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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 v24.12:Flashings; Flashing-Design

Flashing Design

8.2 Flashing Design

Transverse flashings run across the roof slope at right angles to the ribs of the roof and longitudinal flashings run down the roof slope.

Ingress of rain into the roof or wall cavity via the flashings can be caused by the pressure differential between the air outside and that inside the roof or wall cavity. The pressure differential caused by wind gusting fluctuates greatly, so a gap should be created behind the outer edge of a flashing to provide a pressure cushion. Longitudinal flashings are best designed with a pressure equalisation gap to balance varying pressures and prevent capillary action. (See 4.10.6 Capillary Action.)

The preferred maximum production length of flashing is 6–8 m, depending on profile strength. As any sealed lap secured by rivets or screws effectively becomes one length, provide expansion joints where required. Flashings are similarly restricted in length as roof and wall cladding sheets and are subject to the same requirements and expansion provisions. (See 7.3.2 Roof Cladding Expansion Provisions.) Inadequate provision for flashing expansion can also cause roof noise

Avoid wet contact between the edges of flashings and concrete, plaster or butyl rubber (See 4.9 Compatibility.) When notched flashings are used, the cut edge must not touch the pan, as that can cause corrosion from abrasion.

8.2A Notched Flashing

Notched Flashing (with Screw)

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8.2B

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Flashing Edges

8.2.1 Flashing Edges

The dressed soft edging or the downturn of a notched flashing acts as a baffle to wind and rain at the outside edge of a transverse flashing, as does the stop-end of the cladding at the upper end. The void in between these two barriers acts as a pressure equalisation gap.

Soft edging can be supplied in material compatible with the roofing material. This may be a naturally malleable metal, or metal perforated to enable it to be shaped to the profile, see 17.3 MRM Soft Edge Standard.

Stop ends must be provided for all pitches in all wind zones

In extreme circumstances, a profiled or notched closure can be used between the stop end and the downturn or soft edge. When closures or filler blocks are used, they must be adhered to the profile or secured by the cladding fasteners to ensure they remain in position.

Additional ventilation requirements must be considered when using profiled closures, see 10.10 Ventilation Pathways.