Understanding Damp in London Homes
London's housing stock includes a large proportion of Victorian and Edwardian terraced and semi-detached properties, many of which were not built with modern moisture management systems. Damp is a common presenting problem — but diagnosing it correctly is essential before any treatment is applied.
Incorrectly treated damp can waste thousands of pounds and leave the underlying problem unresolved.
Types of Damp
Rising Damp
Rising damp occurs when groundwater travels upward through masonry via capillary action. It is most commonly identified by:
- A tide mark on internal walls, typically up to 1 metre high
- Salts crystallising on the wall surface (efflorescence)
- Damp patches that persist even in dry weather
- Damage to plaster at low level
Rising damp requires a damp proof course (DPC). Most properties built after 1875 have a slate or bitumen DPC. Where this has failed or is absent, an injected chemical DPC is typically recommended.
Penetrating Damp
Penetrating damp enters the building horizontally through the external fabric. Common causes include:
- Failed pointing or mortar joints in brickwork
- Damaged or blocked gutters and downpipes
- Cracked render
- Failed window or door seals
- Parapet walls and flat roof details
This type of damp is identified by patches that correspond with rainfall events and appear at any height on the wall.
Treatment: Repair the defect at source. No amount of internal waterproofing will resolve penetrating damp if the external cause is not fixed.
Condensation
Condensation is the most common form of damp in London homes and is frequently misdiagnosed as rising damp. It occurs when warm, moist air contacts cold surfaces.
Signs include:
- Black mould, typically at corners and in poorly ventilated rooms
- Moisture on windows in the morning
- Damp that worsens in winter and improves in summer
Treatment: Improved ventilation (mechanical ventilation with heat recovery for bathrooms and kitchens), upgraded insulation to raise surface temperatures, and occupant behaviour changes. Damp-proof treatment applied to walls will not resolve condensation.
Damp Proofing
Damp proofing refers to measures that resist the passage of moisture through building elements. Common methods include:
- Chemical injection DPC: A silicone-based product injected into the mortar course at low level to create a horizontal barrier
- Tanking: An internal or external waterproof render or coating applied to below-ground walls
- Cavity drainage membranes: A dimple membrane fixed to internal walls to manage water ingress in basement situations, typically used with a sump pump
Waterproofing
Waterproofing refers to more comprehensive systems designed to resist hydrostatic pressure — water under active pressure from the ground. This is particularly relevant for:
- Basement conversions
- Below-ground habitable space
- Retaining walls with significant groundwater pressure
BS 8102 sets out the three approaches to below-ground waterproofing:
- Type A: Tanked (applied coating or membrane, internal or external)
- Type B: Structurally integral (reinforced concrete construction designed to resist water)
- Type C: Drained (cavity drainage system with sump pump)
For habitable basements, a combination of types is often recommended for redundancy.
What to Expect From a Damp Survey
A professional damp survey should:
- Use a calibrated moisture meter to take readings at multiple points
- Identify the type of damp (rising, penetrating, condensation)
- Consider thermal bridging and ventilation
- Investigate externally as well as internally
- Produce a written report with recommendations and costings
Be cautious of any surveyor who diagnoses rising damp without ruling out condensation or penetrating damp, or who recommends expensive treatments before addressing simpler possible causes.
RCB Design & Build
RCB assesses damp issues as part of the pre-construction survey process for refurbishment projects. We commission independent specialist surveys where appropriate and ensure that any damp treatment is correctly specified and sequenced within the wider project programme.