What is it?
Screed is a thin layer of cement-based material laid over a concrete slab (or over insulation, or over underfloor heating pipework) to provide a flat, level surface ready for the final floor finish.
- 1Screed
- 2Underfloor heating pipework
What does it do?
It levels out the top of the slab, which is rarely flat or accurate enough on its own to take a finish like tiling or timber directly, and provides the wearing surface floor tiles, timber, or carpet are laid onto. Where underfloor heating pipes are buried within it, the screed also acts as the thermal mass that stores and evenly spreads the heat from the pipes into the room above.
Where is it used?
Ground floors and upper floors alike, wherever a level surface is needed above a structural slab and before the final floor finish goes down.
Types
Screed is generally either bonded (fixed directly to the slab below, used where floor build-up depth is tight) or unbonded/floating (laid over a separating layer, insulation, or underfloor heating, so it isn't fixed to the structure beneath and can move independently of it). A floating screed needs to be thicker than a bonded one to span without cracking, since it isn't relying on the slab underneath it for support.
How to specify
The choice between bonded and floating screed follows directly from the floor build-up: if there's insulation or underfloor heating pipework beneath the screed, it has to be floating, since nothing bonds it to the slab. Thickness is set from that choice, a floating screed needs more depth than a bonded one, and needs coordinating early, since it affects the overall floor build-up height and, at a doorway, the level the threshold has to resolve.
Other considerations
Screed needs time to dry before a moisture-sensitive floor finish (timber, some vinyls) goes down on top of it. Rushing this is a common cause of finish failures. Movement joints may be needed in larger areas to stop the screed cracking as it dries and shrinks.
Interacting elements
- Concrete slab: the screed is laid over the slab, bonded or floating
- Rigid insulation: where present, sits beneath a floating screed
- Threshold strip: the screed's level at the doorway is what the threshold detail has to resolve