What a sagging or bouncy floor is actually telling you

Common causes of sagging or bouncy floors

Undersized or over-spanned joists are common in properties built before span tables were standardised, and these floors are often stable but simply springier than a modern build. Notching or drilling of joists for pipework and cabling over the years, sometimes beyond what the timber can safely lose, is another frequent cause of localised weakness.

Timber decay where a joist end sits in a damp external wall is a more serious cause, often linked to poor sub-floor ventilation, blocked air bricks, or damp penetrating the wall at that point. Settlement of an internal sleeper wall or support pier beneath the joists can also cause a floor to dip in one specific area rather than across the whole room.

• Joists sized or spaced adequately for older standards but springy by today's expectations
• Excessive notching or drilling for services weakening individual joists
• Joist end decay from damp penetration or blocked sub-floor ventilation
• Settlement or failure of a sleeper wall or support pier
• Loose or failed strutting between joists increasing flex

Causes specific to South Devon's older stock

Many solid-walled cottages and terraces in the area have suspended timber ground floors that rely entirely on air bricks for ventilation, and these are easily blocked by raised external ground levels, planting or render carried down over them during past repairs. Once ventilation is restricted, moisture builds up under the floor and joist ends nearest the damp wall are usually the first timbers affected.

Coastal humidity generally keeps sub-floor voids damper for longer after wet weather than an inland equivalent, which is another reason joist decay tends to start at the perimeter of a room near external walls rather than in the centre.

Telling a structural sag apart from a cosmetic dip

A floor that has always had a slight, consistent slope and shows no recent change is usually just a characteristic of the original build and not a cause for concern. A floor that has visibly changed, that bounces noticeably when walked across, or that has a sag concentrated in one small area rather than spread evenly, needs a proper look underneath.

Checking the sub-floor void directly, where access allows, for timber that feels soft, damp air, or visible fungal growth is the most reliable way to separate an old, settled floor from one that is actively deteriorating.

What a proper repair involves

Where joists are sound but under-strength for their span, the usual fix is sistering new timber alongside the existing joists to add stiffness without disturbing the floor above. Where decay is found at a joist end, the affected section is cut out and replaced, and crucially the cause of the damp is addressed at the same time, whether that means clearing blocked air bricks, lowering external ground levels, or repairing a defective wall.

Any repair to a suspended timber floor should also check and restore sub-floor ventilation, since replacing decayed timber without solving the moisture source simply resets the clock on the same problem.

When to act

A stable, long-standing dip with no recent change is generally low priority and can be left as a characteristic of the building. A floor that has started to sag more noticeably, feels soft underfoot, or is near a damp external wall should be checked promptly, since timber decay spreads while conditions stay damp.

Musty smells from under the floor or visible fungal growth on skirting are a clear signal to get the sub-floor void inspected without delay, as decay in one joist can spread to neighbouring timbers if left.

Common questions