A screed is the levelling layer between a structural floor and its finish, and it is where underfloor heating pipes and floor insulation usually live. Getting the build up right matters because everything above it is finished work: a floor that is not level, not dry or not insulated cannot be corrected without lifting what is on top.
Sand and cement against liquid screed, and liquid screed cost per m3
A traditional sand and cement screed is laid and levelled by hand, is cheap in material, is slow to lay over a large area and needs a longer drying time before floor finishes go down. A liquid or flowing screed, usually calcium sulphate based, is pumped, self levels, goes down far faster, can be laid thinner over underfloor heating and conducts heat better, which is why it dominates where there is underfloor heating. It costs more per cubic metre and it has to be protected from moisture and, with some products, sanded before a finish is bonded to it.
Drying time is the thing that catches people
Screeds dry rather than simply cure, and the drying is measured in weeks rather than days, longer for thicker sections. Laying a floor finish onto a screed that is still wet is the commonest cause of a failed floor: adhesives fail, timber cups, vinyl lifts. The reliable approach is a moisture test rather than a date in a programme, and it is the item most likely to be compressed when a project is running late.
What goes under it, including underfloor insulation
In a modern ground floor: hardcore, a sand blinding, a damp proof membrane, rigid insulation, then the screed, with edge insulation isolating it from the walls so heat does not escape at the perimeter and the screed can move. The building regulations set the performance the floor has to achieve when it is newly built or renovated, which is what drives the insulation thickness rather than a fixed number. In a suspended timber floor, insulation goes between the joists supported on netting or battens, with the void below kept ventilated.
How much to replace floorboards, and floor sanding cost against it
Replacing floorboards is priced on area, the board type and how much of the structure is exposed once they are up, and on a period house matching reclaimed boards costs more than new timber. Sanding an existing timber floor is much cheaper than replacing it and is the right answer far more often than people assume, provided the boards have enough thickness left above the nail heads to take a sanding. Both are dusty disruptive jobs best done before anything else is finished.
Questions people ask about floor screed
How long does screed take to dry?
Weeks rather than days, and longer for thicker sections. Rely on a moisture test rather than a date, because laying a finish on a damp screed is the commonest cause of a failed floor.
Is liquid screed better than sand and cement?
It goes down faster, self levels, can be thinner over underfloor heating and conducts heat better. It costs more and needs protecting from moisture. For underfloor heating it is usually the better answer.
How thick should floor insulation be?
It follows the performance the floor has to achieve under the building regulations rather than a fixed figure, so it depends on the insulation used and whether the floor is new or renovated.
Is sanding a floor cheaper than replacing it?
Considerably, and it is the right answer more often than people assume, provided there is enough thickness left above the nail heads.