Waterproofing the building envelope: one system, four layers

When people say a building is "waterproofed", they usually mean one thing: the membrane under the tiles, or the coating on the balcony. In reality the envelope keeps water out with several separate systems, installed by different trades at different times, and each one is governed by different rules.

Most leaks do not happen in the middle of any of those systems. They happen at the joins between them. Understanding the layers is the first step to making sure nobody assumes someone else has dealt with the join.

Layer 1: horizontal surfaces (roofs, balconies, podiums)

Anything flat or nearly flat has to hold water out while it waits to drain. For buildings covered by NCC Volume One (apartments and other Class 2 to 9 buildings), clause F1D5 requires a roof, balcony, podium or similar horizontal surface to have a waterproofing membrane of materials complying with AS 4654.1, designed and installed in accordance with AS 4654.2. Clause F1D4 adds that exposed joints must be protected as set out in the installation standard, and that joints must not sit beneath planters or water features. See Part F1 of NCC 2022.

We cover failure modes in more detail in box gutters and flat roofs and facade and balcony waterproofing.

Layer 2: walls and openings

Walls shed water; they do not seal against it. The system is a series of overlapping layers (cladding, flashings, wall wrap, sealants) that direct water down and out, plus drainage so any water that gets behind the cladding can leave.

• Houses (Class 1): For masonry, Part 5.7 of the Housing Provisions sets cavity widths, flashing and damp-proof course requirements, and weepholes (open perpend joints) at maximum 1.2 m centres in the course above flashings, so water collecting in the cavity can drain out.
• Other buildings: Part F3 sets the performance requirement (F3P1) that roofs and external walls must prevent the penetration of water that could cause unhealthy conditions or damage to the building.

Window and door heads, sills, penetrations and cladding terminations are the usual weak points. We list them in where buildings leak.

Layer 3: the ground interface

Water also arrives from below. Part F1 covers this with separate clauses:

• F1D6 Damp-proofing keeps ground moisture away from floor timbers and from walls above the damp-proof course, using materials such as those complying with AS/NZS 2904.
• F1D7 Damp-proofing of floors on the ground requires a vapour barrier under slabs and floors on ground or fill, referencing AS 2870.
• F1D8 Subfloor ventilation sets minimum ventilation openings and ground clearances by climatic zone.

For houses, Part 5.7 requires the damp-proof course or a flashing serving as one to be not less than 150 mm above adjacent ground level in most conditions. Garden beds, paving and render that bridge the damp-proof course undo the whole detail, which is a common renovation mistake.

Layer 4: below-ground walls and basements

This is the layer that surprises people. As we read Part F1 of NCC 2022, it has no deemed-to-satisfy provision dedicated to waterproofing below-ground walls, such as basement walls and retaining walls behind habitable space. The clauses above cover surface water, rising damp and horizontal membranes, but not tanking of a wall in contact with soil.

That does not mean it is unregulated. The building still has to meet the performance requirements, so below-ground waterproofing is typically designed by a specialist consultant as a performance solution and accepted by the certifier, rather than copied from a standard detail. For an owner or developer, the practical points are:

• The design should exist on paper before excavation, and say what system is used, how it is drained and how it is protected during backfill.
• Penetrations, construction joints and the junction with the slab need specific details.
• The membrane is inspected before it is covered by backfill or drainage, because afterwards it cannot be seen.
• Ask the certifier how compliance is being demonstrated.

Where the layers meet

Each junction between layers belongs to more than one trade:

1. Balcony membrane to wall cladding. The membrane must turn up behind the cladding and be protected by flashings. Waterproofer, cladder and glazier all touch it.
2. Slab edge to wall base. The damp-proof course, vapour barrier and slab edge treatment come from different trades at different stages.
3. Wet-area membrane to wall lining. Inside, wet area waterproofing must connect with the wall assembly around it. The 2021 edition of AS 3740 is the standard referenced by NCC 2022 for domestic wet areas.
4. Basement wall to slab and podium. The below-ground system must connect to the horizontal membrane above it.

The fix is to name an owner for each junction in the construction program, and to inspect it before the next trade covers it. That is the idea behind hold points and the handover record, and the reason our stage-by-stage inspections follow the envelope from slab to roof.

Legal context in NSW

Under the Home Building Act 1989, section 18E provides a six-year statutory warranty period for major defects, which include a failure of waterproofing, and two years for other defects. For work covered by the Design and Building Practitioners Act 2020, section 37 sets out a duty of care to avoid economic loss caused by defects. Whether those provisions apply to your building depends on the work and the building class, so confirm the position with a lawyer. Our article on statutory warranties explains how the warranty periods work in practice.

Questions worth asking

1. Which standards and NCC clauses apply to each layer on this building, and which edition?
2. Who is responsible for each junction between layers?
3. Is there a below-ground waterproofing design, and who certifies it?
4. Which membranes will be inspected before they are covered, and how is that recorded?
5. What certificates, product data sheets and photos do I receive at handover?

How we approach it

We plan the envelope as one system, with each layer and junction assigned an owner, a standard and an inspection point. If you are planning a build, a renovation or a remediation, get in touch.

Sources

• NCC 2022 Volume One, Part F1 Surface water management, rising damp and external waterproofing, Australian Building Codes Board (F1P2, F1P4, F1D4 to F1D8)
• NCC 2022 Volume One, Part F3 Roof and wall cladding, Australian Building Codes Board (F3P1)
• NCC 2022 Housing Provisions, Part 5.7 Weatherproofing of masonry, Australian Building Codes Board
• NCC Navigator: Waterproofing in houses, Australian Building Codes Board (AS 3740:2021, Part 10.2)
• Home Building Act 1989, section 18E, NSW Legislation via AustLII
• Design and Building Practitioners Act 2020, section 37, NSW Legislation via AustLII

This article is general information, not legal or technical advice for your project. Requirements depend on the building class, the approvals and the products used, so confirm them with your certifier, a lawyer and the relevant standards. Full website disclaimer