External waterproofing for roof decks, podiums and planters

What is external above-ground waterproofing?

External above-ground waterproofing is the membrane system that stops water passing through weather-exposed horizontal surfaces over spaces that must stay dry. Typical examples are trafficable roof decks, apartment balconies over living areas, podium slabs over car parks or shops, and planter boxes built into a slab.

These areas take wind-driven rain, ponding, ultraviolet light, thermal movement, foot traffic and, in planters, wet soil and roots. Defects stay hidden under tiles, pavers or soil until water shows up below. Our guide to why balconies leak looks at the failure patterns in detail, and our guide to AS 3740 wet areas covers the internal equivalent.

Which standards and NCC clauses apply?

Two Australian Standards work as a pair. AS 4654.1-2012 sets requirements for the membrane materials, and AS 4654.2-2012 sets requirements for the design and installation of external above-ground membrane systems for roofs, balconies and similar areas. Standards Australia’s catalogue notes the membranes covered are also suitable for landscaped areas, but the standard is not intended for below-ground structures, swimming pools or spas. Both parts are currently marked as pending revision.

The Western Australian Building Commission’s 2014 industry bulletin records that the NCC has referenced AS 4654 Parts 1 and 2 as Deemed-to-Satisfy (DTS) provisions since 1 May 2013. The current clauses are:

• NCC 2022 Volume One, F1D5. A roof, balcony, podium or similar horizontal surface part of a building must have a membrane of materials complying with AS 4654.1, designed and installed to AS 4654.2.
• NCC 2022 Volume One, F1D4. Exposed joints in the drainage surface must be protected to Section 2.9 of AS 4654.2 and must not sit beneath or run through a planter box, water feature or similar.
• NCC 2025 Volume One, F1D6 and F1D7. The same exposed-joint and membrane rules, renumbered, with additions described below.
• NCC 2022 Volume Two, H2D8. For houses, external waterproofing of flat roofs, roof terraces, balconies and similar surfaces over internal spaces must use membranes to AS 4654.1, designed and installed to AS 4654.2.

The membrane clauses do not apply everywhere. Under F1D2 of NCC 2022 and NCC 2025 Volume One, they do not apply to a roof with a covering that meets the roof covering DTS provisions, to balconies and podiums with timber decking or other perforated flooring, or to balconies and podiums located directly above the ground. H2D8 in Volume Two has its own exclusions, including spaced timber decking and certain concrete slabs with a step-down of at least 50 mm.

What did NCC 2025 change for concrete roofs and balconies?

NCC 2025 restructured Volume One Part F1 as “Water management” and added new DTS provisions for concrete external areas. Under F1D4 of NCC 2025, a concrete roof, balcony, podium or similar part of a building must have:

• A drainage system with overflow. Drainage complying with F1D3, which calls up AS/NZS 3500.3, incorporating sufficient overflow provision.
• Falls in the slab. The structural substrate graded with a minimum fall of 1:80 in the plane of the substrate.
• A step-down. A minimum 70 mm step down from the internal floor’s structural substrate to the external structural substrate, where applicable.
• A perimeter hob. A monolithic hob, integral with the structural substrate and at least 70 mm high, at the outer perimeter, unless that edge drains directly to a gutter.

F1D7(2) adds that, on concrete roofs and balconies, the membrane is applied directly on a structural substrate complying with F1D5, and F1D6 requires exposed joints to be located on the ridge line or high point of the substrate. In practice that means falls are formed in the slab, not built up in a screed under the membrane.

How should falls, outlets and overflows be designed?

Water must leave the surface by a planned route, even when the main outlet is blocked. AS 4654.2 deals with drainage and overflows within its design and installation requirements, and NCC Volume One requires stormwater drainage to be designed to AS/NZS 3500.3.

Good practice on site looks like this:

• Fall to the outlet at membrane level. Water that gets under tiles or pavers runs on the membrane, so the membrane surface needs its own fall, not just the finish.
• Outlets sized and accessible. Use outlets designed to bond or clamp to the membrane, with access for cleaning.
• An overflow on every enclosed area. A balcony, roof deck or planter with upstands on all sides needs an overflow that operates before water reaches the door sill or the top of the membrane.
• Visible overflows. A visible overflow warns that the main outlet is blocked.

In Victoria, the Building and Plumbing Commission’s practice note SD-02 gives guidance on balcony design and stormwater drainage, and balcony water ingress has been a focus of its Class 2 and Class 3 inspection campaigns.

How high do upturns and door terminations need to be?

The membrane must turn up walls, upstands and door frames far enough that water, including wind-driven rain, cannot flow over the top of it. AS 4654.2 sets minimum vertical termination heights in Table A1 of Appendix A, and they vary with the building’s wind classification. Treat those figures as minimums and confirm the height for the project’s wind class.

The WA Building Commission’s bulletin highlights door and window sills as a common complaint. Where doors open onto a waterproofed area, the membrane system should include a sub-sill flashing, and the external finished floor level must finish below the membrane’s termination height. Where a set-down or hob is not possible, for example for step-free access, the bulletin suggests forming a gutter in the substrate immediately in front of the opening.

The top edge of every upturn needs a termination that stops water getting behind it. Common methods are a cover flashing, a reglet cut into the wall, or a pressure flashing, following the membrane manufacturer’s details.

What extra detailing do planters and podiums need?

Planter boxes are permanently wet, heavy and full of roots, so they punish weak detailing. Key points are:

• No exposed joints under planters. NCC Volume One prohibits exposed joints beneath or running through a planter box, water feature or similar.
• A root-resistant system. Use a membrane the manufacturer states is suitable for planters and root contact, or add a separate root barrier over it.
• Drainage cell and filter fabric. A drainage layer over the membrane, with geotextile to keep soil out, lets water reach the outlet.
• Inspection access. Each planter outlet needs a way to check and clear it without digging out the soil.
• Membrane above soil level. Turn the membrane up the planter walls above the finished soil and mulch level.

Podiums add structural movement, penetrations and heavy loads. Protection boards over the membrane stop damage from backfill, pavers and later trades, and every penetration needs a puddle flange or a manufacturer’s detail.

Trafficable or non-trafficable: which membrane?

The answer depends on what sits on top. A trafficable membrane is designed to be walked on directly, usually with a wearing coat, and is common on small roof decks and plant platforms. A non-trafficable or protected membrane sits under tiles, pavers, screed, soil or ballast, which shield it from foot traffic and ultraviolet light.

Choose a system with evidence it suits the exact use: exposure, traffic, planting, substrate and movement. Mixing manufacturers within one system can void warranties and weaken the evidence of compliance. Where a product is outside AS 4654, it needs a Performance Solution supported by evidence of suitability.

How should external membranes be inspected and tested?

External membranes need inspecting before they are covered, because once tiles, pavers or soil go down, defects are expensive to find. The WA bulletin stresses that practitioners should monitor each stage of installation to confirm compliance with the NCC and AS 4654.

The NCC clauses above do not themselves require a flood test. Many specifications, certifiers and manufacturers ask for one on balconies, roof decks and planters: outlets are plugged, the area is filled to a set depth below the upturns and left for a set period, and the soffit below is checked for leaks. Agree the depth, duration and record first, within the structure’s design load.

In NSW, waterproofing is a building element under the Design and Building Practitioners Act 2020, so on regulated Class 2, 3 and 9c buildings the waterproofing design must be prepared and declared by a registered design practitioner before work starts. Waterproofing is also a licensed trade in NSW and Queensland, among other states. Our guide to common waterproofing defects covers who carries liability when membranes fail.

External waterproofing checklist

• Confirm the edition. Check whether NCC 2022 or NCC 2025 applies to the project.
• Check the falls. Confirm the substrate falls to outlets, at 1:80 in the slab where NCC 2025 F1D4 applies.
• Design overflows. Provide a visible overflow for every enclosed balcony, deck and planter.
• Set termination heights. Use AS 4654.2 Table A1 for the project’s wind class.
• Detail every door. Include sub-sill flashings and keep the finished floor below the membrane.
• Locate joints. Keep exposed joints out of planters and, under NCC 2025, on high points.
• Match the system. Use one manufacturer’s system suited to traffic, exposure and planting.
• Protect the membrane. Install protection boards before backfill, pavers or other trades.
• Inspect before covering. Photograph membrane, upturns, outlets and penetrations.
• Record any flood test. Note depth, duration, date and result in the handover file.

Frequently asked questions