Screed types · checked 29 October 2026
Bonded, unbonded and floating screed explained
What the screed sits on decides how thick it has to be. Here's how to tell which your floor needs.
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Figures checked against each source on 29 October 2026. How we check
A screed is bonded, unbonded or floating depending on what it sits on. Bonded screed grips the slab. Unbonded screed sits on a membrane and isn't stuck down. Floating screed sits on insulation and is kept apart from the walls and pipes.
The construction sets the minimum depth. For sand and cement the BS 8204 figures are 25 mm bonded, 50 mm unbonded and 65 mm floating in a house.
The three, defined
| Construction | What it sits on | Main risk | Sand and cement minimum |
|---|---|---|---|
| Bonded | The slab, prepared mechanically so the screed grips it | Losing the bond if the slab isn't prepared | 25 mm, 40 mm normally specified |
| Unbonded | A separating layer, or a slab not prepared for bond (contaminated, or with a damp-proof membrane on top) | Curling and hollowness at joints, cracks and edges | 50 mm, 70 mm specified |
| Floating | Thermal or acoustic insulation, completely separated from walls and pipes | Needs firm, flat insulation (Cemfloor asks for at least 100 kPa) | 65 mm in housing, 75 mm otherwise |
Sources: Mineral Products Association, Mortar Data Sheet 22: Screeds (Issue 3, October 2023); Cemfloor Complete technical data sheet (2023). Checked 29 October 2026.
Liquid screeds go thinner
| Construction | Gyvlon (calcium sulphate) | Cemfloor (cement liquid) | Roadstone FlowPlus |
|---|---|---|---|
| Bonded | 25 mm | 20 mm | – |
| Unbonded | 30 mm | 30 mm | 60 mm on slip membrane |
| Floating, domestic | 35 mm | 35 mm | 45 mm |
| Floating, commercial | 40 mm | 40 mm | 60 mm |
Sources: Gyvlon flowing screed general data pack (Anhydritec); Cemfloor Complete technical data sheet (2023); Roadstone FlowPlus cement liquid screed brochure (RS.BRO-19 Rev 01, March 2022). Checked 29 October 2026.
Which one your floor needs
- Over insulation or underfloor heating: floating. That's most new houses and extensions.
- Over a sound slab, with little height to spare: bonded. The MPA calls it the preferred method for heavy use or a rigid flooring, with the least risk of cracking, curling and hollowness.
- Over a slab with a damp-proof membrane on top, or a contaminated slab: unbonded.
- Over an old floor in a period house: see lime screed before choosing any of these.
Source: Mineral Products Association, Mortar Data Sheet 22: Screeds (Issue 3, October 2023). Checked 29 October 2026.
Edges
A floating screed has to be completely separated from the walls, columns and pipes, which is why an edge strip goes round the room before the screed. Roadstone asks for edge strips at all walls and columns; Gyvlon for at least 8 mm, or 10 mm with underfloor heating.
Sources: Mineral Products Association, Mortar Data Sheet 22: Screeds (Issue 3, October 2023); Roadstone FlowPlus cement liquid screed brochure (RS.BRO-19 Rev 01, March 2022); Gyvlon flowing screed general data pack (Anhydritec). Checked 29 October 2026.
Questions people ask
What is a bonded screed?
A screed that is bonded to the slab. The slab is prepared mechanically so the screed grips. For sand and cement the MPA gives 25 mm minimum and 40 mm normally specified.
What is an unbonded screed?
A screed laid on a separating layer, such as a damp-proof membrane, or on a slab not prepared for bond. Sand and cement minimum 50 mm, 70 mm specified.
What is a floating screed?
A screed laid on thermal or acoustic insulation and completely separated from the walls and pipes. Sand and cement: 65 mm in housing, 75 mm otherwise.
Which is the strongest?
The MPA says fully bonded screeds have the least risk of cracking, curling and hollowness, and are preferred for heavy use or rigid floorings.
Sources (4)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Mineral Products Association, Mortar Data Sheet 22: Screeds: mortar.org.uk
- Cemfloor Complete technical data sheet: cdn.shopify.com
- Gyvlon flowing screed general data pack (Anhydritec): specifiedby.com
- Roadstone FlowPlus cement liquid screed brochure: roadstone.ie
Get your floor priced
Four taps about the job and one screeding contractor who covers your county rings you. Or call 087 162 1846, lines open 8am to 8pm.