Basement and Cellar Damp Investigation
Below-ground walls are wet because they are below ground. The question is whether the water is coming through, condensing on the inside, or arriving from a drain or pipe, and which waterproofing approach is proportionate.
Basement and cellar damp affects Victorian and Edwardian lower ground floors across London, converted cellars, basement flats and new habitable basements alike. Water can enter laterally through the earth-retaining wall, rise through the floor slab, run in at a light well or coal chute, come from a leaking drain or supply pipe outside, or simply condense on cold walls in a space with no ventilation. The cost and disruption of the answers range from a vent and a dehumidifier to a full cavity drain membrane system with a sump and pump, so the diagnosis matters.
What we establish
- Where water is entering, and whether it is lateral penetration, ground water under pressure, surface water from light wells and steps, a drain or pipe leak, or condensation
- Moisture profiling of retaining walls and the floor, with salt testing to distinguish ground salts from other sources
- Humidity, dew point and ventilation, because many “leaking” cellars are condensing
- Condition and performance of any existing tanking, membrane, sump or pump
- External factors: ground levels, paving falls, gullies, downpipes, light-well drainage and nearby drains, with a camera survey where a drain is suspected
- Whether the space is used, or intended to be used, as habitable accommodation, which changes the standard it must meet
Waterproofing approaches, in plain terms
British Standard BS 8102 describes three types of below-ground protection: Type A barrier protection such as cementitious tanking or render applied to the wall; Type B structurally integral protection, normally for new construction; and Type C drained protection, the cavity drain membrane with channels, a sump and a pump that manages water rather than holding it back. Which is appropriate depends on the water source, the structure, the intended use and the consequences of failure. Our report sets out what we found and the approach that fits, so that a waterproofing contractor can design and price it. Where the intended use is habitable and the risk is high, we will tell you that a specialist waterproofing designer should be involved.
Independence below ground
Basement waterproofing is expensive, and the companies that install it also sell the survey. DML does not install waterproofing. Our team or any specialist contractor can quote against our scope, and because our fee is for the investigation, the report is as happy to say “ventilate and fix the gully” as it is to specify a membrane system.
We investigate across Greater London and Surrey. See areas we cover.
Frequently Asked Questions
My cellar walls are wet but there is no standing water. Is that a leak?
Often it is lateral damp through the wall or condensation on cold masonry. Moisture profiles, salt tests and humidity readings tell them apart.
Can I convert my cellar into a habitable room?
Possibly, but a damp cellar needs a designed waterproofing system, ventilation and often drainage works first. Our investigation gives you the starting facts; a waterproofing designer and building control come next.
Is tanking or a membrane better?
Neither is universally better. Tanking holds water back and can fail under pressure; a drained membrane manages water but needs a working sump and pump. The right choice depends on the water source and the use of the space.
The previous owner had it tanked and it is wet again. Why?
Barrier systems fail where water pressure builds, where the floor-to-wall junction was not treated, or where the source was a drain or pipe that was never fixed. We test the source first.
Do you install waterproofing?
No. DML investigates and reports. Waterproofing and drainage works are quoted separately by our team or a specialist of your choice.
Related services
Related reading: Rising damp treatment: what to expect from start to finish · When you need a CCTV drain survey
Wet basement or cellar?
Independent below-ground damp investigations across London and Surrey.
Last reviewed: 23 October 2026. Independent investigation and written reports. Repairs are quoted separately.