What Happens During a Damp Proof Course (DPC) and Does It Work?

A Damp Proof Course (DPC) is a key method used to prevent rising damp in buildings by stopping moisture from penetrating through the walls. Rising damp occurs when moisture from the ground moves up through the porous material of walls, leading to structural damage and mold growth. A properly installed DPC acts as a barrier to halt this process. Here’s how it works and whether it's effective.

The specific steps involved in installing or repairing a damp proof course depend on the type of DPC and the severity of the damp problem. The most common methods are physical DPC and chemical DPC. Here’s an overview of what happens in each process:

Physical Damp Proof Course

A physical DPC involves inserting an impermeable material (such as plastic, bitumen, or slate) into the wall to form a continuous barrier that prevents moisture from rising. It’s usually installed during the initial construction of the property but can also be added to older buildings during repairs.

Steps involved in physical DPC:

• Wall Preparation: If a physical DPC needs to be retrofitted, workers may need to remove sections of the lower wall to insert the new material.
• Insertion of DPC: An impermeable membrane is inserted into the mortar joint between bricks at the base of the wall, approximately 150mm above ground level. This creates a waterproof barrier.
• Repointing: After the membrane is installed, the wall is repaired and repointed to restore its original structure.

Chemical Damp Proof Course

Chemical DPC is the most common method for treating rising damp in existing homes. It involves injecting a chemical solution into the walls that creates a waterrepellent barrier.

Steps involved in chemical DPC:

• Drilling Holes: Holes are drilled into the walls at regular intervals (usually along the mortar joints) at the base of the wall, typically about 150mm above ground level.
• Injecting Damp Proof Solution: A siliconebased or resinbased chemical solution is injected into the holes. This chemical penetrates the bricks or masonry, creating a waterproof barrier that prevents moisture from rising.
• Wall Resealing: After the solution is injected, the holes are sealed and the wall is rerendered or replastered as necessary.
• Drying Time: It can take several months for walls to fully dry out after the chemical DPC has been installed, as residual moisture trapped in the wall will gradually evaporate.

A properly installed damp proof course can be highly effective at stopping rising damp and protecting a building from moisture damage. However, its effectiveness depends on several factors:

Correct Installation

Whether a physical or chemical DPC is installed, the process must be carried out correctly for the barrier to work. Any gaps or improperly treated areas could allow moisture to bypass the DPC, causing the damp to return. If installing a chemical DPC, care must be taken to ensure the correct quantity of chemicals is injected evenly across the treated area.

Type of DPC

Physical DPC is generally more durable since it involves inserting an impermeable membrane, but it’s typically only feasible during the construction phase or major renovations. Chemical DPC is a practical solution for existing buildings, but the quality of the product and the expertise of the installer are crucial for longterm effectiveness.

External Conditions

In some cases, damp problems arise due to external factors such as water ingress from damaged gutters, leaking pipes, or poor drainage around the foundation. If these issues aren’t addressed, even a properly installed DPC might not fully resolve the problem. For example, if rainwater consistently seeps into the wall due to cracks or poor waterproofing, the wall will remain damp.

Plaster Replacement

After installing a DPC, especially with chemical injection, it's often necessary to remove and replace the old plaster affected by damp. If the old plaster is left in place, it could still contain salts and moisture, allowing the damp to persist or reoccur on the surface.

You can tell that your DPC is working effectively if:

• No More Damp Patches Appear: After the walls have had time to dry out, you should see no new signs of damp patches on internal or external walls.
• Improved Humidity Levels: The home should feel drier, especially in previously affected areas. Reduced humidity levels often lead to a noticeable improvement in air quality.
• No Mold or Mildew: Mold and mildew thrive in damp environments. If the DPC has worked, these signs should disappear over time.

There are instances where a DPC may not fully solve the damp problem, typically due to underlying issues:

• Bridging: If soil, debris, or paving bridges the DPC by rising above the treated area, moisture can bypass the barrier. Regularly check the external ground level to ensure it remains below the DPC.
• Internal Moisture Sources: Issues like leaking pipes, condensation, or poor ventilation can also cause dampness, even if a DPC is in place. It’s crucial to identify and address all sources of moisture within the home.
• Preexisting Water Damage: If the walls were extensively damaged by damp before the DPC was installed, it may take longer for them to fully dry out and recover. In severe cases, parts of the wall may need to be replaced.

Conclusion

A Damp Proof Course is an effective and longterm solution for preventing rising damp when properly installed. Whether using a physical membrane or a chemical injection method, a DPC can stop moisture from moving up through your walls, protecting your home from structural damage and health risks associated with damp and mold.

However, for the DPC to work, it must be installed correctly, and other sources of dampness (such as leaks or condensation) should also be addressed. If these factors are managed, a damp proof course can be a valuable and lasting solution for maintaining a dry, healthy home.