Sagging or Bouncy Floors: Causes & Fixes

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.¹ 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.¹

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) is more common than dry rot but less serious.¹ It needs timber to stay wet and does not spread through masonry.¹ 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.²

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. Notches cut by plumbers or electricians in the wrong place can also significantly weaken a joist: LABC guidance limits notches to the top or bottom edge, between 0.07 and 0.25 of the span from a support, and no deeper than 0.125 times the joist depth (35 mm at most).³

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 holding the Property Care Association’s CSTDB qualification (Certificated Surveyor of Timber and Dampness in Buildings, which replaced the older CSRT in 2018) can provide a full written report with moisture readings, species identification, and a treatment or repair specification.⁴ Cost: £200–£500.

Repair Options and Costs

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:

1. Cut out decayed timber, usually with a safety margin into sound wood set by the surveyor
2. Strip plaster and render from adjacent masonry to expose mycelium threads
3. Treat masonry the fungus has reached with a masonry biocide, or with fungicidal plugs, pastes or gels
4. Ensure all moisture sources are eliminated (rising damp, plumbing leaks, blocked airbricks) before reinstatement
5. Re-build with pre-treated timber; use joist hangers rather than building ends into masonry where possible

The Property Care Association treats masonry biocides as a control measure rather than a cure: the long-term solution is removing excess water from the building fabric.¹ Its guidance also says joist ends embedded in affected walls should be removed and independently re-supported.¹

Reputable specialist contractors commonly issue long guarantees on dry rot treatment (often 20 years or more), ideally backed by insurance. A cheap cash-in-hand job without a guarantee is false economy.

Building Regulations

Straightforward like-for-like repair of a few joists does not normally 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 a registered building control approver. Part L can also apply: stripping a ground floor down to its joists and rebuilding it counts as renovating a thermal element.⁵ If more than 50% of the floor is renovated, the whole floor should be improved to at least 0.25 W/m²K where that is technically, functionally and economically feasible.⁵

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, Approved Document C recommends ventilation openings of at least 1,500 mm² per metre run of external wall or 500 mm² per m² of floor area, whichever gives the greater opening area⁶
• 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.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.