How Do I Fix a Damp Cellar? Does It Need Tanking?

Cellars are below ground, so the soil around them is often damp, sometimes wet, and occasionally under real water pressure. Expecting the walls of an old cellar to be bone dry without help is unrealistic. The question is how dry you need the space to be.

For a cellar used for storage or as a coal store, the aim is usually to manage moisture rather than stop it completely. Make sure any air bricks and vents are clear, get air moving through the space, keep cardboard and timber off the walls and floor, and consider a dehumidifier or a mechanical ventilation unit. Also fix the outside: leaking gutters and downpipes over a cellar wall, and broken drains, put a lot of water into the ground right where you don’t want it.

If you want to use the cellar as a habitable room, a gym or a home office, you need a waterproofing system designed for the job. In the UK this is covered by BS 8102, the British Standard for protecting below-ground structures against water ingress. A good designer will assess the risk, the ground water, and how dry the finished room needs to be.

There are broadly two approaches for an existing cellar:

• Tanking (barrier protection). A waterproof render, slurry or coating bonded to the walls and floor that holds the water back. It can work, but it has to resist the full water pressure, and if it cracks or loses bond, water gets through.
• Cavity drain membrane (drained protection). A studded membrane fixed to the walls and floor that lets any water that comes through the wall drain behind it to a channel and a sump, where it’s pumped out. It doesn’t try to hold the water back, it controls it. On many old Yorkshire cellars this is the more forgiving system.

Pumped systems need maintaining. Sump pumps should be serviced and ideally have a backup and an alarm, because a failed pump in a converted cellar is a flood waiting to happen.

Whichever route, don’t rush to tank a cellar just because it’s damp. Get it surveyed by someone qualified in structural waterproofing so the system fits the building and how you want to use it.