Bouncy and springy floors: why it happens and whether it matters

A floor that moves when someone walks across it is unnerving, and the internet's answer is usually either 'that's normal in an old house' or 'call someone immediately'. Neither is much use. The engineering position is more specific and, for most people, more reassuring: bounce is almost always a stiffness problem rather than a strength problem, and the two are not the same thing.

Strength and stiffness are different questions

Strength is whether a floor can carry its load without failing. Stiffness is how much it deflects and vibrates while doing so. A timber floor can be comfortably strong enough and still bounce enough to rattle a glass on a table, and that is the situation in a very large number of British houses.

Design practice reflects this. Floor joists are commonly checked against a serviceability deflection limit of around span over 360 - roughly 11mm over a 4 metre span - not because more than that is dangerous, but because more than that is noticeable and annoying. Older floors were never designed against that criterion at all, which is why they bounce.

What is normal

Perceptible movement in a suspended timber upper floor in a Victorian or Edwardian house is normal. Those joists were sized by rule of thumb and long tradition, spans were often generous, and the floor was covered in boards rather than a stiff sheet. Add modern furniture and a heavier lifestyle and you get a floor that does its job and complains about it.

Some bounce is normal. What is not normal is bounce that has changed - a floor that has recently become springy, or has become springy in one place.

What actually causes it

• Joists that are shallow for their span - the underlying cause in most old houses, and the one that has been there since the day it was built
• Missing or removed strutting - the herringbone or solid blocking between joists that makes them act together. It is regularly cut out for pipework and never replaced, and its absence makes a dramatic difference to how a floor feels.
• Notching and drilling - plumbers and electricians cutting through joists in the wrong place, particularly deep notches near mid-span, which removes a disproportionate amount of a joist's stiffness
• A span that has been lengthened by someone removing a wall below without appreciating it was supporting the floor above
• Decay or insect damage at the joist ends where they bear into a damp external wall - a genuine strength issue, not just a stiffness one
• Removed or collapsed sleeper walls beneath a suspended ground floor
• Added load the floor was never designed for - stone or tiled finishes, a large bath, heavy shelving, a home gym
• In more modern housing, engineered joists or thin chipboard decking with fixings that have worked loose, which produces a different, sharper kind of movement

What is potentially serious

A springy floor becomes a structural question rather than a comfort question when any of these appear:

• The bounce is new, or has got noticeably worse over months rather than years
• It is localised - one spot, rather than the whole floor - which points at a single damaged or failing member
• There is visible sagging, or the floor slopes towards the middle of the room
• Cracking has appeared in the ceiling below, particularly along the line of a joist or across it
• You can feel or hear movement at the wall junction, or see a gap opening between the skirting and the floor
• There is damp, staining or a musty smell near the joist bearings, which raises the possibility of rot at the ends
• A wall downstairs was removed at some point and the floor above spans further than it used to
• You have recently added weight - a stone floor, a large bath, heavy storage in a loft

What you can check yourself this week

• Walk the room slowly and map where the movement is. Whole floor, or one area? Worst at mid-span, or near a wall?
• Put a long spirit level or a straight edge across the floor in both directions and look for a dip.
• Get into the space below if you can - a cellar, or a ceiling void - and look at the joists for notches, drilled holes, missing strutting and damp at the bearings. A photograph of what you find is worth a great deal to an engineer.
• Look at the ceiling below for new cracking, particularly straight cracks following a line.
• Ask what has changed. New bathroom, new kitchen, wall removed, floor tiled, rewire? The cause is very often the last piece of work someone did.
• Bounce a couple of test spots with a full glass of water on the floor and note whether it is the whole structure moving or just the deck.

When to call an engineer

Stated as a threshold: if the bounce is longstanding, even across the room, with no sagging and no cracking, it is a comfort problem and you can treat it as one. If it is new, localised, accompanied by sagging or ceiling cracking, or if it followed a wall being removed or a heavy finish being laid, get it looked at.

Also worth an engineer's involvement before you spend money: if you are planning to lay a stone or tiled floor upstairs, or fit a large bath, the sensible order is to check the floor first. Retrofitting stiffness under a finished floor costs several times what designing for it would have.

How it gets fixed - and what it costs to know

The remedies are usually undramatic: replacing missing strutting, sistering or doubling joists, adding a properly designed intermediate support, repairing notched members, or replacing decayed joist ends. Occasionally the answer is a new beam, and occasionally the answer is that the floor is fine and you want a stiffer covering.

We would inspect the floor, get under it where that is possible, work out what is actually happening, and set out the options with the calculations behind them. We do not do the joinery and we take no commission from anyone who does, so if the honest answer is that the floor is safe and merely annoying, that is what you will be told.

A structural survey and report is a fixed £580 + VAT, with the report inside 10 working days of the inspection, guaranteed. A focused inspection of one floor is a smaller job and is quoted individually.