COP v3.0:external-moisture;

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.

6.1 NZBC Clause E2 External Moisture (Extract) 

6.1.1 E2 Objective 

The objective of this provision is to safeguard people from illness or injury which could result from external moisture entering the building.

6.1.2 E2 Functional Requirement 

Buildings should be constructed to provide adequate resistance to penetration by, and the accumulation of, moisture from the outside.

6.1.3 E2 Performance Requirements 

  1. Roofs shall shed precipitated moisture and snow.
  2. Roofs and external walls shall prevent the penetration of water that will cause dampness or damage to the building elements.

Other performance requirements quoted relate to transmission of ground moisture, areas below suspended floors, condensation in concealed cavities, and construction moisture.

6.2 Compliance 

In the context of COP Roof drainage, NZBC E1 requires that water discharging into a gutter must have less than a 2% probability of entering a building.  This means that those drainage paths inside the building envelope, including valleys and internal gutters, must be designed to accommodate a 50-year rainfall intensity over a short period.  Roof Drainage Design discusses how the COP addresses this in more detail.

The above caveat equally applies to the limitations on run length of roof cladding, drainage capacity around penetrations, and discharge of spreaders, which are dealt with in sections XXXX

Gutter catchments outside the envelope, which can discharge freely without entering the building, can be calculated using less rigorous safety factors

6.3 Related Topics 

NZBC E2 – External Moisture is primarily focussed on a buildings’ external envelope being able to resist weather infiltration.

Acceptable Solution E2/AS1 also has some information on gutter sizing and durability, but these are also covered in E1 – Surface Water and B2 – Durability.

E2/AS1 also has some prescriptive solutions for structure regarding roofing spans which are omitted from B1 - Structure.

For clarity, this Code of Practice deals with the following topics discretely: