What does a soft or spongy spot in a floor mean?
A soft patch is not the same as a bouncy floor. It is a local loss of strength with a short list of causes, and finding which one decides whether it is a board repair or a structural job.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 4 min
- Safety
- warning
Short answer
Press and listen at the edges of the soft area to map it. A soft patch confined to one board bay is usually a wet or rotted deck; one that follows a joist line is a failed joist or a rotted joist end; one under a bathroom or an external wall is a leak or damp until proved otherwise. Lift and look — you cannot diagnose a soft spot from above.
The word people use is bouncy, but there is an important distinction. A floor that deflects gently and evenly across the room is doing what a timber floor does, and the guide on that is separate. A patch that gives way more than the floor around it is a local failure, and local failures in floors are nearly always water.
Safety
What you'll need
- a torch
- a screwdriver or bradawl to probe timber
- a moisture meter (optional)
- a cordless drill and screws (optional)
Step by step
- Map the soft area with your foot and a straight edge.Walk it slowly and mark the boundary with masking tape. A patch confined between two joist lines is a deck problem. A soft strip running the length of a board line is a fixing problem. A patch that dips towards a wall points at a joist end.
- Work out what is above and around it.A bath, shower, WC, basin, washing machine, boiler, radiator, or an external wall or door threshold. Almost every soft floor sits under or beside one of them, and that is the leak to look for.
- Get underneath if you can.From a cellar, a crawl space or the ceiling below in a flat conversion. Ten minutes with a torch from below tells you more than an hour of guessing from above.
- If you cannot get under, lift a board.Lift at the edge of the soft area, not the middle, so you land on sound timber. Set the saw depth to the board thickness only and scan for cables and pipes first.
- Probe the timber with a screwdriver.Sound timber resists a firm push. If the point sinks in easily, the timber has decayed. Probe the joist top, the joist sides and the joist end nearest the wall — the end is where rot starts because that is where the damp is.
- Identify what you are looking at.Dark, wet, crumbly and stringy is wet rot from a live water source. Dry, cracked into cubes, with white or grey growth and a mushroom smell is a more serious problem that needs proper identification. Powder and small round exit holes is woodworm. Swollen and delaminating with no smell is a chipboard deck that got wet.
- Fix the water first.Replacing timber without stopping the source guarantees you will do it again. Trace and repair the leak, clear the blocked airbrick, or resolve the raised ground level, then let the area dry before rebuilding.
- Repair to the level the diagnosis justifies.A wet chipboard sheet or a soft board is a straight replacement. A joist that is soft over a short length can be sistered — a new timber bolted alongside the sound part. A joist end rotted where it bears into a wall is a structural repair and a job to get properly specified.
If it doesn't work
Soft only in a bathroom, worst near the bath or shower
Cause: A long-running slow leak at a waste, a trap or failed sealant, soaking the deck and joists beneath — Fix: Find and fix the leak before touching the floor. Then lift the covering, cut out the wet deck, dry the void thoroughly and replace with moisture-resistant board.
Soft strip along one board only, springs back up
Cause: The board has come off its fixings or the joist beneath is missing at that point — Fix: Lift and look. If the joist is there, screw the board down properly. If there is no bearing, add a noggin or batten between joists to carry it.
Soft close to an external wall, worse in winter
Cause: Damp bridging into the joist ends, often from a raised path, a blocked airbrick or failed pointing — Fix: Deal with the external cause first — lower the ground level or clear the airbrick — then have the joist ends assessed once the wall has dried.
Whole floor moves gently and evenly, no local patch
Cause: Normal deflection in a timber floor, or joists at their span limit — Fix: This is a different problem with a different answer — see the guide on a bouncy floor rather than treating it as a soft spot.
Soft with a musty smell and no obvious leak
Cause: Stagnant, humid underfloor void with blocked ventilation — Fix: Clear the airbricks, remove anything stacked against them outside, and check nothing has been stuffed into the void. Then reassess the timber once the void has dried.