Rising damp, groundwater wicking upward through masonry, is the primary moisture mechanism in pre-1970 brick homes across Wollongong, Corrimal, Thirroul, Dapto and older parts of the Illawarra. Without an effective damp proof course (DPC) to block this upward movement, ground moisture travels through the base of walls, depositing salts and sustaining mould growth that cannot be permanently resolved without addressing the DPC failure.
This guide explains how DPCs fail, how to identify the problem, and what the remediation options are.
What Is a Damp Proof Course?
A damp proof course is a horizontal barrier built into masonry walls, typically at or just above ground level, that prevents moisture from rising through the wall material. In older Illawarra homes, DPCs were constructed using:
- Slate, quarried stone sheets laid across the wall, used pre-1920s
- Engineering brick, a dense, low-permeability fired brick used as a DPC course, common 1900-1940
- Lead sheet, used particularly in higher-quality construction 1900-1960
- Bituminous felt, a tar-impregnated membrane, common 1940-1970
- Early polymer membranes, various synthetic materials, 1960s onwards
Modern construction uses continuous polymer or bituminous membrane DPCs that are highly effective and durable. The problem is that older materials, particularly bituminous felt and some early polymer types, deteriorate over 40-60 years. Slate and lead are more durable but can crack or gap at joints, especially in older homes that have settled. Some older homes have no DPC at all.
How DPCs Fail
Material deterioration, bituminous felt oxidises and becomes brittle over time. The membrane develops cracks and pin holes. Each small failure allows a moisture pathway through.
Physical bridging, any material that contacts the outer wall below the DPC level and extends above it creates a moisture bridge around the DPC. Classic examples: garden beds raised above DPC level, concrete paving or paths poured against the wall base, render that extends below the DPC level.
Cracking from building movement, foundations settle, especially in Illawarra’s varied soil types. As the building moves, the DPC layer can crack or gap at points of stress.
Chemical attack, salts in the soil or groundwater can attack some DPC materials over time. This is more common in coastal areas of Wollongong where marine salts are present in the ground.
Construction omission, a significant proportion of pre-1940 Illawarra homes simply have no DPC. In these cases, there is nothing to fail, the wall was never protected in the first place.
Identifying DPC Failure vs Other Moisture Problems
Rising damp from a failed DPC has characteristic signs that distinguish it from condensation or penetrating damp:
| Sign | Rising Damp | Condensation | Penetrating Damp |
|---|---|---|---|
| Tide mark on wall | Yes, classic sign | No | Sometimes |
| Location on wall | Lowest 500-1,000 mm | Upper walls, windows | Any height |
| Efflorescence (white salts) | Yes, heavy | No | Occasionally |
| Pattern | Horizontal band | Follows cold surfaces | Follows water path from above |
| Season | Year-round (worse in wet season) | Winter only | During and after rain |
| Damp at floor level | Yes | Not typically | Depends on source |
| Smell | Strong musty/earthy | Musty | Musty |
Moisture meter readings confirm rising damp: readings will be highest at the base of the wall and reduce as you move higher. A professional inspection with a calibrated moisture meter at multiple heights is the definitive diagnostic tool.
The Mould Pattern From Rising Damp
Mould growing from a failed DPC appears as a band rising from the bottom of walls, following the moisture front upward. It’s most pronounced:
- At the base of external walls in ground floor rooms
- Behind skirting boards (the space between skirting and plaster holds moisture)
- In wall recesses, alcoves and built-in cupboards on external walls
- At wall-to-floor junctions on external walls
The mould is characteristically patchy and appears to rise from the floor rather than from a point source above. In brick homes, the external face of the brick often shows efflorescence (white salt deposits) at the same height band as the internal mould.
Why the Illawarra Has Elevated DPC Failure Rates
Wollongong’s heritage residential suburbs, Thirroul, Bulli, Corrimal, Mount Keira, Wollongong CBD, Figtree and parts of Dapto, contain a high density of pre-1960 housing with original DPC materials now 60-120 years old. The combination of:
- Age-related material deterioration
- Relatively high groundwater levels in lake-adjacent and floodplain areas
- Garden renovation and paving that bridges DPCs without owners realising
- Humid coastal climate that means ground moisture levels stay relatively high year-round
…creates a region with above-average DPC failure rates and consequently above-average rising damp and mould problems in its older housing stock.
Treatment Options for Failed DPC
1. Physical DPC Replacement (Traditional)
The original solution: saw-cutting through the mortar bed at DPC level, removing the old material and inserting a new damp proof course (typically a continuous polymer membrane). This works well but is:
- Disruptive (wall must be cut through brick by brick in short sections to maintain structural integrity)
- Expensive ($200-$500 per linear metre of wall)
- Not feasible in all wall configurations
2. Chemical Injection DPC
The most common modern retrofit solution. A silicone or silane-based fluid is injected under pressure into a series of drilled holes along the mortar course at DPC level. The fluid impregnates the masonry and polymerises into a water-repellent zone that acts as a new DPC.
Effectiveness: good for most situations. Not suitable for very high water tables (the injection zone can’t overcome hydrostatic pressure). Should be installed by a company with experience in the specific products used. Typically carries a 20-year guarantee.
Cost: $150-$350 per linear metre including replastering.
3. Cavity Drain Membrane (Controlled Drainage)
Rather than blocking moisture entry, a dimpled plastic membrane attached to the internal face of the wall creates an air gap that allows moisture to run down to a drainage channel at floor level, where it’s directed to drain. This approach manages the moisture rather than stopping it, and is most appropriate where water table pressure is too high for injection DPC.
4. Electro-Osmotic Systems
An electromagnetic system that theoretically redirects the electrical charge driving capillary rise. Evidence for effectiveness is limited and contested. Not a first-line recommendation by most building practitioners.
After DPC Repair: Dealing With the Mould
A repaired DPC stops new moisture entry but does not remove the salt-contaminated plaster, mould-affected skirtings, or mould in the wall base. After DPC repair:
- Remove salt-contaminated or mould-affected plaster to at least 600 mm above the high-tide mark
- Allow the masonry to dry, this takes 1-12 months depending on the severity of saturation
- Apply a salt-retarding render or renovation plaster designed for rising damp situations (standard renovation plaster can be pushed off by salt crystallisation)
- Treat and prime with appropriate products before repainting
- Replace skirting boards that have been moisture-affected
Full mould remediation of the internal surfaces should happen at the time of DPC repair, not before, cleaning mould from plaster that is still rising-damp affected just allows regrowth.
Frequently Asked Questions
How do I know if my home has a DPC? Ask your local council or do a historical property search, homes built after 1960 in NSW were required to have DPCs under building regulations. For older homes, visual inspection of the outer wall at ground level may reveal a line of different material (often blue-grey engineering brick or a visible membrane). A building inspector or damp specialist can confirm and assess its condition.
Can I just paint over rising damp mould with anti-mould paint? No. Salt crystallisation from rising damp pushes off any coating applied over damp or salt-contaminated plaster. Anti-mould paint applied to rising-damp-affected walls will bubble, peel and fail within months. The DPC and salt-contaminated plaster must be addressed first.
My house was built in the 1940s with no DPC, what are my options? Chemical injection DPC is the most practical option for most older homes without a DPC. The drill holes are placed in the first mortar bed above ground level, and the injection creates an effective new barrier. Combined with removing salt-contaminated plaster and re-rendering, this is a lasting solution.
How much does DPC repair and replastering cost in Wollongong? A typical terraced or semi-detached home with rising damp affecting three ground-floor rooms: $4,000-$12,000 for chemical injection DPC, plaster hack-off, replastering and repainting. Add mould remediation treatment to the internal wall surfaces as part of the same scope. Get quotes from damp specialists, not general plasterers, the product knowledge matters.
Is rising damp a defect that must be disclosed when selling in NSW? Vendors must disclose material defects they are aware of. Rising damp is a material defect. Undisclosed rising damp after a property sale has been the subject of numerous NSW property disputes. A vendor who knows about rising damp and doesn’t disclose faces potential rescission of the contract or a damages claim. See our guide on pre-sale mould disclosure.
Persistent mould at the base of your walls in an older Illawarra home? Get the right diagnosis before spending money on the wrong treatment. Contact Illawarra Mould Removal for a rising damp and mould assessment.