The Clause That Changed Everything
E2 (External Moisture) is the Building Code clause that governs the weathertightness of New Zealand buildings — the requirement that buildings exclude wind-driven rain, surface water, and groundwater from entering the building envelope in ways that would damage the structure or cause unhealthy conditions. E2’s importance in the New Zealand regulatory landscape cannot be overstated: it was the inadequacy of weathertight construction practice (combined with problematic design) that caused the leaky building crisis of the 1990s–2000s, and E2 compliance remains the most scrutinised aspect of building consent applications for residential construction.
The Acceptable Solution: E2/AS1
The Building Code compliance pathway for E2 is the Acceptable Solution E2/AS1 — a detailed technical document that specifies design and construction details for a wide range of cladding systems and building configurations. E2/AS1 is prescriptive: if you follow it, you achieve compliance. Deviating from it requires an Alternative Solution — a demonstration by a competent person that the proposed approach achieves the same level of performance as E2/AS1.
E2/AS1 covers: the design requirements for roof, wall, and floor details; the materials approved for use in weathertight construction; the maximum complexity ratings for different building configurations; and the specific requirements for cladding systems including monolithic plaster, weatherboards, brick veneer, and other common cladding types.
Complexity Ratings
E2/AS1 uses a risk matrix — combining factors including roof pitch, roof-to-wall junctions, window and door configurations, and cladding material — to generate a complexity rating for each wall face of a building. Higher complexity ratings require additional weathertightness measures or use of specific proven cladding systems. Buildings with high complexity ratings that use monolithic cladding systems — the combination associated with the worst leaky building outcomes — are subject to the most scrutiny.
The Practical Implication
For builders, understanding E2/AS1 compliance requirements — particularly the flashing and cavity requirements for the cladding systems you typically install — is fundamental to the legality and quality of your work. Cladding installation that doesn’t comply with E2/AS1 creates weathertightness risk that can result in damage to the building, claims against you as the builder, and Building Code compliance problems that affect the Code Compliance Certificate.