COP update

COP Recommendation: Design for Natural Ventilation
In the revised clause Internal Moisture, the COP requires designing and installing roof cladding to allow natural ventilation in residential buildings, including skillion, sarked, and low-pitched residential roofs.

It is a common misconception that roofing underlay acts as a drainage plain, channelling condensation from the underside of metal roof cladding to the gutter. Most condensation, however, form on the underside of the underlay.

Any condensation that does form on the underside of the roof and falls onto the underlay generally only tracks down the underlay as far as the next purlin, where it is trapped and is absorbed by the underlay, leading a prolonged time of wetness.

Underlay does not compensate for unreliable weatherproofing and ventilation design.
The COP suggests some basic design strategies to provide a clear path for air to enter, travel through and exit the roof-ceiling cavity.

Ensure that insulation does not impinge on the underside of the roof — especially at the eaves.
Position insulation so there is a gap of at least 20 mm between the insulation and the underside of the roofing. Insulation that absorbs condensation from the roof become less effective as thermal insulation. Wet insulation also retains moisture, leading to typical issues with durability and mould. 

Make the roof underlay discontinuous at the apex.
Warm air naturally rises but doesn’t move laterally much unless a strong wind blows into the roof vents. Providing an effective exit point for the rising hot air is much more effective under typical conditions than side-to-side ventilation.
Use ventilated soft edges on ridging and apex flashings.
Using ventilated soft edges on ridging and apex flashings with corrugate and low trapezoidal roofing, provides a weathertight exit for moist air in the roof cavity to balance fresh air entering at the eaves
The latest updates to the NZ Metal Roof and Wall Cladding Code Practice (COP), is effective 1 May 2019.

1. The main clause Roof Ventilation was renamed Internal Moisture and completely revised and updated.

2. The main clause Installation was extensively revised and updated.

3. Some clauses form Site Practice were moved to be included in the more relevant clause Installation. Site Practice now contains mainly Safety, and it will be revised and updated in accordance with Worksafe: Working at heights in New Zealand.

4. Some clauses in Testing were renamed and renumbered to fix broken links and reflect content more accurately.

Summary of other changes in this update

  • Reformatting of extract from NZBC B2, for consistency.
  • Some clauses from Internal Moisture were deleted and new clauses created to enhance clarity. Information contained in the deleted clauses are now discussed in other relevant clauses.
  • Other changes are Category 1 updates, which fixed minor errata such as grammar, spelling, and layout.

A complete list of changes is available at:

Revisions are shown in three categories.
Category 1 – Minor Errata

Updates come in three levels/categories 

Category 1 - Minor Errata
Correction to spelling, grammar or formatting that have no bearing on the substance of the clause. These changes will be recorded on the website only and not individually included in an 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. The substantial recommendations are not altered, although references may have been changed.
 These changes will be recorded on the website and will be cited in an emailed update.

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. These changes will be recorded on the website, and explained with detail in an emailed update.

PDF version
In response to requests for a downloadable PDF version, a new PDF document is created quarterly, with every update.
The online version of the NZMRM Code of Practice is always the most up-to-date version and prevails over any saved or printed version.