Below ground there is no damp course to repair, because the whole wall is in contact with wet soil. Cellar damp proofing is therefore not a line of injections but a system, and there are two families of them. One holds the water back with a barrier; the other lets it in and takes it away. They cost differently, fail differently, and suit different buildings, and the choice between them is the decision worth spending time on.
Tanking: holding the water out
A cementitious or membrane tanking system is applied to the inside face of the wall and floor to form a continuous barrier. It is cheaper, takes less headroom, and gives you a wall you can plaster and fix to. Its weakness is that it has to be perfect: water under pressure will find any junction, service penetration or shrinkage crack, and repairing a failed tank means taking the finish off to get to it.
Drained cavity: letting the water in and taking it away
A studded membrane is fixed to the wall and floor, leaving a drained void behind it that runs to a perimeter channel and a sump with a pump. Nothing has to be watertight, because the system is designed around water arriving. It costs more, eats headroom, and commits you to a pump with a power supply, a maintenance regime and, in most designs, a battery backup.
How the building decides which
A dry cellar in free-draining ground with no history of standing water is a candidate for tanking. A cellar that has flooded, sits below the water table for part of the year, or is in clay that holds water against the wall wants the drained cavity, because a barrier in that situation is being asked to resist pressure rather than moisture. A survey that does not discuss groundwater at all has not made the choice, it has made an assumption.
Cellar tanking, and what tanking a cellar involves below ground
Tanking in its original sense means applying a barrier to the inside of the structure so water cannot pass through it, usually a cementitious render system or a membrane bonded to the wall. Below ground that barrier has to resist water under pressure, so the preparation matters more than the product: the wall is hacked back to sound masonry, salts and old finishes come off, and every junction, especially the one between wall and floor, is detailed. Done properly it works. Done over a surface nobody prepared, it fails at the junctions first, which is why the trade largely moved to drained systems.
Tanking a wall: where wall tanking still makes sense
Above ground and in small areas, a tanked render is often the sensible answer rather than an overhaul. A single wall below an external ground level, the base of a wall in a basement stairwell, or a room where a drained system has nowhere to discharge are the cases where a barrier is the practical choice. The two things to establish before anyone starts are where the water is coming from, because a barrier over an unresolved source moves the damp rather than stopping it, and what the finish will be, since a tanked wall needs a compatible plaster and paint or it blows within a year.
Questions people ask about cellar damp proofing
Does cellar damp proofing need building regulations approval?
Waterproofing on its own generally does not, but converting a cellar into a habitable room does, and that brings in ventilation, means of escape and structural work. If the conversation has moved from dry storage to a bedroom, building control is involved.
What maintenance does a drained cavity system need?
The channels need rodding access and the sump and pump need testing periodically. It is a system with moving parts and it is the maintenance, rather than the installation, that people underestimate. Ask what the service interval is and what the pump failure plan is before you sign.
Can I just paint the walls with a waterproof coating?
Not reliably. Coatings sold for the purpose can resist moisture but not hydrostatic pressure, and below ground the two are hard to tell apart until the coating lifts. It is the cheapest option and the one most likely to be redone.