Basement Waterproofing Retrofit, Hampstead

A waterproofing retrofit of an existing but chronically damp basement beneath a Victorian terraced house in Hampstead, converting it from unusable storage into a dry utility room, plant room, and study. A Type C cavity drain membrane system with dual sump pumps was installed without the need for full planning permission.

Background photography is illustrative of our North London portfolio work, not a claim that it shows this exact property.

Project Type

Basement Waterproofing Retrofit

Duration

8 weeks

Project Value

£38,000

The Challenge

The basement had suffered from persistent water ingress for over 20 years. Previous attempts at tanking with cement-based systems had failed, leaving areas of delaminated render and active seepage. The existing floor level was too low in places for a conventional drain channel detail, and the ceiling height was only 2.1m — borderline for habitable use.

Our Solution

A Type C cavity drain membrane system was chosen as the most reliable solution for an existing wet basement. The floor slab was broken up in sections and a perimeter drain channel installed at slab level, discharging to a dual sump chamber with Grundfos pumps and a high-level alarm. The floor was re-laid at the lowest practicable level to maximise headroom. The ceiling was finished at 2.1m with a suspended grid to conceal services.

Results

✓Type C cavity drain system installed — independently inspected and certified
✓Dual Grundfos sump pumps with high-level alarm and battery backup
✓Basement converted from unusable to dry usable space
✓45m² divided into study, utility room, and plant room
✓Ceiling height achieved: 2.1m throughout
✓Six-month post-completion monitoring confirmed zero water ingress
✓No planning permission required — works entirely within existing footprint

Key Features

✓Type C cavity drain membrane — full perimeter and floor
✓Dual Grundfos sump pumps with battery backup
✓New floor slab at optimised level for maximum headroom
✓Suspended ceiling concealing cavity drain membrane and services
✓Utility room — new WC, laundry plumbing, and electrical board
✓Independently certified by waterproofing specialist surveyor

Specification

Item	Specification
Cavity drain membrane	Delta MS Drain, 8mm studded HDPE, full floor and perimeter walls
Sump system	Dual Grundfos Unilift CC 7 pumps, 300L GRP sump chamber, float switch alarm
Floor slab	A142 mesh reinforced C25 concrete slab, 100mm, DPM below
Ceiling	Metal grid suspended ceiling system, 30mm cavity, Gyproc tiles, 2100mm finished height
Wall finish	Delta PT membrane plaster base, Gyproc Thistle bonding coat and skim
Electrics	New consumer unit, LED downlighters, double sockets throughout, IP65 to wet areas

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