DIY Damp Proof Injection: How to Install a Damp Proof Course

Understanding Damp Proof Injection and Why It Matters

Rising damp is one of the most persistent problems affecting older buildings, and installing a chemical damp proof course (DPC) is often the only lasting solution. Damp proof injection works by creating a water-repellent barrier within the mortar course of a wall, halting the upward capillary movement of ground moisture. Whether you are dealing with tide marks on internal plaster, rotting skirting boards, or peeling wallpaper, a properly installed injected DPC can resolve the root cause rather than just masking the symptoms. This guide walks through the entire process, from diagnosing the problem to selecting the right injection kit and completing the installation correctly.

Before any injection work begins, it is essential to confirm that rising damp is actually the issue. Many cases of dampness in walls are caused by condensation, leaking pipes, or bridging from external ground levels. Diagnosing damp issues below a damp proof course requires careful inspection with a moisture meter and an understanding of how damp travels through masonry. If the damp is rising from the ground and stopping at a height of around one metre, with a characteristic tide mark and white salt deposits, then an injected chemical DPC is likely the correct remedy.

How Chemical DPC Cream Works in Masonry

Chemical DPC products are typically silicone-based or silane/siloxane formulations that react with the alkaline environment of mortar and brick to form a hydrophobic barrier. The injected liquid travels through the pores and capillary channels of the masonry, lining the internal surfaces so that water can no longer pass upward. The treatment zone is normally applied in a single mortar course approximately 150 mm above external ground level.

Key Properties of DPC Injection Fluids

• Silicone-based creams: Thick consistency that stays in place after injection, ideal for DIY use and vertical or overhead applications
• Silane/siloxane liquids: Lower viscosity, penetrate deeper into porous masonry, often used by professionals with pump equipment
• Water-based formulations: Lower odour and safer for indoor use, though they may require longer drying times
• Solvent-based formulations: Faster penetration and quicker curing, suitable for dense or damp masonry

When a DPC Injection Is the Right Solution

Not every damp wall needs a chemical DPC. The injection method is specifically designed to address rising damp where the original physical DPC has failed, was never installed, or has been bridged by building alterations. Common scenarios include:

• Pre-1900 solid brick or stone walls built without any physical damp proof course
• Older buildings where the original slate or bitumen DPC has deteriorated or been punctured
• Extensions or alterations where the floor level has been raised above the existing DPC
• Internal plaster that has been applied across the DPC line, creating a bridge for moisture

Selecting the Right DPC Injection Kit and Tools

Choosing the correct DPC injection system is critical to achieving a lasting result. Modern injection kits come in two main formats: cartridge-based creams that fit a standard skeleton gun, and liquid systems that require a pressure pump. For most DIY installers, cream-based systems are the more practical option because they offer better control, less mess, and do not require specialist pumping equipment.

Essential Tools for DPC Injection

Beyond the injection kit itself, you will need a reliable drill with a 12 mm masonry bit, a tape measure and pencil for marking hole positions, a vacuum cleaner to remove dust from drilled holes, and personal protective equipment including goggles, gloves, and a dust mask. Some manufacturers supply a drilling template with their kits, which ensures holes are placed at the correct height and spacing for the specific product being used.

Preparing the Wall Surface

Before drilling begins, strip back any plaster, render, or wall coverings to expose the bare masonry at the level where the DPC will be injected. The treatment zone should be clear of debris, and the mortar joints should be in reasonable condition. If the existing mortar is loose or badly weathered, it may need raking out and repointing before the injection can be carried out effectively. Understanding the difference between plaster, render, and screed helps ensure the correct repair materials are used for the surrounding finishes.

Step-by-Step DPC Injection Procedure

The installation of an injected damp proof course follows a methodical sequence that must be adhered to for the treatment to be effective. Rushing any step or skipping the preparation work will compromise the result.

Drilling the Injection Holes

Holes are drilled into the mortar joint (never through the brick or stone itself) at a downward angle of around 30 degrees to help the fluid travel deeper into the wall. The spacing between holes depends on the wall thickness and the product manufacturer’s recommendations, but typical intervals are 100 mm to 120 mm apart for solid walls. For cavity walls, only the inner leaf requires treatment, and the holes should not penetrate through to the cavity.

1. Mark the drilling positions along a single mortar course, using the manufacturer’s template if supplied
2. Drill to the depth specified in the kit instructions, usually around two-thirds of the wall thickness
3. Blow out or vacuum all dust and debris from each hole, as any blockage will prevent proper absorption
4. Insert injection nozzles or packers if required by the system being used

Injecting the DPC Fluid

Once the holes are prepared and cleared, the injection process can begin. For cream-based systems, attach the cartridge to the skeleton gun and insert the nozzle fully into the hole. Squeeze the trigger steadily and withdraw the nozzle slowly as the hole fills. The cream should visibly purge from the hole as it reaches capacity, indicating that the void is fully saturated.

For liquid systems using packers, connect the pressure pump and inject until the fluid returns around the packer or the pressure gauge indicates the required dosage. Work systematically along the wall, injecting each hole in sequence, and then repeat the process to top up any holes where the fluid has absorbed further into the masonry.

Post-Injection Sealing and Drying

After injection, the holes can be sealed with a mortar plug or left open depending on the product instructions. The wall must then be left to dry naturally for a period of six to twelve weeks before any replastering takes place. During this drying phase, keep the area well ventilated and consider using a dehumidifier to accelerate moisture removal. Proper cavity wall insulation and damp prevention methods can complement the new DPC by reducing the risk of condensation and interstitial dampness forming after the treatment.

Replastering After DPC Injection and Long-Term Maintenance

Once the wall has dried adequately, replastering is the final stage that restores the interior finish and protects the treated area. This step is just as important as the injection itself because standard gypsum plasters can draw residual salts from the masonry and fail within months if applied directly.

Selecting the Correct Plaster System

For walls treated with a chemical DPC, a renovating or breathable plaster system is essential. These plasters are formulated to tolerate low levels of residual moisture and to resist salt migration. A typical system comprises:

• A bonding coat or scratch coat applied directly to the masonry
• A top coat of renovating plaster, applied once the base coat has set
• Decoration with breathable paint or a water-based emulsion, avoiding vinyl or oil-based finishes

For particularly damp or salt-laden walls, an air gap membrane system can be installed before plastering. This creates a physical separation between the masonry and the new plaster, allowing moisture to evaporate harmlessly behind the membrane. Waterproofing basements and cellars using tanking and membrane systems follows similar principles and provides useful reference for below-ground applications.

Monitoring and Maintaining the New DPC

An injected chemical DPC should last the lifetime of the building if properly installed, but it needs certain conditions to remain effective. External ground levels must be kept at least 150 mm below the DPC line, and any flower beds, patios, or pathways that bridge the DPC should be excavated and lowered. Gutters and downpipes must be maintained to prevent water splashing up against the wall above the DPC level.

Common Problems After DPC Installation

1. Persistent damp patches caused by bridging from external ground levels that were not addressed before injection
2. Salt staining on new plaster where a non-breathable paint or wallpaper has been used
3. Condensation forming on cold walls where ventilation is inadequate, mimicking rising damp symptoms
4. Failed injection patches where holes were not drilled at the correct spacing or to sufficient depth

A thorough inspection regime, checking both the internal plaster condition and external ground levels annually, will catch any potential issues before they develop into serious damp problems. Keeping a photographic record of the wall before and after treatment can help identify changes over time.

Damp proof injection remains one of the most effective and cost-efficient methods of dealing with rising damp in solid wall construction. While the process requires careful attention to detail and patience during the drying phase, the results can transform a damp, musty interior into a warm, dry living space. For any property owner tackling this job, investing time in proper preparation and selecting quality materials will repay itself many times over in the decades to come.