COP v3.0:structure; snow-loads

1 Introduction 

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, www.metalroofing.org.nz/cop, as Code of Practice Online (MRM COP Version 3).

The most recent updates to the NZ Metal Roof and Wall Cladding Code of Practice were published on 1 June 2023.

Substantial change to recommendations:

The main section 3 Structure has been comprehensively revised.

Other important updates:

  • A new clause discusses 12.3 Flashing Buckling in more detail, while the old clause has been renamed Compression Timber.
  • Standing Seam Cladding has been renamed 15.4 Tray Roofing and the entire clause and its sub-clauses have been reviewed.
  • The definition of standing seam roofing (a form of tray roofing installed on solid sarking using traditional seaming tools) has been clarified.

The rest of the updates consisted of minor corrections and edits.

For more detailed information, see 19.1 2023 – June.

2 Glossary 

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.

 

 

3 Structure 

The information in this section explains the various factors used in calculating  design loads and in resisting those loads to ensure roofs are structurally sound and meet the objectives of NZBC B1 Structure.

Topics include:

  • NZ Building Code Clause B1 (Extract)
  • Standards
  • Performance
  • Profiles
  • Modes of Failure
  • Fasteners

The designer must be familiar with the performance requirements of the New Zealand Building Code (NZBC). Loads can be calculated in accordance with the appropriate standard and maximum spans and fastening patterns specified according to manufacturers’ literature or the generic tables listed in this Code of Practice.

It is the responsibility of the roofing contractor to install roof cladding according to the design and raise any concerns with the designer before commencement.

 

4 Durability 

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

5 Roof Drainage  

This section considers the design of water drainage from the time it hits the roof cladding to the time it enters the downpipe. As the design considerations are similar, this also includes the discharge from gutters and troughs within the roof plane, valleys, internal gutters, and external spouting.

The Roof Drainage section gives guidance for compliance with NZBC Clause E1 Surface Water. It describes how to drain rainwater effectively from roofs and gutters.

 

The Code of Practice provides several interactive calculators:

5A Capacity Calculations

Gutter Capacity 5.4.7 Gutter Capacity Calculator
Valley Capacity (both symmetrical and asymmetrical) 5.5.7 Valley Capacity Calculator
Downpipe Capacity 5.7.3 Downpipe Capacity Calculator
Maximum Area Above Spreaders 5.8.1 Maximum Area Above Spreader Calculator
Maximum Area Above Penetrations 9.4.4 Maximum Area Above Penetration Calculator
Maximum Run 7.1.4 Maximum Run Calculator

6 External Moisture Overview 

The primary function of a roof is to shed external moisture. The Code of Practice deals with External Moisture in four sections, allowing for more detailed discussion. The solutions in the COP relate to all buildings and are not limited to buildings within the scope of NZS 3604.

Included in the COP under External Moisture:

6 External Moisture Overview provides an extract from NZBC E2 External Moisture. It highlights the Objectives, Functional Requirements, Performance Requirements, and Limits of the NZBC Clause E2. The second half highlights the scope and extent of Acceptable Solution E2/AS1.

7 External Moisture Roofing discusses the external moisture requirements and strategies for dealing with external moisture where it concerns metal roof and wall cladding.

8 External Moisture Flashings discusses strategies of managing external moisture with a specific focus on flashings.

9 External Moisture Penetrations focusses specifically on managing external moisture and preventing leaks around penetrations.

7 External Moisture Roofing 

This section should be read in conjunction with 6 External Moisture Overview.

8 External Moisture Flashings 

This section should be read in conjunction with 6 External Moisture Overview and  9 External Moisture Penetrations.

The purpose of a flashing is to divert water away from any point of entry and to make a building weatherproof.

Flashings are not only required to weather the many junctions on a roof or wall structure but are often a highly visible part of the roof and wall cladding design. Therefore, they perform an important role in the aesthetic appearance of the building.

It takes longer to make and install flashings than fixing roof or wall cladding, so designers should be aware of the cost effects of design complexity.

9 External Moisture Penetrations 

This section should be read in conjunction with 6 External Moisture Overview and 8 External Moisture Flashings.

A penetration is any hole cut in a roof or wall cladding to accommodate projections such as pipes, ducts, chimneys, roof lights doors and windows.

This section focusses on roof penetrations only. The type of penetration design is determined by:

  • the size of the hole,
  • shape,
  • the roof pitch,
  • the type of roof,
  • the catchment area,
  • placement on the roof, and
  • aesthetic requirements.

Designers are urged to consider what type of penetration design matches the building application and their customer’s needs, and detail accordingly, rather than allow the installer to make an on-site decision.

Many of the penetration details drawn in the Code of Practice (COP) are included in a step by step How -To  Guide published by the Roofing Association of New Zealand.  A copy of this may be obtained by contacting info@roofingassn.org.nz.

10 Internal Moisture 

The science of internal moisture control is concerned with the need to manage and control condensation, mould growth, and corrosion.  
The outdoor 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 non-residential roof and wall cladding, which may also be affected by excessive internal moisture. 

 

11 Natural Light 

Natural Lighting covers the use of translucent roofing material for providing interior illumination. These typically take the form of profiled sheeting, stand-alone units, or flat sheet systems.

Skylights profiled to match the roof sheeting may be made from glass reinforced polyester or polycarbonate. Proprietary stand-alone skylights may have polycarbonate, glass, or acrylic panels. Single or multi-skinned sheet products can be used in conjunction with proprietary support systems to replace glass in conservatory type situations.

Main headings include:

  • Compliance
  • Profiled roof lighting
  • Stand-alone roof lighting
  • Sheet systems

12 Fitness For Purpose 

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.

13 Site Practice 

Guidelines for site practice, including:

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

13 Safety 

This section highlights some aspects of the New Health and Safety at Work Act 2015 (HSWA 2015), including:

  • responsibility,
  • working at height in New Zealand, and
  • working on roofs.

HSWA 2015: Objective

The objective of The Health and Safety at Work Act 2015 is protecting the health, safety, and wellbeing of workers, and other people.

Information used in this section was retrieved from http://www.legislation.govt.nz/act/public/2015/0070/55.0/DLM5976660.html and worksafe.org.nz.

14 Installation 

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

 

15 Other Products 

Other roofing products, include:

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

 

16 Maintenance  

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

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

 

17 Testing and MRM Standards 

The NZMRM employs specific testing methods and standards to evaluate the performance of metal roofing components and accessories.

Structural testing of profiled metal closely follows the standards AS 1562 and AS 4040, but differs in minor areas to suit NZ requirements.  This testing is used to demonstrate compliance with NZBC B1 – Structure

Compliance with NZBC B2 – Durability is demonstrated by testing of profiled metal and fasteners.  This testing is based on AS/NZS2728 and AS/NZS 3566 respectively, again with some modifications.

Also included, are the MRM soft edge standard, and testing and performance standard for pre-painted coil.

18 Useful Information 

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.

19 Revision History 

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 19 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:

19A Revision Categories

Category

Description

Notification

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