Basement and Cellar Damp: Tanking, Injected DPC or Membrane?

Why London basements get damp

Much of London sits on London Clay, which lets water through very slowly. Rainwater soaks into the made ground and silt above the clay and sits there, pushing sideways on basement and lower ground walls. Add old drainage, leaking gutters and downpipes, raised paths and front steps over vaults, and below-ground rooms take on water from several directions at once.

GOV.UK groups damp into four types: condensation, which is the most common, penetrating damp, rising damp and traumatic damp from leaks. Rising damp is real, but conservation bodies such as SPAB describe it as rarer than commonly perceived and often misdiagnosed. Condensation in a cold basement can look like damp coming through the wall, and waterproofing will not cure it: the Property Care Association's code of practice says condensation will not be eliminated by structural waterproofing.

Get the diagnosis right first

The Property Care Association (PCA) trains surveyors in structural waterproofing (the CSSW qualification) and keeps a register of waterproofing design specialists. Its 2022 code of practice sets out a minimum survey: how the space will be used, the structure and its condition, ground conditions and drainage, and ventilation, with a written report stating the waterproofing grade the design will achieve.

Be wary of a diagnosis based only on a moisture meter. Surface meters can suggest damp but cannot prove its source, and salts in old plaster give false high readings, a point made in both PCA and Historic England research.

BS 8102 grades: what level of dry do you need?

Grade	What it allows	Typical use
1A	Some seepage and damp areas	Car parks, plant areas
1B	No seepage, but damp patches tolerated	Basic utility areas
2	No seepage; damp from condensation tolerable	Stores and plant rooms (minimum)
3	No water ingress or damp areas; heating and ventilation designed in	Bedrooms, kitchens, any habitable room

A store under the front steps needs at least Grade 2. A room you will live, sleep or keep valuables in needs Grade 3, which also means planning the heating and ventilation, not only the waterproofing.

The three methods compared

Cementitious tanking (Type A, barrier)

A waterproof render or slurry bonded to the walls and floor, such as Sika-1 render systems (sprayed or trowelled in coats to at least 18mm) or Vandex slurries (about 1.5mm to 3mm depending on water pressure). The wall-floor junction and joints are sealed first. It needs a sound substrate that can take the water pressure, careful curing (Vandex quotes about 28 days before plaster or decoration), and a breathable, non-gypsum finish. Nothing can be screwed through it later: Sika warns that puncturing the system can compromise it, so shelving and fittings need bonded fixings or pre-formed pockets.

Cavity drain membrane (Type C, drained)

A studded plastic membrane fixed to the walls and floor lets water run behind it to a drainage channel and sump, where a pump removes it. It copes with poor or uneven walls and relieves water pressure, and fixings go through sealed plugs. The cost is permanent reliance on pumps: NHBC guidance asks for a primary pump, a battery-backed second pump and an alarm, and the PCA notes that failures caused by missed servicing are not covered by guarantees.

Injected damp-proof course

A chemical cream injected into a mortar course, to BS 6576, to stop moisture rising from the ground. It only treats rising damp. Walls should dry for at least 14 days before replastering with a salt-resistant render, and walls heavily loaded with salts can stay damp after injection. Ground levels outside should sit about two brick courses below the damp-proof course.

Fix the outside too

• Clear and repair gutters, downpipes and gullies near the basement
• Check that paths, patios and raised beds are not above the damp-proof course
• Repair or waterproof front steps and pavement vaults that sit over below-ground rooms
• Clear planted walls and drainage channels that have silted up
• Heat and ventilate the space, or condensation will return whatever is done to the walls

Building regulations

Turning a cellar into a habitable room needs building regulations approval, covering ventilation, escape, stair headroom and smoke alarms as well as the waterproofing. Replastering more than half of an external wall can count as renovating a thermal element under Part L. Waterproofing an existing store like for like is not usually treated as notifiable work, but confirm with building control.

What it costs

There are no official cost figures. These ranges come from trade and lead-generation websites in 2025 and 2026 and are indicative only; London is commonly 15 to 25% higher.

Method	Indicative range
Injected damp-proof course	About £70–£120 per metre of wall, excluding replastering
Cementitious tanking or slurry	About £40–£100 per m²
Cavity drain membrane	About £80–£150 per m², plus the sump
Twin pump with battery and alarm	About £2,000–£3,500
Pump servicing	About £150–£300 a year

Ask for a 10-year guarantee from a PCA member, ideally insurance-backed so it holds if the installer stops trading.

Frequently Asked Questions

Sources

• Property Care Association: code of practice for waterproofing existing below ground structures (June 2022)
• Property Care Association: waterproofing and tanking advice
• PBC Today: summary of changes to BS 8102:2022
• GOV.UK: damp and mould, understanding and addressing the health risks
• Sika: Sika-1 Pre-bag LC render product data sheet
• NHBC: waterproofing basements and other below ground structures
• Historic England: assessing moisture in porous traditional building materials

This guide is general information, checked against the sources above on 3 November 2026. Every house is different, so take advice on your own property before starting work.