Where this starts
A floor has to do three things and they are checked separately: carry the load without failing, not sag too far, and not move in a way people notice. The third is the one that produces this complaint, and it is the one least likely to have been designed for in an older or a self-built floor.
DEFLECTION is distance. A joist bends under load, and how far is set by its depth above all — depth counts enormously more than width, because stiffness rises with the cube of the depth. Deflection shows as a visible dip, as doors and skirtings that no longer line up, and as cracking where a partition sits on the floor.
VIBRATION is a frequency. A floor is a spring with a mass on it, and if its natural frequency falls into the range that footfall excites, a footstep at one end is felt at the other — a glass ripples, a shelf rattles, somebody walking past a sofa is noticed through it. That can happen in a floor whose deflection is well inside every limit, which is why a structurally sound floor can feel entirely wrong and why modern design checks the two separately.
SO THE FIRST OBSERVATION IS WHICH ONE. Put a glass of water on the floor and walk normally past it. Movement in the water with no visible movement in the floor is vibration. A visible dip underfoot, a door that binds, a gap opening between the floor and the skirting, is deflection.
WHAT MAKES A FLOOR WORSE OVER TIME is a short list and worth going through, because most bouncy floors were not always bouncy: strutting or blocking removed or never fitted, a joist notched or drilled for services, a wall taken out that was quietly supporting it, a heavy stone floor laid over it, or a bearing that has moved.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Joists too shallow or spanning too far
Most commonGeneral springiness across a whole room rather than one spot, sometimes with a visible dip mid-span, most often in an older floor or a converted space.
How to check: The joist depth and the clear span. Depth dominates stiffness, so a shallow joist over a long span is springy however sound it is.
No strutting or blocking between joists
Most commonA floor that feels bouncy under one person's step and transmits movement sideways, with the joists visibly free to twist when viewed from below.
How to check: Look between the joists from below. Nothing between them means each joist is working alone rather than sharing load with its neighbours.
A joist notched or drilled for services
CommonLocalised movement near a bathroom, a kitchen or a heating run, with notches cut in the tops of joists for pipes — often deep, and often near mid-span where it matters most.
How to check: Where the pipes and cables cross the joists, and how deep the notches are. A notch in the wrong place removes much of the joist's strength.
A support removed below
CommonOnset following building work — a wall taken out, a through-lounge formed, a chimney breast removed — with the bounce in the area that lost its support.
How to check: Whether anything was removed underneath. A wall that looked non-structural may have been carrying the floor whether or not it was carrying the roof.
A vibration problem rather than a strength one
CommonA floor that barely deflects but transmits footfall across the room, commonly on long-span engineered joists in a newer building, with drinks rippling and shelves rattling.
How to check: Put a glass of water down and walk past it. Water moving with no visible floor movement is frequency rather than stiffness.
A bearing that has moved or decayed
Less commonMovement concentrated near a wall rather than mid-span, sometimes with a gap at the skirting, and occasionally with soft or damp timber where the joist enters the wall.
How to check: Whether the movement is at the ends rather than in the middle. A joist end sitting in damp masonry is a different problem entirely.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates vibration from deflection in ten seconds, and the two have different causes, different remedies and different levels of concern.
- 2
It is the commonest thing missing and the cheapest thing to add, and it makes joists share load rather than each carrying its own footstep alone.
- 3
A notch near mid-span, or one cut deeper than guidance allows, removes a large share of a joist's stiffness in the place it can least afford it.
- 4
Most bouncy floors were not always bouncy. A wall taken out, a chimney breast removed, or a stone floor laid over boards are the usual events.
- 5
Mid-span is the joist; at the bearing is the support, and a soft or damp joist end is a decay question rather than a stiffness one.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the floor has become bouncier after building work, since something that was carrying it may have been removed and the load has gone somewhere.
- Where there is a visible dip, a gap opening at the skirting, or doors in the room have started to bind, all of which indicate real deflection rather than liveliness.
- Where joists have been notched or drilled deeply, particularly near mid-span, because the remedy is a repair to the member rather than stiffening the floor.
- Where any joist end is soft to a probe or damp where it enters the wall, which is decay at the bearing and is not a stiffness question at all.
- Where a heavy finish — stone, thick tile, a large bath — is planned over a floor that is already lively, since the load and the added mass both change the answer.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.