Damp In A Cellar Or Basement

What's happening?

A cellar is below ground, so its walls and floor are in contact with soil that holds water, and the water can arrive under pressure rather than only by capillary action. That is what makes it a different problem from damp in a wall above ground: the remedy is not to stop water entering the masonry (often you cannot), but to manage where it goes once it does. Two approaches exist. Tanking applies a barrier to hold water back, which works where the structure can take the pressure and the barrier is continuous. Cavity drain membranes take the opposite approach: they accept that the wall will be damp, hold the water behind a studded membrane and drain it to a sump. Which is appropriate depends on the water, the structure and how the space will be used, and getting that judgement wrong is how cellar conversions fail.

What usually causes it?

• Groundwater pressure against walls and floor, seasonally or after heavy rain
• Surface water not being taken away: blocked gullies, a soakaway too close, or ground falling towards the house
• No damp proof membrane in the cellar floor, or a membrane not joined to the wall treatment
• Failed or incomplete earlier tanking, breached at a service entry or a floor-to-wall junction
• A leaking drain, water main or rainwater pipe running through or beside the cellar
• No ventilation, so moisture evaporating from the walls has nowhere to go and condenses
• A converted cellar lined and finished over damp masonry with no drainage provision behind it

Is it dangerous?

The damp itself is not, but three things around it deserve care. A persistently damp, unventilated cellar is where dry rot establishes, and cellars contain the timber holding up the floor above. Standing water and electrical installations do not mix: a cellar consumer unit, socket or pump circuit in water is an electrical matter and wants isolating. And where a cellar is being converted to habitable use, that is building work with its own requirements, not a decorating job.

Can I fix it myself?

The outside is where the accessible wins are. Clear gullies, make sure downpipes discharge into a drain rather than onto the ground beside the house, and check that the ground falls away from the wall rather than towards it. Improving ventilation to a cellar used for storage helps a great deal. What is not DIY: tanking, cavity drain membranes and sump systems are engineered systems whose failure is expensive and hidden, and applying a waterproof coating to a cellar wall that has water pressure behind it typically blows off and takes the plaster with it.

When should you call a damp proofer?

Call a damp proofer with basement experience before any lining or conversion work, and when water is appearing on the floor rather than only damp on the walls. Ask which approach they propose and why, because tanking and cavity drainage are different answers to different conditions. Call promptly where there is standing water near any electrical installation, or where a musty smell suggests decay in the floor timbers above.

What will the damp proofer actually do?

• Establish where the water is coming from and whether it is arriving under pressure
• Check external drainage, ground levels and any services passing through the structure
• Assess the structure's condition and whether it can take a tanking system at all
• Specify the appropriate system (tanking, or a cavity drain membrane with drainage channel and sump), and say why
• Install with the wall and floor treatments continuous at the junction, and detail service penetrations
• Provide ventilation and, where a sump is used, set out the maintenance and failure alarm it needs

Who fixes this

• Basement tanking

Questions people ask

Sources & References

1. Property Care Association: Waterproofing and tanking
2. GOV.UK, Site preparation and resistance to contaminants and moisture: Approved Document C
3. Property Care Association: Dry rot
4. GOV.UK: Building regulations approval