Concrete and screed
Hard, flat and heavy, then unforgiving about moisture and about laitance. The surface is opened up, primed and levelled, and a bonded covering goes down onto it beautifully once it is dry.
The base under the covering
Every covering copies the floor it is laid on. A hollow reads through sheet vinyl within a week, a hump wears a bright line across a carpet, and a floating floor knocks over a dip until somebody lifts it. So the base is settled first, on its own line.
The base is whatever the covering lands on: a concrete slab, a sand and cement screed, floorboards, chipboard deck or a ply overlay. Its job is narrow. It has to be flat within tolerance, sound underfoot, dry, clean and free of anything that stops an adhesive taking. A covering fixes none of those faults, it repeats them.
What a straight edge finds
A two metre straight edge is laid across the room in several directions, then the gap under it is read with a rule. That gap is the whole argument.
Deviation is read at the worst point, not the average one. A floor can look flat across a room while a single dip at a doorway still shows through the finished covering. Boards are walked and listened to at the same time, since a squeak under the foot is a fixing that has let go, and it will still be there under the new floor.
| What the gap shows | What it means | What is done |
|---|---|---|
| A shallow dish | Slow settlement across a bay or a doorway. | Feather filler, taken out wide, then sanded back to nothing. |
| A sharp hump | A proud joint, a fixing, or old adhesive left standing. | Ground or scraped back until the straight edge sits down on it. |
| A general roll | The whole slab moves out of level across the room. | Self-levelling compound over a primer, poured and spiked out. |
| A soft patch | A screed that never gained strength, or a wet spot. | Cut out, made good, then left until it is hard and dry. |
| Movement underfoot | Loose boards or a deck that has stopped gripping its joists. | Refixed into the joist line, then a ply overlay where it is still lively. |
Getting a base ready, in order
The order matters more than the products. Filling a dusty floor wastes the filler, and levelling a damp one wastes the day. So the sequence runs one way, and each stage waits for the one before it.
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1
Strip back to the base
The old covering, the old underlay, the tack strip, the staples and the tape all come off. Adhesive ridges are scraped while they are still proud of the surface.
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2
Clean it, then read it
The floor is swept and vacuumed until a hand comes off it clean. Then the straight edge goes down and the gaps are written on the sheet, room by room.
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3
Fix what moves
Loose boards are screwed back into the joist, not nailed. A deck that still flexes takes a ply overlay, screwed at close centres across the whole room.
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4
Prime, then fill or level
Primer seals the surface and stops it drinking the compound dry. Small dips take a feather filler; a whole room out of level takes a poured compound.
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5
Wait, then check again
Compound is left until it is hard right through, not until it looks dry. The straight edge goes back down afterwards, then any last high spot is sanded off.
Where the survey finds more than a day of this work, the base is quoted on its own line, separately from the fitting.
Moisture, and why a floor waits
Moisture is the fault that hides. A slab can feel dry to the hand while it still holds water deep down, then that water finds the adhesive and lifts the covering.
A ground-floor slab with no barrier in it is checked before anything is bonded to it. A box is taped down onto the surface and left, then the reading is taken from inside the box rather than off the top of the slab. A fresh screed carries its own water as well, and that water leaves slowly, from the top down, at a rate the calendar does not hurry.
Where the reading is too high for the adhesive, the base takes a surface barrier first and the covering waits for it. Where the reading is fine, the work goes ahead the same day. Timber and engineered boards are treated the other way round: the packs are stood in the room until they settle to the heat and the moisture there, then they are opened and laid.
Underfloor heating
A heated base is brought up and back down before a covering is laid over it, so the screed has already moved once. The system then stays off while the floor goes down, then it is brought up a step at a time until it reaches its working heat. A covering is only laid over heating when its label allows it, and the underlay is chosen to pass the heat rather than hold it.
Three bases, three arguments
Floorboards
Lively, gapped and rarely flat across a whole room. Boards are refixed first, then a ply overlay carries a sheet covering; a floating floor can go straight over them with a board-grade underlay.
Chipboard deck
Flat from new, then vulnerable at the joints once it has been wet. Swollen edges are taken back, loose sheets are screwed down, then a thin overlay evens out what is left.
What the survey finds is written onto the job sheet with the rooms, so the covering choice is made against a base that is already understood. From there the work carries on to the covering page, then out to the edges page where the floor is closed at every doorway.