A bouncy or sagging floor is unsettling — literally and figuratively. In most UK homes with suspended timber ground floors, some flex underfoot is normal. A floor that springs noticeably with every step, dips visibly in the middle of a room, or creaks excessively is telling you something is wrong structurally. Catching this early is important: deteriorated joists can worsen quickly if damp is the underlying cause, and in severe cases pose a genuine safety risk.
How Suspended Timber Floors Work
Ground-floor suspended timber construction (common in UK homes built before 1960 and in many Victorian-era properties) consists of:
- Sleeper walls — low brick “dwarf walls” built off the concrete oversite, supporting the joists at regular intervals
- Floor joists — typically 50 × 200 mm or 50 × 175 mm regularised C24 timber (in modern work), or equivalent sawn sections in older construction, spanning between the external walls and sleeper walls
- Floorboards or sheet flooring — fixed to the top of the joists; traditionally 19–25 mm tongue and groove softwood boards, now often 18 mm or 22 mm T&G chipboard
The void beneath a suspended floor must be ventilated through airbricks in the external walls to prevent moisture build-up. If airbricks are blocked or the ventilation is inadequate, the timber becomes vulnerable.
Causes of Bouncy or Sagging Floors
Dry Rot
Dry rot (Serpula lacrymans) is the most destructive cause of structural floor failure. It thrives in warm, damp, poorly ventilated conditions and digests the cellulose in timber, leaving a dry, cuboidal-cracked residue that has almost no structural strength. Dry rot can spread through masonry and under plaster, well beyond the originally infected timber.
Signs of dry rot: cuboidal cracking on joist surfaces; white or grey cotton-wool growth on affected timber; orange or rust-coloured spore dust; a distinct musty smell from under the floor.
Wet Rot
Wet rot (most commonly Coniophora puteana) requires continuously wet conditions and does not spread through masonry. Affected timber becomes soft, darkens, and loses strength — but it will not spread once the moisture source is removed. Wet rot is more common where a burst pipe, leaking WC, or blocked airbrick has kept timber damp.
Woodworm
Furniture beetle (Anobium punctatum) and, less commonly, house longhorn beetle larvae bore through structural timber, reducing its section. Light infestation in a well-dried timber is often historic and inactive. Active infestation produces fresh cream-coloured bore dust (frass) below exit holes.
Overloaded or Undersized Joists
Older properties sometimes have joists spanning beyond their engineered capacity, particularly if the original span tables were not followed or if a sleeper wall has subsided or been removed. Notching by plumbers or electricians in the wrong location (within the middle third of the span) can also significantly reduce joist stiffness.
Joist End Bearing Failure
Joists built into external masonry walls are susceptible to end-bearing failure where moisture penetrates the wall and rots the last 100–200 mm of the joist sitting in the brick pocket. The joist appears sound in the middle but has lost its bearing at one or both ends.
Settlement or Sleeper Wall Movement
Sleeper walls can settle if built on poor ground or if the mortar has deteriorated. A single sleeper wall dropping by even 10–15 mm creates a visible dip mid-span.
How to Diagnose the Problem
From above: Lift a floorboard (choose one at the worst point of deflection). Shine a torch into the void and assess:
- Are joists visibly sagged or split?
- Is there any visible growth (white or grey mycelium threads suggest dry rot)?
- Are airbricks clear? Can you see daylight through them?
- Is there standing water or wet soil visible?
From below (if access allows): Some properties have external airbrick grilles or an access hatch; crawling under the floor with adequate lighting gives a clear view of all joist spans.
Probe test: A bradawl or screwdriver pushed firmly into joist surfaces reveals decay — sound timber resists penetration; decayed timber is soft and the tool sinks easily.
Professional survey: A damp and timber specialist (CSSW or CTIS qualified) can provide a full written report with moisture readings, species identification, and a treatment or repair specification. Cost: £200–£500.
Repair Options and Costs
| Repair | Typical Cost | Notes |
|---|---|---|
| Sistering (adding new joist alongside old) | £500–£1,500 | Effective for isolated damage; access via lifted boards |
| Partial joist replacement (1–4 joists) | £800–£2,000 | Includes lifting floorboards and making good |
| Full suspended floor replacement (one room) | £2,000–£5,000 | New joists, insulation, and boarding; 20–25 m² |
| Dry rot treatment and eradication | £1,500–£5,000+ | Specialist contractor; includes masonry sterilisation |
| Wet rot treatment (limited area) | £500–£1,500 | Removal of wet timber plus biocide treatment |
| Sleeper wall repair/rebuild | £800–£2,500 | Often combined with joist work |
| Airbrick clearing and sub-floor ventilation upgrade | £150–£500 | Add new airbricks where deficient |
| Woodworm treatment (full room) | £300–£700 | Insecticide spray or paste system; guarantee issued |
| Specialist damp/timber survey | £200–£500 | Written report with recommendations |
Regional variation applies — London and South East prices tend to be 20–35 % above the national range.
Sistering Joists: The Most Common Fix
Sistering involves bolting a new full-length joist alongside the existing damaged one, transferring the load entirely to the new timber while the old joist remains in place. It is usually the lowest-cost repair where only a few joists are compromised and the floor void is accessible.
The new joist should be:
- Same depth and width as the original (or sized to current span tables under BS EN 1995-1-1)
- C24 graded structural timber
- Treated with preservative, particularly at any end bearing into masonry
- Firmly bolted through to the existing joist every 400–600 mm
Sistering does not address the underlying cause. If the joist failed due to damp, the moisture source must be eliminated before repair.
When Dry Rot Is Involved
Dry rot repair is more complex than simple joist replacement because the fungus spreads through masonry. A proper specification includes:
- Remove all infected timber with a 500 mm margin beyond visible growth
- Strip plaster and render from adjacent masonry to expose mycelium threads
- Sterilise masonry with boron-based biocide or a specialist fungicide; apply two coats
- Ensure all moisture sources are eliminated (rising damp, plumbing leaks, blocked airbricks) before reinstatement
- Re-build with pre-treated timber; use joist hangers rather than building ends into masonry where possible
Reputable specialist contractors issue 20–30 year guarantees on dry rot treatment backed by insurance. A cheap cash-in-hand job without a guarantee is false economy.
Building Regulations
Straightforward like-for-like joist replacement does not require Building Regulations approval. However, if you are:
- Altering the structural configuration (changing spans, removing sleeper walls)
- Adding a new floor over a previously open-plan ground floor
- Changing the floor construction type (e.g. converting suspended timber to solid concrete)
…you will need Building Regulations approval from your local authority Building Control or an Approved Inspector. Thermal performance requirements under Part L may also apply if the floor area is significant.
Preventing Future Problems
The most effective long-term prevention is maintaining adequate sub-floor ventilation:
- Keep airbricks clear of soil, paving, and vegetation — check them twice a year
- Ensure new external works (paths, drives, raised beds) do not cover or obstruct airbricks
- If additional airbricks are needed, the Building Regulations guidance in Approved Document C recommends a minimum of 1,500 mm² of free ventilation per metre run of external wall
- Consider adding a DPC (damp-proof course) between sleeper walls and joists where none exists
- Have any plumbing runs under the floor inspected if you suspect a slow leak
A ventilated, dry sub-floor void will keep suspended timber in sound condition for the life of the building.
