Is My DPC Failing? Signs Your Damp Proof Course Needs Attention

The damp proof course — or DPC — is one of the most talked-about elements in damp diagnosis, and one of the most misunderstood. "Your DPC has failed" is a common explanation offered by damp proofing companies. In many cases it’s accurate. In many others, it isn’t — and understanding the difference could save you thousands.

What Is a Damp Proof Course?

A damp proof course is a horizontal barrier built into the base of a wall — typically 150mm above external ground level — that prevents ground moisture from rising up through the wall structure by capillary action.

In properties built after around 1875, a physical DPC is usually present. The material varies by era:

• Slate — Victorian and Edwardian properties. Effective and durable but can crack over time, particularly if the building has moved.
• Engineering brick — a highly dense, low-absorption brick used as a DPC layer in some older properties
• Bitumen felt — 1920s onwards. Remains effective in most cases but can become brittle with age
• Plastic or polyethylene — modern construction. Highly effective and essentially permanent

Properties built before around 1875 typically have no DPC at all. These buildings were designed as solid, mass-masonry structures that managed moisture through absorption and evaporation — a completely different strategy that requires a completely different approach when problems arise.

Signs That Are Often Blamed on DPC Failure

The following symptoms are frequently attributed to a failing DPC by damp proofing companies:

• Damp patches at low level on ground floor walls
• A tide mark — a horizontal stain at the upper extent of moisture
• Paint bubbling or plaster blowing at low level
• Salt crystallisation (efflorescence) on wall surfaces
• Musty smell at ground floor level

These symptoms are consistent with rising damp, which can result from DPC failure — but they’re also consistent with several other causes that have nothing to do with the DPC.

What Actually Causes These Symptoms?

Bridged DPC — the most common cause of apparent DPC failure. The DPC itself is intact and functioning, but external ground levels have risen above it — through a raised path, a flower bed built up against the wall, a new extension, or soil that has accumulated over decades. Water bypasses the DPC entirely by running down the external wall face and entering below DPC level.

The fix is removing the bridge — lowering the ground level, cutting back the flower bed, installing a drainage channel. Not chemical injection.

Hygroscopic salts — if a property has had previous rising damp or penetrating damp that has since been resolved, the soluble salts drawn into the plaster during that episode remain permanently. These salts are hygroscopic — they absorb moisture from the atmosphere — meaning the wall can produce elevated moisture readings and a damp appearance indefinitely, even though no moisture is currently entering the structure.

Penetrating damp — defective gutters, failed pointing, or cracked render allowing rainwater to enter the wall. The moisture tracks down to low level internally and is misread as rising damp.

Genuine DPC failure — yes, this does happen. Slate DPCs can crack with building movement. Bitumen felt can deteriorate. In these cases the DPC is genuinely not functioning, and some form of remediation is appropriate.

When Is DPC Replacement or Injection Actually Warranted?

Genuine DPC failure requiring remediation is warranted when:

• Investigation confirms that moisture is rising through the wall structure (not tracking down from above, not caused by a bridge, not hygroscopic)
• The tide mark, salt deposits, and moisture readings are consistent with capillary rise
• External ground levels are below the existing DPC — ruling out bridging as the cause
• Thermal imaging and calibrated readings confirm structural moisture rather than surface effects

Even then, the appropriate treatment depends on the construction type. Chemical injection into a solid brick wall may be appropriate in some cases. Applying it to a lime-mortared rubble wall is likely to be ineffective and potentially harmful to the structure.

Get an Independent Assessment Before Spending Anything

If you’ve been told your DPC has failed, get an independent survey before commissioning any work. Richard Bull MISSE carries out independent damp surveys across Derby, Nottingham, Leicester, and the wider Midlands. Call 07983 550 662 or use the contact form.