Why is my floor bouncy or springy?

A floor that bounces is deflecting more than it should under load. Usually the joists are over-spanned or undersized for what is on them, a stump or pier has settled, a bearer has decayed, blocking is missing, or something has been removed that was helping to carry the load. Bounce is a stiffness problem rather than a strength problem — most bouncy floors are not about to fail. But it is the building telling you something.

The advice you find online for this is almost entirely American and British — sistering joists in a basement, adding a mid-span beam under a crawlspace, sill plates, drywall, and span tables from British Standards. Australian suspended floors are built differently: bearers and joists on stumps or piers, sized to AS 1684, over a ventilated subfloor rather than a basement, in a country where termites are a live factor in why timber loses stiffness.

What bounce actually is

Two different things get called the same word.

Deflection is how far the floor sags under a load and springs back when the load moves. Every timber floor deflects; the question is how much. Structural design limits deflection not because a floor might collapse but because a floor that moves too much feels wrong, cracks finishes, and makes furniture rock.

Vibration is the springy, resonant feeling you get when someone walks across a room and the glasses rattle. It is related to deflection but not the same thing — a floor can be strong, safe, and still unpleasantly lively, particularly over long spans.

Most complaints are about the second. It is a comfort and serviceability issue, and it is genuinely fixable.

Bouncy, or unsafe?

Worth separating early, because the responses are very different.

Bouncy but sound: the movement is springy and even, it returns immediately, it happens across a whole room rather than one spot, and nothing has changed recently. Floors like this have often been doing it since the house was built.

Warrants investigation: the bounce is new or getting worse; it is localised to one area; the floor feels soft or spongy underfoot rather than springy; there is a slope or dip; you can hear cracking; doors and skirtings nearby have moved; or it appeared after work was done in the house.

Soft and spongy is the word that matters. Springy is stiffness. Soft is usually decay.

The Australian causes

Joist span or size. Timber framing is sized from published span tables in AS 1684, which relate member size and species to span and load. A floor built to the limit of what the table allows will meet the standard and still feel lively. Older houses may predate the tables entirely, and additions are sometimes framed by eye.

Stump or pier settlement. The most common cause in older housing. Timber stumps rot at ground level, concrete stumps can move on reactive clay, and either way the bearer above loses support. This produces a localised dip and bounce rather than a whole-room springiness. On reactive soils this is seasonal — see cracks and sticking doors on reactive clay.

Bearer decay. Bearers sit lower than joists, closer to the ground, and are the first thing to suffer where subfloor ventilation is poor or drainage is bad.

Termite damage. A genuine and specifically Australian cause. Termites hollow timber from the inside, so a joist or bearer can look sound and have very little left. Any new bounce in an older timber-floored house should include a termite check — see what to check as termite season arrives.

Missing blocking or bridging. Short pieces fixed between joists stop them twisting and make the floor act as a group rather than as individual boards. Where blocking was never installed, or was cut out to run services, the floor loses stiffness noticeably.

Notched or drilled joists. Plumbing and wiring runs cut through joists. There are limits on where and how deep, because a notch in the wrong place removes a large share of a joist's strength. Bathroom floors are the classic case — lots of pipework, all of it in one area.

Something removed. A wall taken out during a renovation that was carrying floor load, or a subfloor wall removed to open up storage. See whether you need a structural engineer.

Load that was never anticipated. A stone benchtop, a spa bath, a wood heater, a large aquarium, or a wall of full bookshelves. The floor was sized for domestic loading, and concentrated point loads are a different proposition.

If your floor is on a slab

A slab-on-ground floor does not bounce. If a floor you believe to be slab has any give in it, the movement is in something above the slab — a timber overlay, a floating floor over an uneven substrate, or a raised platform. That is a different and usually much simpler problem.

How to work out which it is

1. Map it. Walk the room and note where the bounce is worst. Whole room, one corner, or a line? A line usually means one joist or bearer. A corner usually means a stump.
2. Check for level. A long spirit level or a marble will show a dip. Bounce with a dip points to support failure rather than span.
3. Get underneath if you can. Take a torch. Subfloors are cramped, sometimes contain hazards in older houses, and are worth approaching carefully — but this is where the answer is.
4. Check the stumps and piers. Look for stumps that no longer touch the bearer, packers that have fallen out, rot at ground level, or footings that have sunk.
5. Probe the timber with a screwdriver at the bearer ends, at stump tops, and anywhere close to the ground. Sound timber resists. If it sinks in, you have decay rather than deflection.
6. Look for mud tubes on stumps, piers and the underside of bearers.
7. Look for notches and holes where pipes and cables pass through joists, particularly under bathrooms and kitchens.
8. Check the subfloor is dry and ventilated. Damp ground, blocked vents and poor drainage are the underlying cause of most decay, and fixing the bounce without fixing that guarantees a repeat.

When to bring in an engineer

An engineer is worth engaging if the bounce is new or worsening, if there is decay or termite damage in a structural member, if the floor has a visible dip, if a wall has been removed, or if you are considering adding significant weight such as a stone bench or a spa.

What an engineer provides is a specification for the fix. Without one you are guessing at sizes, and the common DIY approach — adding a bit more timber and hoping — frequently fails to change how the floor feels because it does not address why it moves.

What the fixes look like

In broad terms, and subject to what the diagnosis turns up:

Re-supporting. Additional stumps or piers under an existing bearer to shorten the span. Usually the cheapest and most effective fix where support has failed.

Sistering. Fixing a matching joist alongside an existing one along its full length. Effective for a single weak or damaged joist; less so for a whole floor that is simply over-spanned.

Adding blocking. Restoring or installing blocking between joists at mid-span. Often the highest return for the least work on a floor that is springy but structurally fine.

Replacing decayed members, alongside fixing the moisture that caused the decay. Doing one without the other is temporary.

Adding a bearer. Introducing a new intermediate bearer and stumps to halve the joist span. The most substantial intervention and the most effective for a genuinely over-spanned floor.

Common questions

Is a bouncy floor dangerous?

Usually not. Deflection limits are set for comfort and for protecting finishes, well below the point of structural failure. Bounce that is new, worsening, localised, or accompanied by soft timber is a different matter and should be looked at.

Why does it only bounce in one spot?

That points at a single member or support rather than the whole floor — a failed stump, a decayed bearer end, or a notched joist. It is generally the easier version of the problem to fix.

Will more screws or nails in the floorboards help?

Only for squeaking. Bounce is in the framing beneath the boards, and fixing the surface down more firmly does not change how far the joists deflect.

My new extension bounces more than the old house. Why?

Often because the old house was framed conservatively by modern standards, while a new floor built exactly to the span tables meets the requirement with nothing to spare. It complies and it still feels different. Worth raising with the builder while you are inside the defects liability period, though be aware that meeting the standard may be the answer.

Can I fix this myself?

Adding blocking or packing a stump is within reach of a capable owner. Anything involving replacing structural members, adding bearers, or working out sizes is not — and in some states structural work carries licensing requirements regardless of who does it.

The floor bounces and the doors have started sticking. Related?

Very likely. Both are symptoms of movement. Whether the cause is subfloor support failure or ground movement under the footings, the two together are worth investigating properly rather than adjusting the doors and moving on.

Where to go next

Nearly everything that makes an Australian timber floor lose stiffness happens in the subfloor, out of sight, and is driven by water or insects.

Read next: what to check as termite season arrives, or whether you need a structural engineer if the movement is new.

An hour under the house with a torch and a screwdriver answers this question more reliably than any amount of walking around on top of it.