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NZ Building Code · WeathertightnessThe E2/AS1 risk matrix, in plain English
The E2/AS1 risk matrix turns “looks fine” into a defensible weathertightness score that decides whether a wall can be direct-fixed or must use a cavity.
The E2/AS1 §3.4 risk matrix is how you prove, elevation by elevation, how weathertightness-risky a wall is — and therefore whether you can direct-fix or must use a cavity. You score eight features, add them up, and the total decides the build. It turns “looks fine” into a defensible number the BCA can check.
The eight factors you score on each elevation
Each elevation gets its own score across eight features. Higher points mean more weathertightness risk — the one exception is eaves width, where wider eaves earn fewer points because they shed more weather off the wall.
- Wind zone (0–2)
- Site exposure (0–2)
- Height to eaves (0–2)
- Roof complexity (0–2)
- Eaves width (0–1) — wider eaves = less risk
- Envelope complexity (0–2) — corners, junctions, parapets
- Wall cladding type (1–2)
- Deck design (0–1)
There is also an overall banding of 0–3 that sits across the assessment.
What the total score means
Once you add the factors up, the total decides how the wall must be built:
- Score above 6 — cavity construction is mandatory.
- Score of 6 or less — may permit direct-fix, but only for specific low-risk profiles. Check E2/AS1 §9 for which claddings qualify.
Most modern NZ builds score above 6, so a cavity is the default outcome.
Why you score every elevation separately
Each elevation is scored on its own, and the worst elevation drives the build. The front and back often differ from the sides — for example, wider eaves on the road frontage, or a parapet on the sun side — so one high-risk face can push the whole job to cavity even if the others come in lower.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
What does the E2/AS1 risk matrix actually decide?
It decides, elevation by elevation, whether a wall can be direct-fixed or must be built with a cavity. You score eight weathertightness features, add them up, and the total drives the build — turning a subjective judgement into a defensible number the BCA can check.
What score means a cavity is mandatory?
A score above 6 makes cavity construction mandatory. A score of 6 or less may permit direct-fix, but only for specific low-risk cladding profiles — check E2/AS1 §9 for which claddings qualify. Most modern NZ builds score above 6, so a cavity is the default.
Which factors are scored, and how many points does each carry?
Eight factors: wind zone (0–2), site exposure (0–2), height to eaves (0–2), roof complexity (0–2), eaves width (0–1, where wider eaves mean less risk), envelope complexity (0–2), wall cladding type (1–2) and deck design (0–1). An overall band of 0–3 also applies.
Why is each elevation scored on its own?
Because the worst elevation drives the build. Faces of a house often differ — wider eaves on the road frontage, a parapet on the sun side — so scoring each elevation separately means one high-risk face can require a cavity even when the others score lower.
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