Floor screed glossary

Bay · Bonded screed · BRE screed tester · Calcium sulphate screed (anhydrite) · Carbide method · Cement-based liquid screed · Commissioning (heat-up) · Curling · D49 mesh · Damp-proof membrane (DPM) · Edge strip · Floating screed · Floor zone · Foamed glass gravel · Force drying · Geotextile · Hemihydrate screed · In situ crushing resistance · Laitance · Levelling screed · Limecrete · Natural hydraulic lime (NHL) · Radon membrane · Relative humidity (RH) · Screed · Slip membrane · Standby radon sump · Strength class (e.g. CT-C20-F4) · U-value · Unbonded screed · Wearing screed

Bay

An area of screed between joints. Smaller over underfloor heating: 40 m² for Roadstone FlowPlus, 300 m² for Gyvlon.

Bonded screed

A screed bonded to the slab, which is prepared mechanically for the bond. Sand and cement: 25 mm minimum, 40 mm normally specified.

BRE screed tester

A 4 kg weight dropped 1 m onto an anvil on the screed four times; the dent shows the screed's resistance.

Calcium sulphate screed (anhydrite)

A flowing screed on a calcium sulphate binder. Needs sanding, testing and priming before most floor finishes.

Carbide method

A moisture test on a sample of the screed. Roadstone gives less than 75% RH, about 2% by the carbide method, for FlowPlus before wood or vinyl.

Cement-based liquid screed

A flowing, self-levelling screed on a cement binder, such as Roadstone FlowPlus or Cemfloor.

Commissioning (heat-up)

Bringing underfloor heating up slowly for the first time and back down, before any floor finish goes on.

Curling

An upward deformation of the edges of a screed (MPA).

D49 mesh

A reinforcing mesh. Gyvlon's comparison lists D49 mesh or fibres as required in a floating sand and cement screed.

Damp-proof membrane (DPM)

A membrane under a ground floor that stops ground moisture rising. Roadstone asks for one below FlowPlus on a ground floor.

Edge strip

A compressible strip round every wall and column, so a floating screed can move. Gyvlon asks for at least 8 mm, 10 mm with underfloor heating.

Floating screed

A screed on thermal or acoustic insulation, completely separated from walls and pipes. Sand and cement: 65 mm in housing, 75 mm otherwise.

Floor zone

The depth between the slab and the finished floor level: insulation, screed and floor finish.

Foamed glass gravel

Insulating aggregate made from recycled glass, laid loose under a limecrete slab. Tŷ-Mawr gives 0.078 W/mK.

Force drying

Using the underfloor heating, or heaters and dehumidifiers, to dry a screed faster. Gyvlon allows it from 7 days.

Geotextile

A breathable fabric membrane between the ground, the insulating aggregate and the limecrete slab.

Hemihydrate screed

A calcium sulphate screed made with a hemihydrate binder, such as Ultraflo.

In situ crushing resistance

How well a levelling screed resists being crushed by loads and traffic. Tested with the BRE screed tester.

Laitance

A surface skin that can form on some screeds. Gyvlon says calcium sulphate screed must be sanded to remove it before bonded floor finishes.

Levelling screed

A screed finished to a set level to receive the final flooring (BS 8204-1).

Limecrete

A floor slab of natural hydraulic lime and aggregate, used in old solid-wall houses.

Natural hydraulic lime (NHL)

The lime binder in limecrete and lime screeds.

Radon membrane

A gas-proof membrane under the floor, required in new buildings in High Radon Areas since 1 July 1998 (EPA).

Relative humidity (RH)

How damp the air over the screed is, measured under a hood. 75% RH is the usual limit before floor finishes.

Screed

A layer of screed material laid in situ on a base, bonded or unbonded, or on insulation, to reach a set level, carry the floor finish or give a wearing surface (BS EN 13318, quoted by the MPA).

Slip membrane

A sheet, usually polythene, between an unbonded screed and the slab, or between a floating screed and the insulation.

Standby radon sump

Pipework from under the floor to outside, required in every building since 1 July 1998. A fan is added if radon levels are high (EPA).

Strength class (e.g. CT-C20-F4)

The BS EN 13813 label for a screed: C is compressive strength and F flexural strength, in N/mm². Roadstone FlowPlus is CT-C20-F4.

U-value

How much heat passes through a floor, in W/m²K. SEAI's target where underfloor heating goes into a refurbished floor is 0.15.

Unbonded screed

A screed on a separating layer, or on a slab not prepared for bond. Sand and cement: 50 mm minimum, 70 mm specified.

Wearing screed

A screed that serves as the floor surface itself.

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); EPA, Radon advice for builders; SEAI Domestic Technical Standards and Specifications (v4.0, July 2026); Tŷ-Mawr, What is the Sublime floor (22 March 2024). Checked 29 October 2026.