How do I deal with damp in a cellar or basement?

A cellar is below the water table for part of the year in much of Britain, so it is the one part of a house where damp is usually structural rather than behavioural. Diagnosis first saves a great deal of money.

Safety

Step by step

1. Do the polythene test.Tape a half-metre square of polythene tightly to a wall and another to the floor. Two days. Water on the room side is condensation; water trapped behind it is coming through the structure.
2. Look outside before inside.Blocked gullies, downpipes discharging at the wall, ground sloping towards the house, a broken drain, or soil piled above the damp-proof course. These cause a large share of cellar damp and cost little to fix.
3. Clear and check the drainage.Gutters, downpipes and gullies around the whole perimeter. Water dumped at the base of a wall has to go somewhere and the cellar is downhill of it.
4. Check any external lightwell or vault.Lightwells fill with leaves and their drains block. A flooded lightwell against a cellar wall is a permanent water source.
5. Ventilate if it is condensation.Airbricks clear and unblocked, cross-ventilation if possible, and a dehumidifier set to about 55% with a continuous drain. That combination handles most condensation cases.
6. Add gentle background heat.A cold cellar condenses everything the warm air above brings down. A tube heater or a low background temperature raises surfaces above the dew point.
7. Keep stored items off the floor and walls.Open shelving, pallets, plastic boxes rather than cardboard. It will not fix damp but it stops the damage while you do.
8. Understand tanking before agreeing to it.A waterproof render or membrane applied to the inside. It holds water back but water still arrives, and if it finds a weak point it will show there instead. Done badly it moves the problem rather than solving it.
9. Understand a cavity drain membrane.A studded membrane that lets water run down behind it into a channel and a sump with a pump. The usual modern approach, because it manages water rather than resisting it. It needs a maintainable sump and a power supply.
10. Be sceptical of injection damp proofing here.Chemical injection addresses rising damp in a wall, not lateral water pressure against a below-ground structure. It is frequently sold for cellars where it cannot work.
11. Get an independent survey for structural water.Preferably from someone not selling the remedial work. A cellar tanking job is a large sum and the diagnosis determines whether it is the right one.
12. Do not seal a cellar you have not dried.Insulating or boarding a damp cellar traps moisture in the structure and grows mould behind the finish. Resolve the water first.

Tips

• The polythene test costs nothing and decides whether this is a ventilation job or a construction job.
• Fix the outside drainage before spending anything inside. It is the cheapest and most commonly overlooked step.
• Get the survey from someone who is not quoting for the work.

Common mistakes

• Tanking before diagnosing — If the damp was condensation, tanking does nothing and the money is wasted. If it is groundwater, the external drainage may have been the cheaper fix.
• Boarding or insulating a damp cellar — It hides the moisture rather than removing it, and mould grows behind the finish where you cannot see it.

If it doesn't work

Questions people ask