Treating Damp Before Repainting London Properties: A Practical Guide

Why Damp Must Be Addressed Before You Redecorate

One of the most common mistakes made in London property renovation is decorating over active damp. The result is always the same: within weeks or months, emulsion begins to bubble and peel, a white salty crust (efflorescence) appears through the paint film, or a tide mark of discolouration returns. The decoration fails not because of a paint problem, but because the underlying cause was never resolved.

London's housing stock — predominantly Victorian and Edwardian brick construction — is particularly susceptible to damp problems. Many of these buildings were constructed without any damp-proof course, or with an original bitumen DPC that has cracked and failed over more than a century. Basement conversions, ground-floor extensions and party-wall areas are the highest-risk zones.

Understanding which type of damp you are dealing with is the necessary first step, because rising damp, penetrating damp and condensation require entirely different remediation approaches.

Rising Damp

Rising damp occurs when groundwater travels upward through the masonry by capillary action. It is most commonly found in ground-floor rooms and basements where no effective damp-proof course exists or where the existing DPC has been bridged (by an internal floor screed or raised external ground levels, for example).

The tell-tale signs are a tide mark on walls at a consistent height — typically between 300mm and 900mm from floor level — accompanied by salt crystallisation through the plaster, peeling or bubbling paint at low level, and in severe cases a visible damp patch that varies seasonally.

DPC injection is the most common modern treatment for rising damp. A cream-based silicone product is injected through a series of holes drilled at regular intervals along the base of the wall, just above floor level. The silicone cream fills the masonry pores and, once cured, creates a hydrophobic barrier that prevents capillary water rising further. This is a specialist job that should be carried out by a qualified remedial damp contractor, not a decorator.

After DPC injection, the wall must be re-plastered using a salt-resistant renovation plaster. The original plaster will be saturated with hygroscopic salts absorbed from the groundwater, and these will continue to attract moisture from the air even after the rising damp is stopped. Hacking off the contaminated plaster and replastering with a dedicated product is not optional — it is the only way to ensure the finished surface is stable.

The wall must then be allowed to dry for a minimum period — typically several months for a thick Victorian brick wall — before final decoration. Your decorator can advise on appropriate moisture reading thresholds before painting proceeds.

Penetrating Damp

Penetrating damp enters a building horizontally through the external fabric: through cracked masonry, failed pointing, defective window and door flashings, leaking gutters and downpipes, or poorly sealed external render. Unlike rising damp, the damp patches from penetrating damp are not constrained to low levels — they appear wherever water is getting in, which may be at any height on any wall.

The remediation approach for penetrating damp is to eliminate the entry point. This may mean repointing failed mortar joints, applying a breathable external render or masonry treatment, replacing defective flashings, or repairing gutterwork. Until the entry point is sealed, no internal treatment will be permanently effective.

Once the external envelope is secure, internal treatment depends on the extent of damage. In many cases, allowing the wall to dry naturally and then redecorating with a breathable paint is sufficient. Where the plaster has been badly damaged or where salt contamination is present, re-plastering with a renovation plaster is the better option.

Breathable masonry paint on the exterior is an important consideration in London's Victorian brick stock. Solid brick walls rely on their ability to absorb and release moisture; covering them with an impermeable coating (some older oil-based masonry paints, or concrete-based renders) traps moisture within the wall and can accelerate decay. We specify breathable silicone or silicate-based masonry paints for all exterior masonry work on solid-wall London properties.

Tanking

Tanking is used where a wall or floor must be made permanently waterproof against hydrostatic pressure — most commonly in basements below the water table or in areas subject to sustained water ingress. It involves applying a rigid, cementitious waterproofing slurry to the internal face of the wall in multiple coats, creating a physical barrier that holds water out under pressure.

Tanking is not suitable as a first-line response to rising damp (it will be defeated by water pressure from below) and it is not the same as using a damp-seal paint product, which is a temporary cosmetic treatment rather than a structural solution.

Where a basement conversion is planned in a London property, a structural waterproofing specialist should be consulted before any other contractor. There are two principal approaches to basement waterproofing — Type A (barrier systems, including tanking) and Type C (cavity drain membrane systems that manage water rather than resist it) — and the correct choice depends on the site conditions and the intended use.

Dry Lining Options

Dry lining — fixing insulated plasterboard to the internal face of a damp wall rather than re-plastering directly — is an option in some situations. It provides a flat, paintable surface quickly and, where the boards are fixed on a treated timber frame with a vapour gap behind, it can help manage residual moisture in the wall.

The limitation of dry lining is that it does not address the underlying cause, and without adequate ventilation behind the boards, concealed mould growth can develop. We would only recommend dry lining as part of a broader remediation strategy where the primary cause has been addressed, not as a standalone treatment.

The Decorator's Role

A good decorator will identify damp problems before starting work and will not paint over surfaces showing active moisture. We carry a moisture meter on all site visits and will advise if readings indicate that decoration should be deferred pending remediation.

For London homeowners planning a redecoration, the sequence is: damp investigation, remediation, replastering (where needed), drying period, then decoration. Compressing this sequence or skipping steps is false economy — the decoration will fail and the remediation will need to be done anyway.