Woodworm in Floorboards: Signs, Treatment & Cost
Spot woodworm in floorboards and joists, see how it is treated, what published UK prices say and when boards need replacing instead of simply treating.
Floorboards and joists are a favourite target for woodworm. They are made of softwood, they sit in cool voids, and nobody looks at them, so problems grow quietly. This guide shows how to spot the signs, how treatment works and when to replace boards.
Signs in floorboards
Look at the top and the underside of the boards:
- Small round holes, usually 1 to 2mm wide
- Fine dust on the floor or on skirting boards
- Boards that feel soft, spongy or springy
- Dust that returns after cleaning
Fresh dust is the clearest sign of a live problem. Our guide to the signs of woodworm lists every clue.
Check the joists
Floorboards hide the real risk, which is the joists beneath. Lift one board near an outside wall, because damp gathers there, and shine a torch along the joists. Look for holes, dust and soft patches, especially where joists meet the wall.
If you do not feel confident, ask a surveyor. Joists carry the floor, so damage there matters more than damage in the boards, although both need a check.
Why floors suffer
Suspended floors sit above a void that needs airflow. When air bricks get blocked, damp air lifts the moisture in the wood. Damp softwood is easier for larvae to eat, so the beetle thrives. Clear the air bricks and fix any leaks to reduce the risk.
How treatment works
Most floor jobs use a water-based spray on the exposed joists and the underside of the boards. Thick or damp joist ends may need boron paste. The team usually lifts some boards to reach the timber, since spray must touch bare wood.
Badly damaged boards are better replaced. If joists are weak, they need timber repair. A surveyor will say which applies.
What it costs
Published guides put a single room at about £300 to £500 and several rooms at £500 to £800 outside London. Replacing a few boards or minor joist sections adds roughly £200 to £500, according to DampHub. See the cost guide for sources.
Treat or replace?
Treat the board if it is sound and the holes are shallow. Replace it if it crumbles, flexes or has lost an edge, because treatment cannot restore lost wood. Replace a joist section only after a surveyor checks the load path.
After treatment
Keep the void dry and ventilated, because dry wood is safer. Seal the new or bare wood, and check the floor each spring. The post on how long treatment lasts explains what to expect.
If you are unsure what you have found, request a survey. Related reading: woodworm in skirting boards and woodworm in roof timbers.
Where you live matters, because older streets hold more old timber. See our local guides for woodworm treatment in Plymouth and woodworm treatment in Exeter.
To go deeper, read about the spray treatment and the structural timber repair, because the right method depends on the beetle.