DPC Injection, Does Chemical Damp Proofing Actually Work?

The Rising Damp Industry

The remedial damp-proofing industry in the UK is worth hundreds of millions of pounds annually. The most commonly sold treatment is chemical damp proof course (DPC) injection, a process in which a silane or siloxane-based fluid is injected into a mortar course at or near ground level to create a water-repellent barrier within the wall.

The sale of this treatment is frequently accompanied by re-plastering with renovating plaster, salt-retarding renders, and tanking products. The combined cost for a modest terraced house can run to £3,000–£8,000.

The problem, documented extensively by the BRE (Building Research Establishment) and acknowledged in guidance published by the Property Care Association (PCA), the trade body for the damp-proofing industry, is that a significant proportion of damp problems diagnosed as "rising damp" are in fact caused by condensation, penetrating damp, plumbing leaks, or the hygroscopic action of salts in old walls. Many of these conditions do not respond to DPC injection, and injecting a wall that does not have genuine rising damp achieves nothing beyond masking symptoms temporarily.

What is Rising Damp, and Is It Common?

True rising damp occurs when ground water is drawn upward through the porous structure of a wall by capillary action. It is characterised by:

• A distinct tidemark (usually between 0.5 and 1.2 metres above floor level)
• Salt crystallisation (efflorescence) at the tide mark, visible as white powdery deposits
• Gradient readings on an electrical moisture meter that decrease consistently from low to high on the wall

However, BRE research (summarised in BRE Digest 245 and subsequent publications) has consistently found that genuine rising damp, in the classic textbook form, is far less common than the volume of DPC injection treatments sold would imply. The more prevalent causes of wall dampness in UK housing are:

The PCA's Own Position

The PCA's Remedial Treatment Standard (RTS) and its published technical guidance acknowledge that diagnosis of rising damp must be based on a full investigation, not solely on moisture meter readings. Electrical moisture meters, the primary diagnostic tool used by most damp contractors, measure electrical resistance, which is affected not only by water content but also by salt content, plaster type, and even metallic inclusions. A reading that shows elevated moisture could be caused by hygroscopic salts in old plaster from a previous dampness event long since resolved.

The PCA states that a competent surveyor should:

• Conduct a full visual inspection and identify all potential moisture sources
• Use carbide (calcium carbide) testing or gravimetric analysis where diagnosis is uncertain, these measure actual water content rather than electrical resistance
• Exclude condensation and penetrating dampness as primary causes before diagnosing rising damp

Not all contractors meeting customers follow this standard.

What a Well-Qualified Diagnosis Looks Like

An independent damp survey from a RICS-registered surveyor or a PCA-accredited surveyor with no commercial interest in selling treatment products is preferable to a survey from a contractor who will carry out the remediation work. The independent surveyor will assess:

• Building fabric and external drainage condition
• Ventilation and relative humidity levels inside the property
• Evidence of past or current water ingress routes
• Salt analysis of wall samples where appropriate

Buyers and DPC Warranties

Many properties sold in the UK come with historic damp-proofing guarantees, typically 20 or 25 years, from contractors, many of whom may now be defunct. These warranties are only valuable if the contractor still exists and the insurance-backed guarantee (IBG) has been maintained.

If you are buying a property and a DPC injection guarantee is offered as reassurance, verify:

• Whether the guarantee is IBG-backed and from an active insurer
• Whether the original diagnosis was competent, if not, the underlying cause may persist despite the treatment

Property Passport UK enables owners to upload damp survey reports, treatment specifications, and guarantee certificates so that buyers can review the full history before committing to a purchase.