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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:Internal-Moisture; Moisture-Sources

10.8 Moisture Sources 

Everyday household activities, heating, indoor plants, pets and construction activities all contribute to indoor moisture.


10.8.1 Occupant Behaviour 

The occupants of the building create a significant amount of water vapour. Therefore, the air inside in a building typically has a higher moisture content than the external atmosphere.

10.8.1B Approximate Amount of Water Vapour from Occupant Behaviour

Occupant behaviourEstimated Amount of Water Released (per 2.5 inhabitants)
Cooking (unventilated)3.0 L / day.
Baths / Showers1.5 L/day. 
Clothes Washing0.5 L/ day.
Clothes drying (unvented)5.0 L/ load.
Dishwashing1.0 L / day.
Portable gas heaterup to 1.6 litres per 1kg of gas burned.
Breathing  (average)3 litres per day.
Breathing asleep (per hour) (average)50 ml.
Perspiration 0.5 litre per day.
Pot PlantsThe same amount as the input


Bathing and showering, cooking, heating, indoor laundries, and unvented clothes drying are the most obvious sources of water; respiration, perspiration, indoor plants, and pets also produce moisture.

Areas for moisture-generating activities should be well ventilated and the entire building should be mechanically ventilated to outside the structure. Proposed changes to NZBC G4/AS1 will require venting to the outside of appliances such as showers, baths, and cooktops.


10.8.2 Heating 

Some other sources of moisture are best avoided altogether, particularly unvented gas heating and kerosene heaters. Burning 1 kg of gas can release 1.6 litres of moisture into the atmosphere.

10.8.3 Mechanical Venting 

Supply-driven and exhaust-driven mechanical ventilation systems can pressurise or depressurise internal atmospheres in different areas of the building. Supply-driven systems can be problematic as increased internal pressures can drive moist air into the attic through openings in the ceiling.

Exhaust-driven systems can de-pressurise internal areas and increase the intake of moist external air, e.g. ground moisture via a vented cavity. Unbalanced mechanical ventilation can also encourage moisture migration by creating negative pressures in the ceiling cavity. These systems need to be well designed and maintained to avoid the risk of affecting internal moisture. A balanced mechanical ventilation system, where both intake and exhaust are connected to the outside, is the preferred system and will be most effective when the thermal envelope of the building is airtight.


10.8.4 Construction Moisture 

During construction, timber can become wet and take some time to dry out. Some activities, such as plastering and painting, also release water vapour.

Concrete floors are particularly prolific sources of moisture. During curing, a 100 mm thick concrete slab releases approximately 10 litres of water vapour per square metre of surface area. The curing period depends on various factors, but as a rule of thumb, a concrete floor cures at a depth-rate of 25 mm per month. Therefore, a concrete slab can affect internal moisture levels for a considerable period.

All new buildings, particularly those with concrete floors, must be kept well ventilated  (at much higher levels than required during normal use) until moisture levels of construction materials have stabilised.

10.8.4A Mould Damage

This building suffered mould damage to underlay and roof truss even before occupants moved in.


10.8.5 Ground Moisture 

Ground moisture can infiltrate living spaces by way of the floor or directly to the ceiling space by way of vented cavities. The cavity should be constructed to prevent the migration of water vapour into the ceiling space. Wet subfloors can be isolated by laying polythene tightly over the surface and taping all joints.  More information can be obtained from the Good Repair Guide: Damp Subfloors (BRANZ).

Concrete floors must be installed over a damp-proof membrane (DPM) to ensure that moisture from within the ground does not penetrate the slab. This membrane can be formed by a polyethene sheet that is taped at the laps and laid over compacted hard fill topped by a sand blinding layer. The DPM must be installed under the full extent of the slab, under any internal or perimeter foundations, extend up to the external edge of the floor slab, and lapped and sealed under the wall damp-proofing system.