● Building code

NCC 2025 waterproofing. Eight changes outside.

11 Nov 20262 min readFORSPEC Technical Team

The short version

NCC 2025 changes external waterproofing on Class 2 to 9 buildings: concrete roofs, balconies and podiums.
The headline numbers: 1:80 fall in the slab, 70 mm step-down at doors, 70 mm hob at the edge, overflow 20 mm below the inside floor.
Bathrooms and houses are unchanged in substance. AS 3740, AS 4654.1 and AS 4654.2 stay at the same editions.

The eight changes on one balcony section.

The Australian Building Codes Board's own study put most of the apartment water defects in its sample down to water sitting under the finish, on top of the membrane. NCC 2025 answers that by moving the fall and the drainage into the structure.

The eight changes

Fall in the slab. The structural slab is graded at least 1:80. A screed or tile bed does not count, except inside a planter box.
Step-down at doors. The outside slab sits at least 70 mm below the inside slab, where applicable.
Hob at the edge. A cast-in hob at least 70 mm high at the outer edge, unless the edge drains straight to a gutter.
Overflows. Enough overflow, with its invert at least 20 mm below the inside floor slab.
Membrane on the slab. On concrete, the membrane goes directly on the structural slab.
Joints on high points. Exposed joints sit on a ridge of the slab, never under a planter or water feature.
Finish that drains. The finish over the membrane is self-draining or fixed directly to the membrane.
Deflection. The engineer allows for ten years of slab deflection, so the falls are still there later.

The 70 mm step-down is a concrete level, not the height of the membrane. The membrane upturn is still measured from the finished surface under AS 4654.2.

What did not change

Bathrooms and wet areas: Unchanged in substance. AS 3740:2021 still applies.
Houses (Class 1): No 1:80 or 70 mm rules. External work still follows AS 4654.1 and AS 4654.2.
The standards: AS 4654.1-2012, AS 4654.2-2012, AS 3740:2021 and AS/NZS 4858:2004 stay as the referenced editions.

When it starts

The adoption table printed in NCC 2025 lists these start dates:

1 May 2026: ACT, Victoria, Western Australia
1 May 2027: New South Wales, Queensland, South Australia
Tasmania: Listed as 1 May 2026. Check with the state regulator.
Northern Territory: Listed as not adopted

States can vary the code and move their dates, and the rule for which edition applies to a job differs by state. In NSW, work on an existing part of an existing building may still use the NCC 2022 provisions. Confirm the edition with your certifier before you design the balcony.

What it means for the waterproofer

Expect falls in the slab on new apartment jobs, and fewer fall screeds under membranes. See screeds on balconies.
Check the step-down and the hob before you start. If they are missing, raise it before the membrane goes on.
Direct-stick tiles and pedestal pavers fit the new finish rule. A floating screed needs the certifier's agreement.