Damp Proofing and Damp Treatment: Diagnose Before You Treat

The three types, and how to tell them apart

Start with pattern, height and season. Where the damp sits on the wall and when it appears tells you more than any meter reading.

Distinguishing the three main types of damp. Type | Typical pattern | Timing | Usual cause. Rising damp — Tide mark up to about 1m, salting, perished skirtings — Constant, worse in wet months — Failed or bridged damp proof course; Penetrating damp — Isolated patch, often mid-wall or high up — Appears during and after rain — Defective render, gutter, flashing, pointing; Condensation — Black spot mould in corners, behind furniture, on cold surfaces — Worse in winter and overnight — Moisture production plus cold surfaces plus poor ventilation

Why most damp is not rising damp

Genuine rising damp is real but relatively uncommon in houses built after the 1870s, when damp proof courses became normal practice. Where it does appear, the usual reason is that the existing course has been bridged rather than that it has failed.

Bridging is mundane and cheap to fix: a raised patio or path above the damp proof course, soil banked against the wall, a rendered plinth carried down over the course, or a cavity filled with rubble. Check these before anyone drills a single hole, because lowering external ground levels solves a great many diagnosed cases of rising damp.

External ground level must be at least 150mm below the damp proof course · Check for render carried across the course, a classic bridge · Look for blocked or missing air bricks under suspended floors · Check gutter and downpipe joints in heavy rain, not on a dry day · Rule out an internal leak: a slow waste pipe mimics rising damp precisely

Condensation, which is most of it

A household produces a remarkable quantity of moisture: cooking, showering, drying clothes indoors and simply breathing. If that moisture cannot escape and meets a cold surface, it condenses, and mould follows within weeks.

The fix is three-part and none of it involves a damp proof course: reduce moisture at source with extractor fans that actually run long enough, increase ventilation with trickle vents and background airflow, and raise surface temperatures with insulation. Modern airtight windows fitted to an otherwise uninsulated old house are a very common trigger, because the draughts that used to ventilate the building have been removed.

Old buildings need breathability, not sealing

Solid-wall properties built before around 1919 have no cavity and no damp proof course by design. They manage moisture by absorbing and releasing it. Applying cement render, gypsum plaster and vinyl paint to such a wall traps moisture inside the fabric, and the damp simply reappears above the treated zone.

The correct approach on these buildings is lime plaster, breathable mineral paint, lime pointing and good external drainage. It is slower and costs more per square metre, and it is the only treatment that works long term.

Indicative treatment costs

Ranges below reflect typical UK pricing in 2026 and assume a correct diagnosis has already been made.

Indicative damp remediation costs (2026). Work | Indicative range. Independent damp survey (not tied to a contractor) — £300 – £600; Lowering external ground levels — £500 – £2,000; Gutter, downpipe or flashing repair — £150 – £900; Repointing an elevation — £1,000 – £3,500; Chemical damp proof course injection, per linear metre — £70 – £120; Replastering after a damp proof course, per wall — £400 – £900; Positive input ventilation unit, installed — £600 – £1,200; Continuous-running extractor fans, per room — £250 – £450; Internal wall insulation, per m² — £90 – £160; Tanking a cellar — £4,000 – £15,000

How to buy a damp survey properly

Pay for an independent survey from someone who does not sell the treatment. A free survey from a damp proofing company is a sales visit, and a protimeter pressed against a salt-contaminated wall will read high whether or not there is any rising moisture, because the salts themselves conduct.

A good report identifies the moisture source, explains the evidence, and recommends the cheapest effective intervention first. If a report recommends injection without ever mentioning gutters, ground levels or ventilation, get a second opinion.

Frequently asked questions

Does rising damp actually exist?

Yes, but it is far rarer than the treatment industry implies. Most walls diagnosed with rising damp are suffering from a bridged damp proof course, penetrating damp or condensation, all of which have different and usually cheaper fixes.

How long does a damp patch take to dry out?

Masonry dries at roughly 25mm per month in good conditions. A wall that has been wet for years can take six to twelve months to dry after the source is fixed, which is why replastering too early is a false economy.

Will a dehumidifier solve damp?

It manages condensation while it runs, and it does nothing for rising or penetrating damp. Treat it as a temporary measure while the actual cause is fixed.

Is black mould dangerous?

It is a health risk, particularly for asthma and other respiratory conditions. Clean it with a fungicidal wash rather than bleach on porous surfaces, then address ventilation and surface temperatures, or it returns.