normal use of the building.

Individual building elements which are components of a building system and are difficult to access or replace must either:

  • all have the same durability; or
  • be installed in a manner that permits the replacement of building elements of lesser durability without removing building elements that have greater durability and are not specifically designed for removal and replacement.
Compliance

4.1.3.1 Compliance 

NZBC Clause B2, Durability, requires fifteen years to perforation for claddings easily accessed for replacement. Fifteen years is also required for internal gutters and downpipes, and five years for external gutters.

NZBC B2 requires 50 years’ durability for flashings that require the removal of cladding above to be replaced, while table 20 of NZBC E2/AS1 only requires 15 years’ durability for such flashings. The COP recommends the higher figure as good trade practice and in many cases, lower life-cycle costing.

Generally, higher durability than the minimum requirements can be achieved by using materials and methods outlined in this COP, with no maintenance of coatings other than washing areas which are not naturally washed by rain. Elements more difficult to replace, or to access for maintenance, should be constructed of more durable material.

Normal Maintenance means work recognised as being necessary to achieve the expected durability of a given building element.

According to B2/AS1, normal maintenance may include:

  • Washing down surfaces subject to wind-driven salt spray and contaminants.
  • Re-coating protective surfaces.
  • Replacing sealant seals and gaskets in joints.

Although roof or wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for flashings. Flashings might be embedded in plaster or behind other building elements, making them hard to replace without removing cladding or other building features such as windows.

Cladding material may be described as hidden, sheltered or exposed. Some flashings may have sections falling into all these categories, in which case the worst case (sheltered) should prevail in material selection.

All metal roof and wall cladding and accessories must be designed and installed to comply with the durability requirements of the NZBC. NZBC requirements relate to performance, however, and do not necessarily relate to aesthetics or cost of replacement. Any pre-painted cladding will change colour over time, and partial replacement would be visible. The roof cladding could have deteriorated, although not perforated, within 15 years and still comply with the NZBC, but customer expectations may not be met.

Good design, correct selection of materials, and good installation and maintenance practices are required to achieve optimum product lifespan.