A damp-proof course (DPC) is the critical moisture barrier built into the base of your walls to prevent ground water from wicking upward into your home. When this barrier fails — or was never installed in the first place — chemical DPC injection is the industry-standard solution. Waterproofing Sydney has performed thousands of DPC injections across the Greater Sydney region, and in this guide we explain exactly how the process works, what to expect, and how to ensure you get a lasting result.
Understanding the Damp-Proof Course
A damp-proof course is a horizontal barrier installed near the base of a masonry wall, designed to block the upward migration of ground moisture through capillary action. In modern Australian construction, the DPC is typically a polyethylene sheet or bituminous membrane embedded in the mortar bed during construction, as required under the National Construction Code (NCC) and Australian Standard AS 3740.
Older Sydney homes — particularly those built before the 1950s — may have a DPC made from slate, lead, bitumen-coated hessian, or engineering brick. Over decades, these materials degrade: slate cracks, bitumen dries out, and mortar joints deteriorate. Some very old properties, especially those with Hawkesbury sandstone foundations, were built with no DPC at all.
When the DPC fails, moisture from the surrounding soil rises through the porous masonry by capillary action, carrying dissolved salts with it. This is rising damp, and it causes progressive damage to plaster, paint, timber, and eventually the structural masonry itself.
How Chemical DPC Injection Works
Chemical DPC injection creates a new, continuous moisture barrier within the existing mortar course by saturating the masonry with a water-repellent chemical. The process has been refined over decades and, when performed correctly, provides a permanent solution to rising damp.
The science behind the process is straightforward: the injected chemical — typically a silane/siloxane cream or a resin-based solution — reacts with the silica in the masonry to form a hydrophobic (water-repelling) layer within the pore structure of the brick and mortar. This layer does not block the pores entirely, which means the wall can still breathe and release trapped moisture as vapour, but it prevents liquid water from migrating upward.
Step-by-Step DPC Injection Process
At Waterproofing Sydney, our DPC injection process follows a rigorous methodology developed through years of experience treating Sydney properties:
- Pre-treatment survey and diagnosis: We begin with a comprehensive moisture survey using calibrated electronic moisture meters (Protimeter MMS3 or equivalent) and, where necessary, gravimetric or calcium carbide moisture testing. This establishes the moisture profile of the wall, confirms the diagnosis of rising damp, and identifies the optimal injection height.
- Preparation of the injection zone: External render or internal plaster in the injection area is carefully removed to expose the masonry. This ensures clean access to the mortar bed and allows visual assessment of the masonry condition.
- Drilling injection holes: Holes are drilled into the mortar bed at 120mm centres (the industry-standard spacing for Australian masonry), angled slightly downward at approximately 10–15 degrees. Hole depth is calculated to penetrate at least 90% of the total wall thickness. For a standard 230mm double-brick wall, holes are drilled to approximately 200mm depth.
- Chemical injection: DPC cream is injected into each hole under low pressure using specialist injection equipment. The cream is designed to diffuse outward from each injection point, merging with adjacent injections to form a continuous horizontal barrier across the full width and length of the treated wall.
- Hole sealing: Once injection is complete, holes are sealed with colour-matched mortar to maintain the structural integrity and appearance of the masonry.
- Contaminated plaster removal: All plaster or render below the injection line — plus a minimum of 300mm above the highest detected moisture point — is removed. This contaminated material contains hygroscopic salts that will continue to attract moisture if left in place, even after a successful DPC injection.
- Salt-resistant re-rendering: A specialist salt-inhibiting render system is applied to the treated area. This typically involves a base coat incorporating a salt-blocking addite, followed by a finishing coat compatible with the desired final surface treatment.
- Post-treatment documentation: We provide a detailed treatment report including pre- and post-treatment moisture readings, chemical specifications, treatment methodology, and a formal waterproofing certificate.
Types of DPC Injection Chemicals
Not all injection chemicals are equal. The choice of product depends on the masonry type, wall thickness, and severity of the damp problem.
| Chemical Type | Active Ingredient | Best For | Penetration | Durability |
|---|---|---|---|---|
| Silane/siloxane cream | Silane and siloxane blend | Standard brick masonry | Excellent in regular brick | Building lifetime (20+ years) |
| Silicone micro-emulsion | Silicone resin | Brick and light stone | Good in medium-density masonry | 15–20 years typical |
| Resin-based injection | Polyurethane or epoxy resin | Thick walls, sandstone, rubble-fill | Superior in irregular masonry | Building lifetime |
| Siliconate solution (older technology) | Sodium or potassium siliconate | Thin single-brick walls | Limited in thick walls | 10–15 years |
For the majority of Sydney homes — standard double-brick construction — silane/siloxane cream is the preferred choice. It offers superior penetration, long-term durability, and consistent performance across varying masonry densities. For properties with Hawkesbury sandstone walls or thick rubble-fill construction, we use resin-based systems that can navigate the irregular pore structure of these materials.
DPC Injection in Sandstone Buildings
Sydney has a significant stock of sandstone buildings, particularly in suburbs like The Rocks, Balmain, Glebe, Paddington, and across the Lower North Shore. Hawkesbury sandstone — the dominant building stone in the Sydney Basin — presents unique challenges for DPC injection.
Sandstone is highly porous but has an irregular pore structure compared to manufactured brick. Standard silane/siloxane creams may not achieve reliable penetration through thick sandstone blocks. For these properties, Waterproofing Sydney uses resin-based injection systems specifically formulated for natural stone. The resin is injected at higher pressure and in greater volume to ensure complete saturation of the stone matrix.
Heritage-listed sandstone buildings may also require approval from the local council heritage officer or the NSW Heritage Council before treatment can proceed. We work closely with heritage consultants to ensure our methods comply with all conservation requirements while still delivering an effective moisture barrier.
Common DPC Injection Mistakes to Avoid
Not all DPC injection work is performed to the same standard. Here are the most common mistakes that lead to treatment failure:
- Insufficient hole depth: Injection holes that do not penetrate at least 90% of the wall thickness leave the inner portion of the wall untreated, allowing moisture to bypass the barrier.
- Incorrect hole spacing: Holes drilled too far apart create gaps in the chemical barrier. The industry standard of 120mm centres is based on the typical diffusion radius of modern injection creams in Australian masonry.
- Wrong chemical for the substrate: Using a silane cream in thick sandstone masonry, or a water-based siliconate in a severely damp wall, will produce an inadequate barrier.
- Failure to remove contaminated plaster: This is perhaps the most common mistake. Even a perfect DPC injection will appear to fail if contaminated plaster is left in place, because the hygroscopic salts embedded in the plaster continue to draw moisture from the air.
- Ignoring external contributing factors: High soil levels against walls, blocked sub-floor vents, leaking downpipes, and poor drainage can all contribute moisture that overwhelms even a well-installed DPC. These issues must be addressed as part of any comprehensive treatment.
What to Expect After DPC Injection
Understanding the post-treatment timeline helps set realistic expectations:
- Immediate effect: The injected chemical barrier begins working immediately, stopping further upward moisture migration from the moment of injection.
- Drying period (1–6 months): Moisture already present in the wall above the injection line must dry out naturally. Drying time depends on wall thickness, masonry type, ventilation, and ambient conditions. A typical 230mm brick wall in Sydney conditions dries at a rate of approximately one month per 25mm of wall thickness.
- Re-rendering (after 2–4 weeks): Salt-resistant render can be applied once the worst of the surface moisture has dissipated. The render system is designed to manage residual moisture during the drying period.
- Repainting (after 4–8 weeks): Final decoration should wait until the render has fully cured and residual moisture levels have stabilised.
Australian Standards and Regulatory Requirements
DPC injection work in NSW is governed by several regulatory frameworks:
- AS 3740 – Waterproofing of domestic wet areas: While primarily focused on wet area waterproofing, this standard establishes principles relevant to moisture management in residential buildings.
- National Construction Code (NCC): The NCC mandates damp-proofing provisions for all new construction and sets performance requirements for moisture barriers.
- NSW Home Building Act 1989: Contractors performing DPC injection work valued over $5,000 must hold the appropriate licence issued by NSW Fair Trading. Unlicensed work is not covered by the statutory warranty scheme.
- NSW Fair Trading statutory warranties: Licensed work carries statutory warranty periods of six years for major defects and two years for other defects, providing homeowners with legal protection beyond any contractual warranty offered by the installer.
Choosing a DPC Injection Specialist in Sydney
When selecting a contractor for DPC injection, consider the following criteria:
- Verify their NSW Fair Trading licence for waterproofing work
- Ask for evidence of specialist training in damp-proofing (not just general building qualifications)
- Request references from similar projects, particularly if your home has sandstone or heritage features
- Ensure they conduct a thorough pre-treatment moisture survey rather than diagnosing by visual inspection alone
- Confirm they provide a detailed written warranty and waterproofing certificate upon completion
- Check that they remove contaminated plaster and apply salt-resistant render as part of the standard scope — not as costly extras
Frequently Asked Questions
How long does a chemical DPC injection last?
Modern silane/siloxane cream injections are designed to last the lifetime of the building — typically 20 years or more. The chemical bonds permanently with the silica in the masonry and does not degrade under normal conditions. Waterproofing Sydney provides a comprehensive written warranty with every DPC injection we perform.
Can DPC injection be done from the outside only?
In most cases, injection from one side of the wall is sufficient, provided the holes are drilled to adequate depth (at least 90% of wall thickness). For very thick walls (over 350mm) or rubble-fill construction, injection from both sides may be necessary to ensure complete coverage. Our pre-treatment survey determines the optimal approach for each property.
Is DPC injection suitable for all wall types?
Chemical DPC injection is effective in brick, sandstone, block work, and most other masonry substrates. It is not suitable for cavity wall construction where the cavity itself is the moisture pathway, or for walls where the primary moisture source is lateral penetration rather than rising damp. Correct diagnosis is essential to selecting the right treatment.
What happens if I do not treat rising damp?
Untreated rising damp progressively worsens over time. Salt crystallisation within the masonry causes spalling and erosion of brick faces. Timber elements in contact with damp masonry — floor joists, wall plates, skirting boards — are at risk of rot and termite attack. Mould growth creates health hazards. Property value declines significantly, and building inspection reports will flag the issue for any prospective buyer.
Do I need council approval for DPC injection?
For standard residential properties, DPC injection does not require development approval or a construction certificate. However, heritage-listed properties may require approval from the local council heritage officer or the NSW Heritage Council. Our team can advise on heritage requirements and assist with any necessary applications. Contact Waterproofing Sydney for guidance specific to your property.
Concerned about rising damp in your Sydney home? Waterproofing Sydney offers obligation-free assessments and detailed quotes for DPC injection across Greater Sydney. Our licensed specialists bring the expertise and equipment needed to diagnose accurately and treat effectively. Book your assessment today.