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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; Ventilation-Pathways

10.10 Ventilation Pathways 

Because of the cooking, washing, and respiration of the inhabitants, the humidity in the ceiling cavity is most often greater than that of the surrounding atmosphere.  The space may also be colder than the ambient air at night due to 10.12.3 Night Sky Radiation

Warm air naturally rises but has little tendency to move laterally, except when a strong wind blows into roof vents or causes substantial differences in air pressure on opposite sides of the building. That is why eave-to-ridge ventilation is more effective under typical conditions than side-to-side ventilation.

Ventilation of a cavity space is desired to reduce the accumulation of condensation and assist in removing excess heat. Natural ventilation via the ribs of metal roof and wall cladding can achieve this adequately in normal circumstances, but additional provisions are often necessary. In all cases, air must flow naturally through the profile crests without barriers such as profiled foam filler strips at the eaves and apex, or impingement of bulk insulation.

Many roofs not overtly displaying the signs of excessive moisture build-up would benefit from an increase in ventilation.

Simple techniques to provide a clear path for air to enter, travel along and exit the roof cavity can include:

  • Eaves:
    • Allow air entry by soffit vents, over-fascia or behind-fascia vents, or by using fleece-lined roofing material.
    • Ensuring that insulation does not impinge on the underside of the roof at the eaves. This can be achieved by using a heeled truss or eaves baffle, or both.
  • Body:
    • Ensuring that the air can travel under the roof.  If insulation is flush with the underside of the purlins it can be done using a vented batten between the purlin and the underlay, or by using fleece lined roofing material.
  • Apex:
    • Making the roof underlay discontinuous at the apex by cutting or slitting the underlay and using a vented soft edge or vented ridge.

Sarked roofs must have a gap in the sarking at eaves and apex, or by an alternative eaves-to-apex passage.


With skillion roof or flat roof construction, the air volume is significantly reduced, so moisture saturation levels are more quickly reached. Also with these roof types, air flow paths are more easily obstructed. In skillion roofs with tongue and groove ceilings, a layer of roof underlay immediately above the ceiling will provide a vapour check and air barrier to compensate for the porosity of the ceiling. For more advice on skillion roof ventilation, see

Roofs continuous over an apex, such as barrel curved roofs or roofs with prickled ridges, must have adequate ventilation to prevent the accumulation of moisture at the apex.

Long low pitched roofs will benefit from increased ventilation which may also assist in minimising thermally induced expansion noise. Ventilation typically increases as the roof pitch increases. Air movement through the crests depends on the spacing and area of the crests, roof pitch, and overall length of the sheeting. Corrugate and trapezoidal roofing provide more ventilation than secret-fix roofing. Ventilation through the crests still depends on air being allowed to enter at the eaves and escape at the apex.

Through or tray section roofs have smaller ventilation channels and may require additional ventilation.

The use of profiled closures at eaves or ridge will create a substantial air barrier and alternative ventilation paths must be created.

Additional roof space ventilation may take the form of:

  • louvre vents in gable-ends,
  • soffit vents,
  • proprietary ridge vents,
  • ventilated soft edge strips on transverse flashings, or
  • mechanical or wind-powered vents positioned close to the apex.

Where eave-vent intakes and ridge-vent exits are both employed, the area of the ridge vents should be less than that of the eave vents. More air escaping at the ridge than entering at the eaves can lower the pressure of the attic cavity and encourage more ingress of moist air from the dwelling area.

In pitches of 30° or less cross venting from eaves to eaves alone is generally enough when combined with natural passive ventilation at the apex.

A common rule of thumb is to have a total ventilation cross-section area equal to 1/300 of the ceiling area. In NZ buildings, much smaller ratios have proven sufficient in most cases.

Increasing roof space ventilation above the insulation has only a small effect on R values. Ventilation of spaces above bulk insulation is not only desirable but prevents insulation losing effectiveness due to absorbing moisture.

Partial filling of a ceiling cavity with bulk insulation in flat roofs can severely reduce the amount of free air available to absorb incoming water vapour, thereby increasing saturation levels. Adding insulation while re-roofing must be done with due consideration; unless the amount of ventilation of the cavity is increased or a vapour check layer is used below the insulation, internal moisture problems can occur.

10.10.1 Methods of Ventilation 

The primary purpose of ventilation is to replace the moist air in the ceiling cavity with drier air from outdoors.

As warm, wet air tends to rise, a vent placed in the soffit or at the lower end of a roof will normally operate as an intake vent and a vent at the apex as an exhaust vent, but wind direction can reverse this relationship. Gable-end vents or vents aligned horizontally will act as an intake or exhaust depending on the wind direction. Soffit Vents 

Soffit vents can be made in a range of styles to suit the application. As wind pressure differentials are highest at the eaves, they are an efficient ventilation solution and they are also very weather resistant. Soffit vents should always be installed to allow free movement of air into the cavity and should not be blocked on the interior side by insulation or other material.


In some applications, vented battens may  be needed to increase airflow Fascia Vents 

Vents above fascia may require re-positioning of the fascia to allow for their depth. Fasica vents should be used in conjunction with a high fronted spouting so that the ends of the sheet and the vent are not exposed to driven rain. Ridge Vents 

Ridge vents, such as continuous or intermittent ridge vents or vented head apron flashings, should always be used in conjunction with intake vents at a lower level.

To prevent creating negative pressures which can draw more moist internal air into the ceiling space, the free surface area of ridge vents should always be less than that of eaves vents.

Saturated water vapour can enter the building when commercial ridge vents are subjected to negative pressures or at times of high humidity associated with mist or fog. Such water vapour can form condensation on the structural framework and appear as a leak. Ridge vents without adequate intake vents can also lead to leakage.
 Vented Soft Edge 

Typical soft edge is either soft aluminium or perforated aluminium a with PIB rubber backer. The latter is more common.  The PIB rubber, when dressed down on a hot day, can adhere to the roofing surface and prevent the escape of air at the apex.  By removing all or most of the PIB backing this is avoided, and the perforations in the aluminium become an effective pathway for trickle ventilation, while still being an effective baffle against wind-blown moisture.  This product is marketed as vented soft edge and is available from all suppliers. Turbine Vents 

Wind-driven turbine vents rely on wind to rotate the fan blades. This creates a low-pressure area, so they draw air from the ventilated area at a greater rate than stationary vents. The amount of air movement can be dampened but is normally uncontrolled; it is developed as a function of wind speed as well as turbine size and efficiency. Turbine vents, unlike commercial ridge vents, are unaffected by wind direction and they are less prone to leaking.

10.10.2 Battens 

Battens may be required to provide  create a speparation space between roofing and underlay. In some applications, they may need to be ventilated to achieve sufficient airflow.

The type and number of fixings required to fix the counter battens to the roof structure must meet the design wind load, or the roof fasteners should be long enough to achieve the required penetration into the purlin below.

Battens or counter battens, if not fixed by extended cladding fasteners, can be fixed using countersunk purlin screws or if fixed with hex headed screws, they should be counter-bored before installation to avoid damage to the roof cladding.

Steel top hat, C or Z sections are also used as counter battens but require an additional insulating spacer to avoid thermal bridging.



10.10.3 Applied Fleeces 

Roof sheeting is now available with a synthetic fleece applied directly to the underside of the profile. This works in the same manner as roofing underlay, absorbing condensation into the fleece and releasing it into the atmosphere when ambient conditions improve.

The fleece follows the contours of the profile. By removing the separate layer of underlay, every crest becomes a pathway for ventilation. On a gable roof, this will approximately double the air changes in a roof over conventional construction, without the height disruption, cost, or inconvenience of a ventilated batten.

Another advantage of these products is that they greatly increase speed of lay of a roof, and avoid delays due to excessive wind. They are available on all corrugate and trapezoidal profiles.