Short answer: a bonded screed is stuck directly to a prepared concrete slab, an unbonded screed is laid on a membrane separating it from the slab, and a floating screed is laid on insulation. Each needs a different minimum depth: typically 25 to 40 mm bonded, 50 mm unbonded and 65 mm floating for a sand and cement screed in a home.
Bonded screed
The slab is prepared by mechanical scabbling or grinding, cleaned, then primed with a bonding agent so the screed and slab act as one. Because the slab carries the load, the screed can be thin. It suits level corrections on existing concrete and commercial floors where depth is limited. Poor slab preparation is the main risk: the screed can lose its bond and sound hollow.
Unbonded screed
A damp-proof membrane or separating layer sits between slab and screed. The screed must carry itself, so it needs more depth. It is used where the slab is contaminated, damp-proofing is needed, or a bond cannot be relied upon.
Floating screed
The usual build-up in extensions and new builds. Insulation sits on the slab, then a slip membrane, then the screed, often with underfloor heating pipes. The screed spans the compressible insulation, so it needs the most depth and an edge strip at every wall so it can move without cracking.
At a glance
| Type | Sits on | Typical minimum (sand and cement) | Common use |
|---|---|---|---|
| Bonded | Prepared, primed slab | 25 to 40 mm | Levelling existing concrete |
| Unbonded | Membrane | 50 mm | Over damp-proofing or contaminated slabs |
| Floating | Insulation | 65 mm (75 mm heavier loads) | Extensions, new builds, underfloor heating |
More detail in our screed thickness guide and on our sand and cement screed page.
Written by Ben Phillips, director of Chelmsford Floor Screeding Ltd, floor screeding contractors based in Billericay with 20+ years of experience across Essex and London. More about us.