New Zealand Building Code Clause E2 (External Moisture)

The New Zealand Building Code (NZBC) is a performance-based code set out in Schedule 1 of the Building Regulations 1992. Clause E2 External Moisture requires buildings to provide adequate resistance to penetration by, and accumulation of, moisture from outside. It applies to roofs, walls and other parts of the building envelope, and is administered by the Ministry of Business, Innovation and Employment (MBIE).

Requirements

E2's performance requirements include:

• preventing water from entering the building
• preventing absorption and transmission of moisture that could cause damage or undue dampness
• preventing water from accumulating within the construction
• allowing any moisture that does enter to dissipate

Compliance documents

Because the NZBC is performance-based, MBIE publishes compliance documents that are deemed to satisfy the code:

• E2/AS1 – the main Acceptable Solution, widely used for timber-framed buildings. It contains details for roof claddings (including profiled metal, membranes and tiles), flashings, penetrations and junctions within its scope limits.
• E2/AS2, E2/AS3 and E2/AS4 – other Acceptable Solutions for specific construction types.
• E2/VM1 and E2/VM2 – Verification Methods, which use testing to demonstrate compliance.

MBIE's current set includes fourth editions of E2/AS1, AS2, AS3 and VM1 effective from 28 July 2025, along with E2/VM2 (2nd edition, 2021) and E2/AS4 (1st edition, 2019).

Designers can also use alternative solutions outside these documents, which the building consent authority assesses case by case.

Context: the leaky building crisis

E2 has been central to New Zealand construction since the "leaky building" crisis of the 1990s and 2000s, when many monolithic-clad homes suffered water damage and decay. Revisions to E2/AS1 introduced cavity construction for many wall claddings and stricter flashing details, and greater scrutiny of roof-to-wall junctions.

Roofing practice

For metal roofing, E2/AS1 is often used together with the industry's NZ Metal Roof and Wall Cladding Code of Practice, which gives far more detail on profiles, fastening, flashings and minimum pitches. Structural wind and snow loads on roofs are addressed under Clause B1 (Structure), which references the AS/NZS 1170 loading standards.