The cause
What causes it
Below-ground rooms are surrounded by saturated soil for much of the British year, and water enters through any available path — cracks in the slab or walls, the wall–floor junction, deteriorated brickwork, or directly through the masonry under hydrostatic pressure. Original damp-proofing systems in older properties were often barrier renders that fail with age. Modern extensions can suffer where the original tanking was poorly applied or has been damaged by movement. Surface water mismanagement at street level — overloaded drains, downpipes discharging close to the building — frequently makes the underlying problem worse.