COP v3.0:durability; capillary-action

The NZ Metal Roof and Wall Cladding Code of Practice (COP) is published by NZ Metal Roofing Manufacturers Inc. (MRM), to provide:

  • acceptable trade practice for the fixing of metal roof and wall cladding and accessories.
  • prescriptive detailing for designers and sets a benchmark for the standard of detailing and workmanship required over and above that required to comply with the NZBC.

The COP does not describe or dismiss alternative methods, which may need specific acceptance by the Building Consent Authorities.

It is published in accordance with current technology, materials, and building codes. The COP will be periodically updated to reflect changes in legislation and standards or improvements in technology and available materials.

The most current Code of Practice is available on the MRM website,, as Code of Practice Online (MRM COP Version 3).

The most recent Update to the COP was on 1 October 2018

  • In this Update there are minor spelling and grammar corrections only - no clauses have been substantially revised, nor any recommendations altered.

See  18 2018 – October for a list of the changes in this Update.

The glossary section provides definitions and descriptions of commonly used terms, as well as illustrations of domestic cladding terminology, industrial cladding terminology, and profile geometry.



Load Design discusses design and installation elements to ensure roofs are structurally sound and meet the objectives for the NZBC B1 Structure.

Topics include:

  • AZ/NZS 1170:2011
  • NZS 3604
  • Understanding loads
  • Fastener performance
  • Profile shape
  • Wind load span graphs and fixing patterns.

Corrosion (B2-Durability) covers considerations for continued performance of roof and wall cladding over the building lifecycle.
Key topics include:

  • Material performance and coatings.
  • Corrosion: evironmetal categories and special climates.
  • Material compatibility — both "in contact with" and "runoff onto".
  • Maintenance for prevention and remediation of corrosion.

The Roof Drainage section coincides with the NZBC Clause E1 Surface Water. It describes how to drain rainwater from roofs quickly and effectively. Topics include:

  • Rainfall Intensity
  • Roof Pitch for Drainage
  • Gutter and Downpipe Design
  • Gutter and Downpipe capacity calculations.

The design and installation of metal roofing is greatly influenced by the laws of physics, which include the conduction and radiation of heat and the convection of air. Both designers and Roofing Contractors should be aware of thisinfluence. This section in the COP deals with:

  • Heat.
  • Thermal Movement.
  • Weather Thightness.
  • Profile Strength.

This section contains acceptable and better flashing details for most possible junctions, but does not require the use of welting, seaming or crimping, which are used with flashing standing seam cladding. These techniques are described in Standing Seam Cladding, but they can be used to substitute any sealed lap in lieu of mechanical fastenings. 

Flashings in this Code does not contain specific details of the interface with alternative roof cladding materials, such as butyl rubber, but the same good practice is applicable.


In this COP a penetration is any hole cut in roof or wall cladding to accommodate projections such as pipes, ducts, chimneys, doors and windows. The type of penetration design is determined by how large the hole is, what shape it is, the pitch and type of the roof, the catchment area, placement on the roof, and how it matches the module of the cladding.

The science of internal moisture control is concerned with the need to manage and control condensation.  
The condition of the outside environment, the building design, and occupant behaviour affect humidity in the living spaces, which ultimately affects humidity in the ceiling space.  
This section of the COP focusses predominantly on managing humidity in the ceiling space of dwellings.
Shorter sections also cover the design of wall cladding. Non-residential roof and wall cladding may be affected by the need to control internal moisture.


Clear Roof Sheeting explores the materials and installation methods available to provide natural lighting solutions for buildings in NZ. Other topics include:

  • Types of roof light.
  • Weathering and Durability.
  • Loadings for roof lights.
  • Thermal movement

In addtion to Corrosion (NZBC: B2 – Durability), other issues which may affect the lifespan or perceived quality of metal roof and wall cladding, include:

  • Oil Canning.
  • Purlin Creasing.
  • Colour Differential.

Guidelines for site practice, including:

  • Safety.
  • Transportation and Handling of Material.
  • Walking on Roofs.
  • Tools of the Trade.

Metal roof and wall cladding should be installed as explained in this section to comply with the NZBC and to satisfy manufacturers' warranties.

To minimize the risk of wind damage to partially clad structures during construction, it is necessary to plan the sequence and method of cladding.

Other roofing products, include:

  • Curved Roofs.
  • Solar Units.
  • Pressed Metal Tiles.
  • Standing Seam Cladding.


Maintenance is defined as 'to keep in good condition or repair', and can be divided into four categories:

  • Normal.
  • Scheduled or Planned.
  • Preventative.
  • Special.


Section 16 sets out the design and performance requirements for sheet roof and wall cladding systems specifically for New Zealand. While it is intended that only those people associated with testing would be concerned with the details of the test procedures it is of general and historical interest as to the reasoning behind the performance requirements.

For this reason, a commentary is provided which follows each part.

Section 16 is divided into general requirements and specific methods of testing sheet roof and wall cladding for point load and also to determine their resistance to wind pressures. (UDL - uniformly distributed load).

These procedures apply to all metals and plastic sheeting however because of their different characteristics some of the performance criteria are different.

Useful tools and tables to do calculations and conversions for roof and wall cladding. Just choose the correct online caculator, input your values, and get the answer.

Policy for Updates to NZ Metal Roof and Wall Cladding Code Practice.

All revisions to clauses of the COP after June 2018 are recorded at 18 Revision History, including a brief explanation of the change.

See sub-clauses for detail of revisions at particular dates.

Revisions are split into 3 categories for simple assessment by users:





Category 1 - Minor Errata

Correction to spelling, grammar or formatting that have no bearing on on the substance of the clause.

Recorded on website only - not individually included in emailed update

Category 2 - Editing and rearrangement

A clause or section of clauses has been rewritten to some extent for better articulation of the existing recommendation.

Existing citations of the COP in project documentation may be less clear as a result, but recommendations are not altered.

Recorded on website, and will be cited in emailed update as either specific or summary information as appropriate.

Category 3 - Substantial change to recommendation

A substantial change in a specific recommendation of the COP has taken place.

A review of existing project documentation against the new clause is considered essential.

Recorded on website, and explained with detail in emailed update