How the two differ in kind
Two different questions get asked of the same floor, and passing one says almost nothing about the other.
DEFLECTION is a static check: under a specified load, how far does the member sag? The answer is compared against a limit expressed as a fraction of the span, and the limits exist to protect things rather than people — plaster and ceilings that crack, doors and windows that bind, partitions that distort, and the visible sag of a long beam. Different limits apply to different loads and different finishes, which is why a member is checked against several at once.
VIBRATION is a dynamic check, and it is about perception. What people notice when they call a floor bouncy is not how far it sags under a piano; it is how it responds to a footfall — the frequency at which it wants to oscillate, how much it moves when excited, and how quickly that dies away. The governing quantities are natural frequency, effective mass and damping, and a floor with a natural frequency close to the rhythm of walking feels alive underfoot in a way that no static calculation reveals.
The two diverge most on exactly the floor people want: long span, lightweight, open plan, with few partitions. Depth gives stiffness, so it passes deflection; but the same floor has low mass and almost nothing damping it, so its response to a footstep is large and persists. That is the floor that satisfies every limit in the code and produces a complaint on the day the owner moves in.
The remedies differ accordingly. Deflection responds to stiffness, which mostly means depth. Vibration responds to frequency — also depth — but also to MASS and to DAMPING, neither of which appears in a deflection calculation. Which is why a heavier topping, a properly fixed ceiling, or blocking and strapping can transform a floor that already passed.
The factors that actually differ
Show
| Deflection limit | Vibration check | |
|---|---|---|
| What it measures | Static sag under a specified load, as a fraction of span. | Dynamic response to footfall — natural frequency, amplitude and how fast it decays. |
| What it protects | Finishes, ceilings, partitions, doors and appearance. | The occupant's perception. Nothing is damaged by a bouncy floor; people simply dislike it. |
| Governing quantities | Stiffness — the member's depth above all, since bending stiffness grows with the cube of depth. | Frequency, effective mass and damping. Mass and damping do not appear in a deflection check at all. |
| Where they diverge | Passes comfortably on a long-span lightweight floor. | Fails on the same floor, because low mass and little damping mean a large, persistent response. |
| Effect of adding mass | Makes it worse — more load, more deflection. | Usually makes it better, by lowering the response to a footfall of fixed energy. |
| Effect of a ceiling below | None worth counting. | Real. A fixed ceiling adds damping and couples the joists together. |
| Effect of partitions | Modest, and usually ignored. | Substantial. Partitions damp a floor considerably, which is why open-plan floors are the problem case. |
| Blocking and strapping | Little effect on a single member's deflection. | Helps, by sharing a footfall between joists instead of loading one. |
| Status in the code | Explicit limits, checked as a matter of course. | Often a serviceability recommendation rather than a hard limit, and frequently not checked at all on a small project. |
| When it is discovered | In design, because everybody checks it. | On handover, by the person walking on it — which is the worst possible time. |
Which one, and when
Choose deflection limit when…
- Checking a member against the code — this is the mandatory check and it is not optional.
- The concern is cracked finishes, binding doors, distorted partitions or visible sag.
- A roof member under snow or a long-term load, where deflection and creep are the issue.
- Sizing a beam or joist in the first place, which starts here.
Choose vibration check when…
- The span is long and the floor is lightweight — the combination that produces complaints.
- The layout is open plan with few partitions to damp it.
- The floor carries something sensitive: a laboratory bench, imaging equipment, a workshop.
- A floor already built feels bouncy despite having passed its deflection check.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How can a floor pass deflection and still feel bouncy?
- Because the two criteria measure different things. Deflection asks how far the floor sags when a static load is placed on it, and the limit protects finishes and appearance. Bounciness is a dynamic response: a footfall injects a brief pulse of energy, the floor oscillates at its natural frequency, and what people perceive is how large that oscillation is and how long it lasts. A floor can be entirely stiff enough to keep its sag within limits while having a natural frequency near the rhythm of walking, very little mass to resist being moved, and almost nothing to damp it — so each footstep produces a noticeable, lingering movement. Long spans, lightweight joists and open-plan layouts with no partitions push all three of those in the wrong direction at once, which is why the modern open floor is where this shows up.
- Why does adding weight help vibration but hurt deflection?
- Because the two respond to mass in opposite directions. Deflection is a static response to load, so more mass means more load and more sag — adding weight always makes deflection worse. Vibration is a response to an impulse of fixed energy: a footstep delivers roughly the same energy whatever the floor is made of, and a heavier floor moves less for that same energy, so the amplitude of the response falls. There is a competing effect, since added mass also lowers the natural frequency, and lowering it toward the walking range is unhelpful — which is why a heavy topping helps most where the floor's frequency is already comfortably above that range. It is also why the fix is designed rather than assumed: a screed topping is added for damping and mass, and its extra load is then checked back against the deflection limits.
- What actually fixes a bouncy floor?
- Depth first, then mass and damping — and on an existing floor the order reverses because depth is the hardest to change. In design, deeper joists raise the natural frequency and reduce the response, which is why the simplest answer to a long span is a deeper member rather than a stronger one. On a floor already built, the practical measures are adding a rigid ceiling fixed to the underside, which both damps and ties the joists together; adding blocking or strutting between joists, so a footfall is shared across several members instead of loading one; adding a topping for mass; and, where access allows, adding an intermediate support that halves the span, which is by far the most effective single change. Partitions built on the floor have the same damping effect, which is why a floor can improve once the building is fitted out.
- What deflection limits apply?
- Several at once, expressed as a fraction of the span and differing by what is being protected and by which load is acting. The pattern is consistent across codes even where the numbers differ: a tighter limit applies where brittle finishes such as plaster are attached, a looser one where they are not; live load alone and total load are checked separately, because a permanent sag matters differently from a sag that comes and goes; and long-term effects are allowed for in timber and concrete, where creep means the deflection under sustained load grows over years. Cantilevers carry their own limits, usually tighter relative to their projection. The local code is the authority, and the practical point is that a member has to satisfy every applicable limit rather than the one that happens to be easiest.
- Is vibration actually checked in practice?
- On significant projects, yes; on small ones, frequently not — which is why the complaint tends to arrive after handover rather than in design review. Deflection is a hard limit that every check covers automatically. Vibration is commonly a serviceability criterion with guidance rather than a pass-fail code requirement for ordinary residential floors, so a joist table that satisfies the code can be used without anyone considering how the floor will feel. Several timber engineering standards and span tables now include a vibration or springiness criterion precisely because of that gap, and following those rather than the bare strength-and-deflection tables is the simplest protection. Where the floor is long-span, lightweight or open plan, it is worth asking the question explicitly rather than assuming the standard check covered it.
- Does this apply to roofs as well?
- Deflection does and matters a great deal; vibration in the footfall sense does not, because nobody is walking on it in normal use. Roof deflection is checked against the same kind of span-fraction limits, with particular attention to two things. Ponding is the first: a flat or nearly flat roof that deflects allows water to collect at the low point, and that water is additional load, which causes more deflection, which collects more water — a progressive mechanism that has caused collapses. The second is creep in timber and concrete under long-term load, which means the sag measured on the day is not the sag in ten years. The dynamic questions on a roof are different ones: wind-induced movement, and plant vibration where equipment is mounted on the structure.
- What about a floor carrying sensitive equipment?
- It is a stricter version of the same dynamic problem and it is designed against numerical criteria rather than against perception. Laboratory balances, microscopes, imaging equipment and metrology instruments each have a tolerance for floor velocity or acceleration, usually expressed as a curve across a frequency range, and the floor is designed so that its response under the expected excitation stays below it. The excitation is not only footfall: nearby plant, traffic outside, and lifts all contribute. Meeting a strict criterion typically means a heavy stiff floor on short spans, isolation of the equipment itself, or both — and it is decided before the structure is designed, because retrofitting a sensitive area onto a long-span lightweight floor is expensive and sometimes not achievable.
- Which check should I run first?
- Deflection, because it is mandatory and because it usually sets the member size, then vibration as the check that can force that size up. The sequence matters on the floors where the two diverge: sizing a joist to the deflection limit and stopping produces exactly the long-span lightweight floor that then feels wrong, whereas running the vibration check afterwards identifies it at the point where a deeper joist costs a little more rather than a lot. If the vibration check is going to be skipped — as it often is on small projects — the cheap insurance is to design the floor a little deeper than the tables demand where the span is long and the layout is open, since depth is the variable both criteria respond to and it is the one that cannot be added later.