What Causes Damp Under Suspended Timber Floors?

A lot of older houses round here have suspended timber floors: floorboards on joists, sitting over a void with soil, a concrete oversite or rubble at the bottom. They work well as long as the void stays dry, and that depends almost entirely on ventilation.

How it’s meant to work

Air bricks in the outside walls, usually on opposite sides of the house, let air flow through the void and carry away moisture rising from the ground. Inside, sleeper walls under the joists are built with gaps, called honeycombing, so air can move right through. The timbers stay dry and happy.

What goes wrong

• Blocked air bricks. Soil, paving, ivy, render or someone blocking them to stop draughts.
• High ground levels outside, which can also mean water getting into the void.
• Extensions and solid floors. A rear extension or a kitchen with a new concrete floor can cut off the airflow from front to back, leaving a dead, stuffy area under the middle of the house.
• Water in the void. Leaking pipes, a high water table in wet winters, broken drains or rainwater from blocked gullies.
• Joist ends in damp walls, where timbers sit in pockets in the brickwork that are being wetted by rising or penetrating damp.

The warning signs

• A musty smell in ground-floor rooms.
• Springy, soft or sagging floors, or boards that have dropped.
• Rusty nail heads in the floorboards, which can be a sign of a damp void over time.
• Gaps opening up under skirtings, or skirtings that are soft and crumbly.
• Mould on the underside of carpets or on the boards.
• Fungal growth or strands visible through a lifted board.

Putting it right

• Clear and add air bricks so there’s cross-ventilation, and use periscope vents where ground levels are high.
• Where an extension has cut off the airflow, ducting can be run under the new floor to reconnect the void.
• Fix any leaks and deal with water getting in.
• Inspect the timbers. Any decayed joists, wall plates or boards should be cut out and replaced, and the wall pockets protected.

If you’re having timber floors insulated, make sure the insulation doesn’t block the ventilation.