The Role of Damp Proof Courses (DPC) in Construction

Laying a damp proof course (DPC) is an important part of building. The 1875 Public Health Act first made it mandatory to install a DPC. Building inspectors will check that the cavity wall's two layers are both protected, and ensure that the DPC strips overlap correctly at joins. They also verify that the damp-proof membrane (DPM) on the internal floor extends into the DPC in the walls, creating a seal that prevents moisture from entering.

Where else are DPCs used?

Damp proof courses aren’t limited to lower sections of wall. Vertical DPCs are also used in certain situations, such as where garden walls meet the main structure, to prevent damp from entering.

Additionally, at roof level, a DPC is often placed in the lower brickwork of chimneys and parapet walls to prevent moisture from seeping down into the property.

In modern buildings, cavity trays formed from DPC material are often incorporated over windows and door openings to help manage moisture.

Why DPCs Are Important for Preventing Damp?

DPCs are essential because many building materials, such as brick, stone, and concrete, are porous and can absorb water from the ground due to capillary action. This can lead to damp issues in the building.

To address potential issues, DPCs became a legal requirement in 1875, although builders had been using some form of moisture barrier for years before that. Early DPCs were made of slate, set in mortar, and later evolved to include materials like lead and bitumen. In modern buildings, frost-resistant bricks are commonly used below DPC with the DPC being plastic, making DPC failure rare in newer structures.

Which Properties Are Most at Risk of DPC Failure?

Older buildings, especially those with solid walls, are most at risk of DPC failure. Victorian homes with timber ground floors are particularly vulnerable to damp problems, which can lead to issues like wood rot or insect damage to floor joists. This is especially the case where timbers are in contact with or built into the wall. In these cases, proper ventilation is crucial to prevent decay.

Properties with concrete floors may experience damp, but it's usually less of a structural concern unless there are high levels of sulphate in the concrete.

Retrofitting a DPC

Older brickwork and mortar joints are more porous than modern materials, and if a DPC is covered (e.g., by rendering), damp may be able to bypass the barrier. Signs of this include:

* Staining on walls in ground floor rooms

* Rotting skirting boards

* Damage to floor timbers

*Blistering or spalling of internal or external finishes

This may present similar symptoms to rising damp.

To fix this, chemical DPCs are often injected into the walls. This involves drilling small holes into the mortar and injecting a chemical or silicone-based solution that hardens to form a barrier against rising damp. However, this method is often applied incorrectly, either being placed too high on the wall or in the wrong material, which can leave paths for moisture to seep through.

It is also possible to retrofit a physical DPC although this is much more successful, it is also much more disruptive.

In buildings with thick stone walls, injected DPCs are often ineffective. It's important to get an independent survey rather than relying on sales-driven advice.

What to Do if Your DPC Has Failed?

If you notice damp in your ground floor walls, don't immediately assume the DPC has failed. First, check if water from outside is accumulating near the base of the walls. Excessive surface water or leaks from gutters or downpipes may be the cause of the problem, which can be fixed by diverting the water or repairing the leaks.

In cases where the old DPC has been bridged (e.g., by raised flower beds, rendered walls or raised patios), removing the source of the water can often resolve the problem without needing to insert a new DPC. Damp in the walls will typically stop rising once the water source is cut off.

Internal leaks can also be the cause of dampness. It is important to treat the root cause of dampness before opting to spend time and money on treatments.

In older peoperties using materials to match the existing construction like lime mortar, along with proper ventilation and re-plastering with a renovating plaster, can help control moisture in a property naturally without requiring changes to the DPC.

For minor damp or condensation issues, dehumidifiers can be useful in controlling the problem.