AEP (annual exceedance probability)
AEP is the probability of a storm event being exceeded in any given year. NCC 2022 replaced ARI with AEP for gutter and stormwater sizing.
Ask Chalkline about this →AEP (annual exceedance probability) is the statistical likelihood that a given rainfall intensity will be exceeded in any single year, expressed as a percentage. A 5% AEP storm has a 5% chance of occurring or being exceeded in any given year, roughly equivalent to a 1-in-20-year event. A 1% AEP storm has a 1% chance, roughly 1-in-100 years.
NCC 2022 replaced the older term ARI (average recurrence interval) with AEP to align with AS/NZS 3500.3:2018. The practical effect: 1% AEP is the same storm event previously called a 100-year ARI, and 5% AEP is the same as a 20-year ARI. The terminology change does not alter the physical storm intensity used for design; it changes how the probability is expressed (verified 2026-05-08: ABCB, NCC 2022 gutter and downpipe requirements).
Also known as: ARI (older term, pre-NCC 2022)
Where it comes up: Gutter and downpipe sizing under Housing Provisions Part 7.4 uses 5% AEP for eaves gutter capacity and 1% AEP for overflow measure capacity. Part H2P1 uses the same thresholds for site drainage requirements.
Category: Compliance / NCC