True basements are not common in the Illawarra, the combination of underlying sandstone, coastal water table and steep escarpment topography makes traditional basement excavation less frequent than in colder climates where basements provide winter warmth storage. However, below-grade spaces do exist in Wollongong, particularly in split-level homes on escarpment slopes, older commercial buildings in the CBD, and some undercroft spaces in Thirroul and Bulli, and when they get wet, mould follows quickly.
Cavity drainage systems are the modern approach to managing moisture in below-grade spaces where conventional waterproofing membranes have failed or are impractical to retrofit. This guide explains how they work, when they’re appropriate and what they cost in the Illawarra.
What Is a Cavity Drainage System?
A cavity drainage system (also called a membrane drain or controlled drainage system) manages water that enters a below-grade space rather than attempting to prevent its entry entirely. The key components:
Dimple membrane: A heavy-duty polythene sheet with protruding studs (dimples) on one face that hold the membrane away from the wall or floor surface by 5-8 mm. This creates a drainage void behind the membrane through which water flows down and away rather than accumulating against the substrate.
Floor drainage channel: A plastic drainage channel with a filter sock, installed at the wall-floor junction, collects water that has flowed down behind the wall membrane.
Sump pit and pump: Water collected by the drainage channel flows to a sump pit at the lowest point of the space. An automatic pump discharges the collected water to an external drain or soakaway when it reaches a trigger level.
Vented system (in some applications): Vents in the upper membrane allow moisture vapour to escape to the outside rather than accumulating inside the drainage void, important for mould prevention within the drainage system itself.
When Conventional Waterproofing Doesn’t Work
The alternative to cavity drainage is tanking, applying a waterproof membrane directly to the outside or inside face of the substrate to prevent water entry. External tanking (applied to the outside of the wall) is highly effective but requires excavation around the building, which is expensive, disruptive and sometimes impossible in built-up areas.
Internal tanking membranes (cementitious or polymer coatings applied to the internal wall surface) can work under limited hydrostatic pressure but fail when:
- Water table pressure exceeds the membrane adhesion
- Substrate cracks allow water entry behind the membrane at high pressure
- The concrete or masonry substrate is too saturated to provide a bond for the membrane
Cavity drainage systems bypass this limitation, rather than holding water back, they manage it. As long as the drainage channel and pump can handle the volume, the system works regardless of water table pressure.
The Mould Connection
When a below-grade space is chronically wet, mould follows in 24-72 hours on any organic surface. Typical mould locations in wet below-grade spaces:
- Plasterboard or timber stud partitions installed against wet concrete walls
- Carpet or timber flooring on wet concrete slabs
- Storage items on the floor (cardboard boxes are particularly susceptible)
- Timber joinery and door frames in the space
- HVAC equipment, water heaters and other plant in wet undercroft spaces
A cavity drainage system eliminates standing water and dramatically reduces the water vapour load in the space. Combined with treatment of established mould, it creates conditions where mould cannot re-establish.
Wollongong Properties Where Cavity Drainage Applies
Split-Level Homes on Escarpment Slopes
Thirroul, Austinmer, Bulli, Corrimal and other escarpment suburb homes built on slopes often have a lower floor that is partially or fully below surrounding ground level on the uphill side. This level receives groundwater seepage from the escarpment above and may experience significant water entry during heavy rain events.
Cavity drainage, applied to the uphill wall face and discharging to the downhill drainage, is often the most practical solution for these spaces, particularly where external excavation would require removing significant landscaping or retaining walls.
Wollongong CBD and Inner-City Ground Floor
Older commercial buildings in Wollongong CBD, particularly those near Market Street and Crown Street in the lower-lying sections of the city centre, sometimes have ground floors that experience seasonal moisture from the shallow coastal water table. Cavity drainage in these spaces manages the moisture without requiring building closure for external waterproofing works.
Undercroft Storage Areas Under Stilt Homes
Homes on steep escarpment lots sometimes enclose the area under the stilted lower floor as storage or utility space. This enclosed below-grade space (on the uphill side) is effectively a partial basement and often becomes wet. Cavity drainage provides a management solution for what is otherwise a difficult-to-waterproof space.
Installation Process
A cavity drainage installation in a typical Wollongong below-grade space follows this sequence:
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Mould remediation first. Existing mould on all surfaces must be treated before the membrane is installed, trapping mould behind an impermeable membrane creates an anaerobic mould environment.
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Prepare substrate. Remove any plasterboard, timber stud partitions or floor coverings that will be enclosed behind the membrane system.
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Install wall membrane. The dimple membrane is fixed to the wall using specialist fixings that maintain the drainage void. Joints are overlapped and sealed. The membrane runs from floor level to at least 150 mm above the highest observed water mark.
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Install floor drainage channel. The perforated drainage channel is set into the floor at the wall-floor junction, typically requiring cutting the slab perimeter with an angle grinder. The channel slope is critical, inadequate slope means water doesn’t reach the sump.
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Install sump and pump. The sump pit is excavated at the lowest point. An auto-pump sized for the anticipated water volume is installed with a check valve and discharge pipe to the external drainage system.
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Install floor membrane. A separate floor dimple membrane is laid over the entire floor area, lapping into the drainage channel. This prevents moisture from the slab evaporating into the space.
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Install new partition and floor finishes. Once the drainage system is in and tested, the space can be re-lined with appropriate materials, moisture-resistant plasterboard, vinyl or tile flooring, aluminium framing rather than timber.
Typical Costs in the Illawarra
| Scope | Estimated Cost AUD |
|---|---|
| Wall membrane only (one wall) | $2,000 to $5,000 |
| Full below-grade room, wall and floor membrane, sump and pump | $8,000 to $20,000 |
| Large undercroft or semi-basement | $15,000 to $40,000+ |
| Add mould remediation to the above | $1,500 to $5,000 additional |
These costs reflect the Illawarra market and include materials and installation but not finishing works (new wall lining, floor coverings, electrical adjustments).
Maintenance Requirements
A cavity drainage system requires minimal ongoing maintenance:
- Pump tested 2-4 times per year (run test cycle, confirm discharge is clear)
- Sump inspected annually for sediment accumulation
- Drainage channel filter sock replaced every 5-10 years (or when flow reduces)
- Overflow alarm system (recommended) checked at each pump test
Pump failure is the main risk, a backup pump or high-water alarm is recommended in spaces where flooding during pump failure would cause significant damage.
Frequently Asked Questions
Does a cavity drainage system stop mould permanently? It removes the water source that drives mould growth. Once mould is remediated and the space is kept dry by the drainage system, new mould cannot establish without a moisture source. The system must operate correctly, a failed pump means water accumulates and mould can return.
Can I install cavity drainage myself? The materials are available through specialist building supplies. However, the drainage channel and sump placement require careful survey to ensure correct falls to the sump, and cutting the slab for the drainage channel requires appropriate equipment and building permit consideration. Professional installation is strongly recommended.
My Wollongong CBD ground floor shop gets wet feet in winter, is cavity drainage appropriate? Possibly, but the first step is diagnosing the source. If it’s groundwater rising through the slab (no hydrostatic pressure from the side), a different approach (perimeter slab edge drainage, slab coating) may be more appropriate. If water is entering through the external wall base from street-level or sub-surface water, cavity drainage is a good fit. A mould and moisture inspection with moisture meter surveys determines the source.
Will cavity drainage affect my building’s structure or council approvals? Cutting the slab perimeter for drainage channels and excavating a sump pit are generally considered minor building works. In most NSW jurisdictions, these are exempt development, but check with Wollongong City Council for your specific property and use. In heritage-listed properties, additional approvals may be required.
How long does cavity drainage installation take? A single room: 2-4 days. A full below-grade space (50-100 m²): 5-10 days. Including mould remediation before installation and re-lining after, allow 2-4 weeks for a complete project.
Wet below-grade space in your Wollongong property? Get a proper moisture diagnosis before committing to a solution. Contact Illawarra Mould Removal for an assessment and recommendations.