Why Getting the Diagnosis Right Matters
Damp in a London property can be distressing and confusing. The visible symptoms, staining, peeling wallpaper, a musty smell, often look similar regardless of the cause. But the treatment for rising damp is completely different from the treatment for penetrating damp, and both are different from the treatment for condensation or an actual plumbing leak. Get the diagnosis wrong and you will spend money on a remedy that does not work, while the real problem continues to cause damage.
The damp proofing industry has a reputation for misdiagnosis, and London homeowners are frequently sold expensive chemical damp proof course injections when the actual problem is something else entirely. Understanding the difference between damp types helps you ask the right questions and avoid unnecessary work.
Rising Damp: What It Is and How to Identify It
Rising damp occurs when groundwater is drawn up through porous masonry by capillary action, in the same way that a sponge soaks up water from a puddle. It affects the base of walls at ground floor level and, in theory, can rise to a height of about one to one and a half metres above floor level, though in practice it rarely rises more than a metre.
True rising damp has distinctive characteristics. The damp area starts at the base of the wall and extends upwards, usually with a relatively even horizontal tide mark. The affected plaster will show salt deposits, white crystalline marks known as efflorescence, caused by salts dissolved in the groundwater being deposited as the moisture evaporates. The damp is persistent regardless of weather conditions and is present all year round.
In London properties, rising damp is most common in older buildings where the original damp proof course has failed or was never installed. Many Victorian and Georgian properties relied on slate or bitumen DPCs, which can deteriorate over time. Properties where external ground levels have been raised above the DPC, for example by paving or building up garden beds against the wall, are also vulnerable.
However, genuine rising damp is far less common than the damp proofing industry would have you believe. A major study by the Building Research Establishment found that the majority of cases diagnosed as rising damp were actually caused by other moisture sources. Before accepting a rising damp diagnosis, always seek a second opinion from an independent surveyor.
Penetrating Damp: What It Is and How to Identify It
Penetrating damp occurs when water passes through the building envelope from outside to inside. It can affect any part of the building, not just the base of walls, and it is typically linked to weather conditions, becoming worse during or after rain.
Common causes of penetrating damp in London properties include defective pointing in brickwork, cracked or missing render, failed or blocked gutters and downpipes, damaged roof coverings including slipped or cracked tiles, defective flashings around chimneys and abutments, failed window seals or deteriorated putty, and porous brickwork that has lost its weather resistance.
Penetrating damp usually appears as a damp patch that corresponds to an identifiable external defect. The pattern is often irregular rather than the uniform tide mark of rising damp. It may be worse on walls that face the prevailing wind and rain, which in London means south-west and west-facing elevations. The damp will worsen during wet weather and improve during dry spells.
In solid-walled London properties, which include most buildings built before the nineteen thirties, there is no cavity to prevent water from passing through the wall. These buildings relied on the thickness of the masonry to absorb rainwater before it reached the internal surface. When pointing deteriorates or the brickwork becomes excessively porous, this protection fails and damp appears inside.
When the Problem Is Actually a Leak
This is where it gets interesting, and where many London homeowners are caught out. A significant proportion of damp problems that are diagnosed as either rising or penetrating damp are actually caused by plumbing leaks.
A leaking pipe under a ground floor can produce damp patches at the base of walls that look exactly like rising damp. A leaking pipe in a wall cavity or behind tiles produces damp that looks like penetrating damp. A slow leak from a waste pipe or a failed shower tray seal can cause localised damp that is easily mistaken for condensation.
There are clues that suggest a leak rather than damp. The affected area may be warm if the leak is on a heating pipe or hot water supply. The damp may be present even in dry weather and unrelated to rainfall. There may be no obvious external defect to explain penetrating damp. The water meter may show usage when all taps and appliances are turned off. And if the boiler pressure keeps dropping, a heating system leak is almost certainly involved.
A proper damp investigation should always include checking for plumbing leaks as part of the diagnosis. Unfortunately, many damp proofing companies do not carry leak detection equipment and will default to diagnosing rising or penetrating damp because those are the problems they are equipped to treat.
Getting the Right Treatment
Rising damp is treated by installing or renewing the damp proof course. Modern methods usually involve injecting a chemical DPC into the mortar course, then replastering the affected area with a salt-resistant renovation plaster. External ground levels may also need to be lowered.
Penetrating damp is treated by repairing the external defect that is allowing water in. This might mean repointing brickwork, clearing gutters, replacing flashings, or applying a water-repellent treatment to porous masonry. Internal decoration should only be done after the external repair is complete and the wall has dried out.
Leak-related damp requires finding and fixing the leak first. No amount of damp proofing will solve a problem caused by a leaking pipe. Once the leak is repaired, the affected area needs to be dried, which may require specialist drying equipment, and then replastered and redecorated.
When to Get a Professional Survey
If you have a damp problem in your London property and you are not sure of the cause, a professional survey is worthwhile. Ideally, choose a surveyor who can investigate all possible causes, including plumbing leaks, rather than one who specialises in a single type of damp treatment. An independent chartered surveyor or a leak detection specialist with experience in damp diagnosis will give you an unbiased assessment and save you from spending money on the wrong solution.