Damp Proof Course (DPC): Everything You Need To Know

Been told you need a new damp proof course? Quoted thousands for chemical injection? You’re not alone — but you might not actually need it. As an independent damp surveyor covering Derby, Nottingham, and Leicester, I see homeowners pressured into unnecessary DPC treatments weekly. Most “DPC failures” aren’t failures at all — they’re bridged, misdiagnosed, or don’t exist in the first place.

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 rising up through the wall by capillary action. In properties built from around 1875 onwards it’s typically slate, bitumen felt, or engineering brick. In modern properties it’s a plastic membrane. Properties built before 1875 generally have no DPC and were designed to manage moisture through breathable lime construction instead.

Why “DPC Failure” Is Usually Something Else

Genuine DPC failure is relatively uncommon. In the vast majority of cases I investigate, the real cause is one of these:

Bridged DPC: external ground levels, paths, patios, or render that have been raised above the DPC allow moisture to bypass it entirely. Lower the ground level and the “rising damp” stops. Fix: £200–£500.

Penetrating damp misread as rising damp: blocked gutters, failed pointing, or cracked render allow water in at wall level and it reads as moisture rising. Fix the external defect and the moisture stops.

Condensation: moisture meter readings on cold walls are often condensation, not rising damp. Particularly common in solid wall properties. No DPC treatment will fix a condensation problem.

Hygroscopic salts from old treatments: previous chemical injection leaves salts in the wall that attract atmospheric moisture and give ongoing high meter readings — even when the original damp problem was resolved years ago.

Chemical Injection DPC: Does It Work?

Chemical injection can be effective for genuine rising damp where there is no DPC or the existing one has genuinely failed — but this is a relatively rare situation. The BRE and independent research have consistently found that chemical DPC is oversold and frequently applied where the cause is something else entirely. If the cause isn’t rising damp, chemical injection changes nothing and costs £1,500–£4,000.

What Proper Diagnosis Looks Like

Genuine rising damp diagnosis requires: a tide mark at consistent height; salt analysis of the wall plaster; moisture readings that decrease with height; and consideration of all other possible causes. BRE Digest 245 and BS 6576 set out the correct methodology. If a surveyor diagnosed rising damp without following this process, the diagnosis should be questioned.

Get Expert Assessment

If you’ve been told you need a new DPC, get an independent survey first. In most cases there’s a cheaper, simpler explanation. Richard Bull covers Derby, Nottingham, Leicester, and the wider Midlands.

📞 07983 550 662
📧 richard.bull@dampdetectives.co.uk
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Richard Bull MISSE, ACIEH — Independent & Unbiased — No Sales Pressure