Damp proof courses (DPCs) are vital barriers that prevent moisture from rising through walls and floors. They protect buildings from structural damage, decay, and health issues caused by dampness. DPCs are available in various types, including physical barriers and chemical treatments.

What Are the Key Differences When Applying a DPC to Internal vs External Walls?

The process of applying a DPC differs depending on whether it’s for internal or external walls.

Internal Walls

• May require removal of plaster to access masonry
• Needs careful planning around finishes and interior fixtures
• Typically uses less invasive methods to reduce disruption

External Walls

• Offers easier access but may be impacted by weather conditions
• Often involves external drilling, which is usually more straightforward
• May require adjustments for external renders or cladding

In both cases, it’s important to assess the wall thickness and material. Internal DPCs focus on minimal visual impact, while external treatments prioritise long-term weatherproofing.

DPC Liquids vs Chemical Injection: Which Is More Effective?

Both DPC liquids and chemical injection methods are designed to create moisture barriers but function differently.

DPC Liquids: These silicone-based treatments penetrate the masonry and form a continuous, water-repellent barrier. They're easy to apply, deliver consistent coverage, and are ideal for smaller areas or where minimal disruption is required.

Chemical Injection: This method involves injecting cream-based or pressurised liquids into drilled holes. The chemicals spread and bond within the wall, making it ideal for dense masonry or walls suffering from severe rising damp.

DPC liquids work best in standard brickwork or smaller jobs, while injection methods are preferred for irregular stone or thicker wall construction.

How to Ensure Proper Curing of a Liquid DPC

Following these steps can help ensure a liquid DPC cures properly and remains effective long-term:

• Maintain stable temperature and humidity levels—avoid extreme weather
• Shield the area from rain and moisture during the curing period
• Provide good airflow to support drying and chemical bonding
• Avoid disturbing the treated section to prevent weak points forming
• Use temporary protective barriers to prevent premature moisture exposure

What Happens If You Apply New Damp Proofing Over a Failed DPC?

Applying new damp proofing over a failed DPC can trap moisture between layers, worsening the damp issue. This could lead to more structural damage, internal condensation, and mould growth.

Adhesion issues may also arise, reducing the effectiveness of the new treatment. It’s essential to address the failed DPC—either by removal or remediation—before applying a fresh system.

How Long Does a Damp Proof Course Last?

The lifespan of a DPC depends on several factors, including:

• Quality of materials and installation
• Environmental exposure
• Building maintenance practices
• Type of DPC used (physical or chemical)

A well-installed physical DPC can last the lifetime of the building. Chemical DPCs typically have a lifespan of 20 to 30 years and may require reapplication depending on conditions.