Screed is the layer between the structure and the floor you actually walk on. It goes over a slab, over insulation, or over underfloor heating pipes, and its whole job is to arrive at one flat level at the right height. Get the level wrong by fifteen millimetres and the doors bind, the thresholds do not line up and the kitchen units sit proud.
Most screeding around here is extensions and knock throughs: a rear extension that needs to meet the existing floor, or a through room where two different floors have to become one. The level gets set off the doorways and the existing finished floor before anything is mixed.
Setting the level before anything is mixed
The first job is deciding where the top of the finished floor has to be, and that is set by what meets it: the hall floor through the doorway, the threshold at the back doors, the bottom step of the stairs. From that, the screed depth follows, and so does whether there is room for the insulation the drawings show.
Datum marks go on the walls all round the room so the level is visible from anywhere in it while the screed is going down. It is a quick job that takes half an hour and it is the difference between a floor that meets the hall flush and one that has a lip across the doorway forever.
On a new slab that usually means a damp proof membrane, insulation boards butted tight and taped, and an edge strip round the perimeter so the screed is not locked to the walls. Where there is underfloor heating, the pipes are clipped down and pressurised before the screed goes over them, so a pipe that has been nicked shows up while it can still be got at.
Over an existing slab it is more often a question of what has to come off first. Old adhesive, bitumen and loose laitance all stop a screed bonding, and a bonded screed that has not bonded lifts in sheets a year later.
A traditional sand and cement screed is mixed on site, laid to the datum and ruled off. It is the right answer for smaller areas, for falls to a gully, and for anywhere a barrow cannot easily get to. It needs more depth than a flowing screed and it takes longer to reach a moisture content a floor covering will tolerate.
A flowing screed is pumped in and finds its own level. It goes down thinner, covers underfloor heating pipes better, and is much faster over a large open area. It costs more per square metre and it needs the room properly sealed at the edges first, because it will find any gap there is.
Screed is not ready because it is hard enough to walk on. It is ready when enough water has left it that a floor covering will not trap the rest. The rule of thumb on a traditional screed is a day per millimetre for the first fifty, then longer, which on a normal domestic depth is comfortably over a month.
That is worth knowing at the point the work is booked rather than the week the flooring is due. Where the programme cannot wait, a faster drying screed or a surface damp proof membrane is the conversation to have, and it is one to have before the screed goes down rather than after.
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Usually the next day on a traditional screed, sooner on a flowing one. Walking on it is not the same as loading it though, so the room stays clear of stacked materials and trestles for a good while after that.
How long before the floor covering goes down?
Longer than most people expect. A traditional sand and cement screed at a normal domestic depth is comfortably over a month before it is dry enough for vinyl or wood. It gets said at the estimate so the flooring is not booked for the wrong week.
Can you screed over underfloor heating?
Yes. The pipes get clipped down and pressure tested first so a damaged pipe shows up while it can still be reached, and the screed goes over them at the depth the heating system asks for.
Do I need insulation under it?
On a new floor, almost always, and the drawings will say what thickness. The thing to check early is whether the level you have to meet leaves room for both the insulation and enough screed on top of it.
