Specification
Depth decisions across bonded, unbonded and floating screeds
A comparative look at how bonded, unbonded and floating build-ups pull depth in different directions, and how to reconcile them on one project.
Depth is rarely decided in isolation from build-up type. A bonded screed direct to a concrete slab behaves very differently from a floating screed sitting on rigid insulation, and the depth that suits one would be wrong for another. Specifiers who move between refurbishment and new-build work often need to hold several logics in their head at once.
On a single project it is common to see all three build-ups in play — a bonded screed in a hallway where floor build-up is tight, a floating screed over insulation in the main living areas, and an unbonded screed over a damp-proof membrane in a ground floor extension. Getting the depth right in each zone, and getting the transitions between zones right, is where much of the specification work actually sits.
This page draws the three approaches together so the trade-offs are visible in one place, rather than scattered across separate system descriptions.
Why the same screed needs different depths
The screed itself does not change between build-up types — what changes is what lies beneath it and how load is transferred. A bonded screed relies on adhesion to a sound, prepared substrate, so the substrate does much of the structural work and the screed depth can be modest. Remove that bond, or introduce a compressible insulation layer, and the screed has to do more of the work itself, which generally means more depth.
This is why a single 'standard depth' answer rarely holds across a project. Depth should follow from build-up decisions, not be fixed first and build-ups fitted around it.
Coordinating depth where build-ups meet
Where a bonded zone meets a floating zone — for example, a hallway transitioning into an insulated living space — the finished floor level must still line up, even though the total build-up depth differs. This usually means adjusting the insulation thickness, the slab preparation, or introducing a step that is deliberately designed rather than accidental.
Getting this coordination onto the drawings early, with actual dimensions rather than generic notes, avoids surprises when the floors are poured and avoids the installer having to make level decisions on site that should have been made at design stage.
Drying time differs by build-up
A bonded screed over a solid slab typically dries faster than a floating screed sitting on insulation, simply because heat and air movement reach it more directly from below as well as above.
If the programme assumes one drying figure for the whole ground floor, the floating zones in living areas are the ones most likely to still be damp when the flooring contractor turns up.
Depth by build-up type — the points that decide it
- Bonded screeds can generally be specified thinner because the substrate provides direct structural support.
- Unbonded screeds need enough depth to behave as an independent slab over the membrane.
- Floating screeds over insulation typically need the greatest depth of the three, as they must span a compressible layer.
- Mixing build-up types on one floor requires careful coordination of finished floor levels.
- The choice of build-up is often driven by damp-proofing and thermal requirements, with depth following as a consequence.
- Liquid screed's self-levelling nature makes it easier to hold consistent depth across mixed build-ups than some traditional materials.
- Structural engineer input matters most where floating build-ups add significant dead load to a slab.
Comparing build-up options for your project?
Talk through bonded, unbonded and floating options with our Exeter team before you commit to a floor build-up.
Or call our landline on 01392 237700
Depth by build-up type — your questions
In short
A comparative look at how bonded, unbonded and floating build-ups pull depth in different directions, and how to reconcile them on one project.
Comparing build-up options for your project?
Talk through bonded, unbonded and floating options with our Exeter team before you commit to a floor build-up.