Bonded screed specification
Choosing the right depth for a bonded screed build-up
Bonded build-ups rely on adhesion to the base, so depth selection is governed by substrate condition and finished-floor tolerance rather than insulation thickness.
A bonded screed is laid directly onto a prepared concrete base with no separating membrane, relying on mechanical and chemical bond to behave monolithically with the substrate.
Because there is no membrane to accommodate movement, depth is usually kept thin and is driven by the condition of the base, the required finished level, and the manufacturer's minimum for the chosen screed.
For a bonded pour, the risk sits almost entirely at the interface: laitance, oil staining or a poorly scarified surface will undermine adhesion long before the numeric depth becomes the limiting factor, so preparation should be inspected and signed off before any screed is mixed.
Depth for bonded build-ups — the points that decide it
- Bonded screeds are typically specified thinner than unbonded or floating build-ups because the base contributes to overall stiffness.
- Substrate preparation, usually shot-blasting or scarifying to remove laitance, is essential for reliable bond and should be specified as a clause, not assumed.
- Minimum depth is set by the manufacturer's datasheet for the specific product and should never be reduced without checking it.
- Depth should be checked against any existing floor finishes, thresholds and door clearances before committing to a figure.
- A bonding agent or slurry coat may be specified depending on the base and the screed manufacturer's system.
- Areas of the slab with variable levels may need a locally thicker bonded layer, which should be flagged on the drawing rather than left to site discretion.
- Bonded build-ups are less forgiving of substrate defects than floating systems, so a condition survey of the base is a sensible precursor to specification.
Why bonded depths are kept lean
Because the bonded layer acts compositely with the slab beneath it, it does not need to carry loads independently in the way a floating screed does. This allows a thinner specified depth, provided the base itself is structurally sound and reasonably flat.
That said, going thinner than the manufacturer's stated minimum for a bonded application risks pinholing, curling at edges, and reduced abrasion resistance, so the datasheet minimum should be treated as an absolute floor, not a target.
Setting the depth alongside finished levels
Depth for a bonded screed is often worked out backwards from the finished floor level required at doorways, skirtings and adjoining rooms, then checked against the base level survey to confirm the resulting layer thickness sits above the manufacturer's minimum across the whole area.
Where the base has significant level variation, a bonded specification may not be appropriate at all, and an unbonded or floating build-up with a separating layer might handle the variation more predictably.
Handover timing on a bonded pour
A bonded screed can dry faster than other build-ups because moisture can also escape through the substrate, but only if the base itself isn't holding residual water from earlier trades.
Naming who keeps the site heated and ventilated during curing stops responsibility drifting between trades once the screeder has left site.
In short
Bonded build-ups rely on adhesion to the base, so depth selection is governed by substrate condition and finished-floor tolerance rather than insulation thickness.
Unsure whether a bonded build-up suits your base?
Send us details of the substrate and we can talk through whether a bonded, unbonded or floating specification fits the project.
Or call our landline on 01392 237700
Depth for bonded build-ups — your questions
Unsure whether a bonded build-up suits your base?
Send us details of the substrate and we can talk through whether a bonded, unbonded or floating specification fits the project.