What are the layers of a floor, and which one is causing my problem?
A floor is a stack of layers, and nearly every fault you can see in the top one was made in a layer below it. How to read the stack and find the layer that is actually at fault.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
From the bottom up: structure, damp barrier, subfloor deck, any levelling or overlay, underlay, then the covering you walk on. A symptom in the covering — squeaking, hollowness, lifting edges, bumps, cold — is almost always generated lower down. Identify the layer that failed before you spend money on the one you can see.
People replace floor coverings to fix problems that the covering did not cause, and then find the new floor develops the same fault within a year. The reason is that a floor is not a surface, it is a build-up, and each layer does a different job. Learn the six layers once and floor troubleshooting becomes systematic instead of guesswork.
Step by step
- Identify the structure.Either joists spanning between walls, with a void beneath, or a solid concrete slab on hardcore. This decides everything else — what can be laid, how damp behaves, whether the floor can deflect, and how sound travels. A few quick tests separate the two.
- Identify the damp barrier.On a slab it is a damp proof membrane, either a sheet within or under the concrete or a liquid coating on top. On a suspended timber floor there is no membrane at all: the barrier is moving air in the void, supplied through airbricks. Both fail in ways that look like a covering problem.
- Identify the subfloor deck.Floorboards, chipboard sheets, plywood, or the trowelled surface of a screed. This is the layer that decides flatness and rigidity. It is also the layer most floor complaints actually come from.
- Note any levelling or overlay layer.Self-levelling compound on concrete, or plywood or hardboard screwed over boards. This layer exists to correct the deck. If the deck was bad and no overlay was added, the covering will show it.
- Note the underlay.It takes up small irregularities, carries most of the acoustic and thermal performance, and sometimes carries the vapour barrier as well. Wrong or missing underlay produces noise, coldness and premature wear rather than an obvious defect.
- Then look at the covering.Laminate, engineered wood, solid wood, LVT, sheet vinyl, tile or carpet. This is the layer you can see and the one you will be tempted to blame.
- Read the fault downwards.Squeaking is a fixing between deck and joist. A hollow patch under tile is adhesive coverage. A bumpy vinyl is deck flatness. Lifting laminate edges are moisture or a pinched expansion gap. A cold floor is insulation and draught. In every case the layer to treat is below the one showing the symptom.
- Only then decide what to buy.If the fault is in the deck or the damp barrier, a new covering buys you a fresh-looking version of the same problem. Fix the layer that failed, then re-cover.
Tips
- Floating means the covering is not fixed to anything below it and moves as a single raft, which is why it needs a free gap at every wall, pipe and threshold. Fixed means nailed, glued or bedded down, and it takes the shape and the movement of whatever is beneath it.
- A doorway, a lifted threshold strip or the gap behind a removed skirting is usually the cheapest place to see the whole stack in section without taking anything up.
- When a fitter quotes for a floor, the honest ones quote for the preparation as well. A quote that is only for the covering and the labour to lay it is a quote that assumes your subfloor is perfect.
Questions people ask
How do I find out what is under my floor without wrecking it?
Lift a threshold strip at a doorway, take a skirting board off a short run, or lift one board in a cupboard. Any of those shows the deck, the underlay and often the structure, and all three are repairable in minutes.
Does every floor have all six layers?
No. A tiled floor bonded to a screed has no underlay and no overlay. Solid boards nailed straight to joists are structure and deck in one. The point of the list is to know which layers are present so you know which ones can fail.