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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:Roof-Drainage; Rainwater-Heads-Sumps-And-Overflows

5.6 Rainwater Heads, Sumps, and Overflows 

Rainwater heads are situated outside the building envelope and sumps are internally located.

They both serve to increase the head of water entering a downpipe, and to provide an overflow capacity to safely discharge water when downpipe capacity is compromised or exceeded. The overflow should be obvious so discharging water warns the occupant that downpipe capacity has been exceeded or the primary downpipe is blocked.




5.6.1 Rainwater Heads 

Rainwater heads must be at least as wide as the gutter and have an overflow (normally a weir type). The cross-sectional area of the overflow must be at least equal to that of the required downpipe size for the catchment being served. The lower edge of the overflow must be at least 25 mm below the sole of the gutter, and the upper edge must be at least 25 mm below the upper edge of the gutter.

5.6.2 Sumps 


Sumps must be at least the same width as the gutter and have an outlet positioned below the sole of the gutter to increase the head of water at the outlet.

Internal sumps must have overflows. These are often a secondary pipe overflow, with the outlet height positioned above the level of the primary outlet. Internal sumps must also be sealed against the sides and sole of the gutter.

An internal sump should have a guard that prevents debris from blocking the outlet. A removable aluminium expanded-metal box can be fitted at a minimum of 40 mm below the sole of the gutter. Because the top is flat, it is unlikely that the entire surface area of the outlet can become blocked, so it is preferable to balloon-type guards. A leaf guard should have a horizontal surface area of at least four times the size of the downpipe outlet area and should be installed at roughly mid-height of the sump depth. Gratings can cause sump blockage, and this can reduce the outlet capacity.

Gratings or guards should be designed so that any debris will float, and hail, or obstructions, such as a tennis ball, will not wedge and block the guard. Gratings or guards should be cleared of accumulated debris regularly as part of normal maintenance.

5.6.3 Overflows 

Overflows must discharge clear of the building to clearly show that downpipe capacity has been exceeded; it should be an obvious indication that the gutters need maintenance.

The overflow opening of a rainwater head from an external gutter must have a cross-sectional area equal to that of the downpipe. The bottom of the overflow must be no higher than 25 mm below the bottom of the spouting.

Where the position of an outlet of a parapet wall gutter is on an outside wall, any scupper outflow should discharge into a rainwater head.