Liquid screed glossary
The words on liquid screed quotes, in plain English.
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Acoustic Flanking Isolation
Compressible polyethylene foam strip fitted around room perimeters and structural columns to decouple a floating liquid screed from walls and prevent impact sound transmission.
Alpha Hemihydrate Binder
High-purity calcium sulphate binder used in pumpable flowing screeds, offering low shrinkage, high flexural strength and rapid thermal transfer over underfloor heating pipes.
Anhydrite
A form of calcium sulphate used as the binder in many liquid screeds.
Bay
The area poured as one piece between joints.
Bonded screed
A screed stuck to a prepared concrete slab. The thinnest kind.
BS 8204-7
The UK code of practice for pumpable self-smoothing screeds, which cement-based liquid screed and hemihydrate liquid screed are laid to.
BS 8204-7 Pumpable Self-Smoothing Screed
The standard governing the design, mixing, installation thickness and surface regularity of pumpable self-smoothing flowing floor screeds.
Calcium sulphate screed
A liquid screed with a calcium sulphate binder: anhydrite or hemihydrate. Often called gypsum screed.
Carbide Method Moisture Test (CM Test)
On-site destructive moisture test where a weighed screed sample is crushed with calcium carbide reagent in a sealed pressure vessel to measure residual moisture as a percentage of dry weight.
Carbide test
A moisture test on a drilled sample sealed with calcium carbide; cement-based liquid screed: about 2% for moisture-sensitive floors.
Cement-based liquid screed
A flowing screed with a cement binder and admixtures.
Commissioning (heat-up)
Bringing underfloor heating up and down slowly before flooring: up 5°C a day, held, then down.
Cover
The depth of screed over the heating pipes: at least 25 mm for liquid screeds.
CT-C20-F4
An I.S. EN 13813 strength class: cementitious screed, 20 N/mm² compressive, 4 N/mm² flexural.
Dapple Bar
T-shaped tubular metal levelling bar passed across freshly pumped liquid screed in two perpendicular passes to release trapped air and bring the surface to a flat, self-smoothing plane.
Dappling
Pushing a dappling bar through wet liquid screed, in two directions, to release air and level the surface.
Floating screed
A screed laid on insulation, separated from the walls by a perimeter strip.
Flow Cone Test
Site quality check where freshly batched liquid screed is poured into a calibrated ring on a dry glass plate to verify a spread diameter between 220 mm and 260 mm before pumping.
Force drying
Drying a screed with heat and dehumidifiers. From day 7 on calcium sulphate.
Force Drying Commissioning Protocol
Controlled heat-up and cool-down cycle described in EN 1264-4 for underfloor heating manifolds, starting at 20 to 25 degrees Celsius flow temperature after initial cure.
Hemihydrate
Another form of calcium sulphate. Hemihydrate liquid screed forms no laitance.
Hygrometer
A meter that reads the relative humidity of air in a sealed hood on the screed.
Hygrometer Hood Test (BS 8203)
Insulated relative humidity box sealed to the top of a cured screed for 72 hours to confirm equilibrium relative humidity is below 75 percent prior to bonding resilient floor coverings.
I.S. EN 13813
The Irish edition of the European standard for screed materials, from NSAI.
Laitance
A weak, dusty skin of fines on the surface of a calcium sulphate screed, sanded off before flooring.
Laser Tripod Datum
Adjustable steel tripods set across the prepared subfloor using a rotating laser level to give the pump crew exact finished floor depth markers during the pour.
Movement Joint Bay Division
Pre-formed expansion profile installed across doorways, between independent heating zones and where aspect ratios exceed 2:1 to accommodate thermal expansion without cracking.
Perimeter strip
Closed-cell foam round the walls that lets the screed move. 8 mm, or 10 mm with heating.
Polyethylene Slip Membrane
Minimum 500-gauge polythene separating sheet taped at all overlaps and turned up against perimeter edging so liquid screed cannot leak down between foil-faced insulation boards.
Relative humidity (RH)
The moisture measure for screeds: 75% RH is the usual limit before flooring.
Skin Sanding & Laitance Removal
Mechanical rotary sanding carried out 7 to 10 days after pouring a calcium sulphate screed to remove weak surface fines and open the pore structure for faster drying and primer adhesion.
Surface regularity (SR1, SR2, SR3)
Flatness classes in BS 8204: 3, 5 and 10 mm under a 2 m straightedge.
Thermal conductivity
How well heat passes through, in W/mK: calcium sulphate about 2.2, sand and cement 1.1.
Thermal Conductivity (Lambda Value)
Measure of heat flow through cured screed in W/mK; dense flowing screeds typically reach 1.8 to 2.5 W/mK because the liquid matrix encapsulates the full circumference of the heating pipe.
Unbonded screed
A screed laid on a membrane over the slab, not stuck to it.
Unheated Floating Depth
Minimum nominal screed thickness over compressible floor insulation when no underfloor heating pipes are present, typically 35 to 40 mm for domestic residential loadings.
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