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External waterproofing

External waterproofing covers above-ground exterior surfaces (flat roofs, balconies, planters) under AS 4654 and NCC H2. How it differs from wet areas and tanking.

By Chalkline. Last updated 28 May 2026. Verified 28 May 2026.

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External waterproofing is the waterproofing of above-ground external building elements: flat and low-pitch roofs, balconies and decks over habitable space, planter boxes, and the positive (water) face of retaining walls. It is governed by NCC 2022 Volume Two Part H2 with deemed-to-satisfy paths through AS 4654.1 (materials) and AS 4654.2 (design and installation), and it is the category most leaks-into-habitable-space defects sit under.

How it differs from the other two categories

Waterproofing in Australian residential construction splits into three categories that use different standards and different details:

  • External (this article): above-ground exterior surfaces under AS 4654 and NCC H2. Examples: membrane flat roofing, balcony membrane, planter boxes.
  • Internal wet areas: bathrooms, laundries, ensuites under AS 3740 and NCC F2. Different membranes, different upstand and floor-grading rules, different waste-junction details.
  • Below-ground (tanking): basements and below-grade walls under different products and details, see tanking.

The three categories are not interchangeable. A membrane rated for an internal wet area is not automatically suitable for an exposed balcony; a tanking system is not designed for foot traffic.

The technical anchors

Two technical numbers run through almost every external-waterproofing decision (verified 2026-05-28, see AS 4654.2 membrane design):

  • Minimum fall of 1:100 built into the substrate before the membrane goes down (AS 4654.2 prescriptive).
  • Minimum 150 mm upstand at parapets, walls, and door reveals (AS 4654.2 Appendix A).

Most external-waterproofing failures trace back to one or both being missed.

Also known as: above-ground external waterproofing, AS 4654 waterproofing.

Category: Waterproofing

See also

Last updated: 2026-05-28. Verified: 2026-05-28. Quarterly review for currency.

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