Waterproofing to final coat

Basement conversions

A basement conversion is a waterproofing project with a room on the end of it. Almost every conversion that goes wrong went wrong below the finishes, and it was usually visible in the specification long before it was visible on the wall.

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CSSW Certified Surveyor in Structural Waterproofing Design to BS 8102
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One team, no handover

The common arrangement is that a waterproofing contractor installs a system and a builder finishes the room. The join between them is where responsibility gets lost: a fixing driven through a membrane, a channel blocked during the fit-out, a penetration formed after the system was signed off. When it leaks, each party points at the other.

We carry the project from survey through to the last coat of paint, which removes that join entirely. It also means the people finishing the room know exactly where the membrane is.

Designed for the use, not just for dryness

A habitable basement needs to achieve Grade 3 conditions under BS 8102, and that is a whole-environment question rather than a membrane question. Ventilation, vapour control, heating and lighting all have to be planned alongside the waterproofing — particularly for high-moisture uses like a gym, a shower room or a spa, where the room itself generates the moisture.

We plan those together, in that order, before any finish is chosen.

What people put down there

Home cinemas, which suit below ground because there are no neighbours and mass on every side. Wine rooms, which suit the stable, cool, dark conditions that make a cellar hard to heat. Gyms and spas, which need serious ventilation designed in. And ordinary living space — kitchens, playrooms, offices and self-contained annexes finished to the same standard as the floors above.

Systems and materials

What we install, and why that one.

We specify Wykamol systems for this work. Every product below carries its own datasheet and, where it exists, a BBA certificate — read them rather than taking our word for it.

Wykamol CM3 mesh membrane with bonded polypropylene mesh face

Walls — direct plaster finish

Wykamol CM3 Cavity Drain Mesh Membrane

CM3 with a polypropylene mesh thermally bonded to the face, so plaster can go straight onto the membrane instead of onto battens or board. That saves the batten depth on every wall, which in a low Victorian cellar is the difference between a room you stand up in comfortably and one you do not. The mesh has to be fixed and lapped properly or the plaster finds the joint.

  • 3mm studs, bonded mesh
  • Direct plaster finish
  • BBA certified
  • Studs at 20mm centres
Roll of Wykamol CM8 mesh cavity drain membrane

Walls — 8mm void, plastered direct

Wykamol CM8 Cavity Drain Mesh Membrane

The combination we use most on habitable conversions: the 8mm drainage void of CM8 with the bonded mesh face that takes plaster or a dot-and-dab finish directly. It isolates a wet wall, keeps the void working, and still gives a plastered room rather than a boarded box. Salt inhibiting and root resistant, which matters against a garden retaining wall.

  • 8mm studs, bonded mesh
  • Direct plaster or dot-and-dab
  • BS 8102:2022 Type C
  • 2m × 10m / 2m × 20m rolls
Roll of Wykamol CM20 20mm studded cavity drain floor membrane

Floors — under the screed

Wykamol CM20 Cavity Drain Membrane

The 20mm profile, laid across the floor before the screed or a floating floor build-up. The deep stud carries water to the perimeter channel and takes the loading of the floor above it without collapsing the void. Floors are where a drained system is least forgiving, because nothing can be inspected once the screed is down — so this is one of the few places we will not substitute a shallower profile to save money.

  • 20mm studs
  • BBA certified
  • Floors, under screed
  • Load bearing void
Wykamol Renovation Plaster bag

Finish coat over treated walls

Wykamol Renovation Plaster

A salt-resistant renovation plaster for walls that have carried damp for decades. Ordinary gypsum plaster over a hygroscopic wall pulls salts to the surface and blows within a couple of winters, which is why so many "damp problems" reappear after a decorator has been in. This is the finish that stops that happening once the waterproofing behind it is right.

  • Salt resistant
  • BBA certified (2025)
  • Over treated masonry

Technical downloads

The paperwork behind the specification.

The full technical library, including the drawings manual →

Common questions

What people ask us about basement conversions.

Do I need planning permission for a basement conversion?

Converting an existing cellar to habitable use often falls under permitted development, but lowering the floor, extending the footprint or excavating a new basement generally needs permission — and in several London boroughs a Basement Impact Assessment as well. Building Regulations apply once the space is habitable either way, covering fire escape, ventilation and structure.

How long does a basement conversion take?

For a typical domestic cellar conversion, several weeks to a few months depending on the extent of structural work, whether excavation is involved and how the finishes are specified. The waterproofing itself is rarely the longest element; structural works and fit-out usually are. We give a stage-by-stage programme rather than a single number.

Is a converted basement damp or cold?

It should be neither. A basement designed to Grade 3 with proper insulation, ventilation and heating is as comfortable as any other room in the house — below ground is thermally stable, which works in your favour once the space is insulated. Damp or cold conversions are the result of a waterproofing or ventilation design that was inadequate for habitable use.

Related

You may also need.

Basement waterproofing Cavity drain membrane Heat recovery ventilation Basement monitoring

Where we work

Across London and Surrey.

Central London North London West London South London Surrey & the North Downs

Request a quote

Talk to us about basement conversions.

Tell us what you're seeing and we'll tell you honestly whether it needs a full system or something much simpler.

Call 07342 179614
WhatsApp 07342 179614
Email info@crystalrenovations.co.uk
Office 4D Heathhurst Road, South Croydon, CR2 0BA

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