Damp in London Homes: Types, Causes, and How to Get the Right Survey

**1. Rising damp**:

*What it is*: Water rising from the ground through masonry by capillary action — the small pores in brick and mortar act like a wick, drawing ground moisture upward. Rising damp produces a 'tide mark' of salt deposits (efflorescence) on the wall, typically up to approximately 1m above floor level. The salts (sulphates, nitrates, chlorides) in the ground moisture are left behind as the water evaporates from the wall surface.

*How common is it really?*: Rising damp is significantly less common than its marketing prominence would suggest. A 2007 review by the Building Research Establishment (BRE) concluded that true rising damp is relatively rare in the UK — many cases attributed to rising damp are in fact condensation, bridged DPCs, or penetrating damp. This matters because: the standard 'remedy' for rising damp (chemical DPC injection, replastering) is expensive (£1,000–£5,000 per room) and if the diagnosis is wrong, the remedy will fail.

*When is rising damp genuinely the diagnosis?*: Rising damp is confirmed where: the moisture meter readings are high at floor level and decrease with height; the wall is wet from the ground side (external face wet); there is no evidence of penetrating damp from above or the sides; the DPC (if present) has failed or is absent (Victorian houses were built without a chemical DPC — they relied on slate or engineering brick DPC courses, which can fail or bridge).

• *The correct remedy for true rising damp*:
• •Install a new DPC — either by chemical injection (a chemical cream or fluid injected into a series of drilled holes) or by cutting a horizontal slot and inserting a physical DPC
• •Remove salt-contaminated render and plaster to at least 300mm above the watermark
• •Allow walls to dry
• •Re-render with salt-resistant plaster (Stormdry, K-Rend, or a Limelite renovation plaster) that accommodates residual moisture

**2. Penetrating damp**:

*What it is*: Water entering the building through the external fabric — typically through: failed pointing or mortar joints; cracks in render; failed flashings (around chimneys, at wall-to-roof junctions); failed window or door seals; gutters overflowing onto the wall; or water bridging the external wall from above (soil or paving above the DPC level).

*How to identify it*: Penetrating damp typically appears as a damp patch that corresponds to a specific external feature — a stain below a gutter, a wet patch next to a chimney breast, dampness around a window reveal. The pattern follows the water entry point.

*The correct remedy for penetrating damp*: Identify and fix the external water source — repoint the mortar, replace the flashing, repair the gutter, clear the blockage, apply external waterproofing to a porous render. Interior treatments (tanking, waterproof plaster) applied without fixing the external source will fail because water pressure behind the tanking will find another path.

**3. Condensation (interstitial or surface)**:

*What it is*: The most common form of moisture problem in London homes. Warm, moisture-laden air contacts cold surfaces (cold external walls, cold window glass, cold floor slab) and reaches its dew point — moisture condenses on the cold surface.

*Surface condensation*: The familiar black mould on bathroom tiles, the moisture on cold window glass, the mould at the base of cold external walls — all surface condensation. Surface condensation is a function of: indoor humidity (cooking, breathing, washing — a 4-person household generates approximately 8–10 litres of moisture per day); inadequate ventilation; and cold surface temperatures (poorly insulated walls, single-glazed windows).

*Interstitial condensation*: Condensation occurring within the construction — inside the wall or floor structure — rather than on the surface. A particular risk where vapour barriers are in the wrong position relative to the insulation, or where warm moist internal air can reach a cold surface within the construction.

*The correct remedy for condensation*: Reduce indoor humidity (mechanical extract ventilation in kitchens and bathrooms); improve ventilation (trickle vents, MVHR — mechanical ventilation with heat recovery); raise surface temperatures (external wall insulation, secondary glazing or double glazing, improved insulation). Applying waterproof coatings to the interior surface of a wall affected by condensation does not address condensation — it simply drives the condensation point to a different surface.

**The problem with some damp surveys**:

The UK damp surveying industry has a well-documented tendency toward misdiagnosis and over-treatment. Some companies offer 'free' damp surveys — but the business model is to generate sales of expensive injection treatments. A 'free' survey from a company that sells damp-proofing treatments is not objective.

**What a proper independent damp survey involves**:

An independent damp survey (from a surveyor who does not sell remedial treatments) should include:

• •*Visual inspection*: External inspection of walls, gutters, downpipes, flashings, pointing, window seals, and drainage. Many damp problems are externally visible if looked for.
• •*Moisture meter readings*: A calibrated moisture meter takes surface readings at regular intervals across the wall. A resistivity meter (the most common type) measures electrical resistance between two probes inserted in the wall surface — lower resistance indicates higher moisture content. Readings must be interpreted carefully: high readings from a resistivity meter can result from salt deposits (which conduct electricity) rather than actual moisture. A true moisture reading requires either a calcium carbide test or a core sample.
• •*Hygrometer readings*: Relative humidity of the internal air — above 70% RH is conducive to mould growth. Above 80% RH and surface condensation is likely on cold surfaces.
• •*Thermal imaging (optional but valuable)*: Thermal imaging camera identifies cold spots on walls and floors — cold spots are condensation risk areas. Thermal imaging also identifies areas where insulation is missing or void in the construction.
• •*Calcium carbide test*: A more accurate moisture test — a sample of the masonry material is mixed with calcium carbide in a sealed container; the gas pressure produced is proportional to the moisture content. This is more accurate than a resistivity meter but more time-consuming.

**Who to use for a damp survey**:

*Independent building surveyor or RICS surveyor*: An RICS-qualified building surveyor with experience in damp and timber diagnosis (membership of PCA — Property Care Association — is a useful additional credential). Independent — not selling treatments.

*Chartered Surveyor RICS Level 3 Building Survey*: If purchasing a property, a Level 3 (Full Structural) survey from an RICS building surveyor covers damp and timber alongside all other building elements. This is the most cost-effective way to get an objective damp assessment before purchase — the surveyor has no financial interest in finding damp that isn't there.

*PCA Surveyor offering treatment*: If using a company that also provides treatments, insist on a paid survey (not a free one) and ask the surveyor to show you: (a) moisture meter readings with exact values; (b) the location of each reading on a plan; (c) the specific diagnosis and why. A good PCA surveyor will stand behind their diagnosis.

**Damp survey costs**:

• •Independent RICS building surveyor — damp investigation: £300–£600 (domestic, whole house)
• •PCA-member damp surveyor with treatment quotation: £200–£400 (may be free but will include treatment recommendation)
• •RICS Level 3 Building Survey (full structural survey): £600–£1,500 (includes damp assessment as part of comprehensive survey)
• •Thermal imaging survey (specialist, optional): £200–£500

**Cost guide for common damp remedies in London**:

| Problem | Remedy | Approximate cost | |---|---| | Failed pointing (penetrating damp source) | Repointing affected area | £300–£1,500 | | Failed flashing at chimney | New lead flashing | £400–£1,200 | | Blocked or failed guttering | Gutter repair/replacement | £200–£800 | | Cracked render (penetrating damp source) | Hack off, re-render | £1,000–£4,000/face | | True rising damp + chemical DPC injection | Injection + replaster | £1,000–£4,000/room | | Condensation: kitchen/bathroom ventilation | Extract fan, ducted to outside | £200–£500/room | | Condensation: whole house | MVHR system | £3,000–£8,000 | | Condensation: cold walls | External wall insulation (EWI) | £80–£150/m² | | Condensation: cold walls (interior) | Internal wall insulation (IWI) | £60–£120/m² |

**Common remedies that do NOT work (or are overprescribed)**:

• •*Chemical DPC injection for condensation or penetrating damp*: Injection treats rising damp specifically — it is not relevant to condensation or penetrating damp and will not resolve those problems
• •*Waterproof interior tanking on an externally wet wall*: Tanking the inside of a wall without fixing the external source simply relocates the problem — water pressure behind the tanking eventually forces the tanking off the wall
• •*Anti-mould paint alone for condensation*: Anti-mould paint suppresses mould growth temporarily but does not address the underlying cause (humidity + cold surface) — the mould will return

**The ventilation-first principle for condensation**:

The majority of London houses suffering from mould growth and surface condensation improve dramatically with improved ventilation — without any structural remedial work. The sequence of investigation should be: 1. Confirm it is condensation (moisture meter, hygrometer, visual assessment) 2. Improve extract ventilation — kitchen and bathroom fans ducted to outside, minimum 15 l/s extract rate (Part F) 3. Add trickle ventilation to windows (trickle vents) where currently absent 4. Monitor for 6 months before considering insulation or structural remediation

In most London Victorian houses where the current mould problem is condensation-related, this sequence resolves the problem without expensive structural treatment.