Liquid Anhydrite Screed: The Complete UK Guide for Extension Builders

Liquid anhydrite screed is the default choice for a UK kitchen extension with underfloor heating: it flows around the pipes without voids and pours fifteen to twenty times faster than sand-cement. It is also the screed type that punishes rushed programmes the hardest. Miss the laitance sand, switch the UFH on too early, or ignore the moisture test, and you will be smashing out the floor to redo it.

What it is and what it's for

Liquid anhydrite screed, also called calcium sulphate screed or flow screed, is a pumpable self-levelling floor screed that arrives in a mixer truck and is pumped through a hose into the extension. It flows around your underfloor heating pipes, finds its own level, and cures to a hard, flat surface. The binder is anhydrite (a dry form of calcium sulphate, chemical formula CaSO4) rather than the Portland cement used in traditional sand-and-cement screed.

The switch to anhydrite matters because it flows completely around the UFH pipes without voids, which a hand-laid sand-and-cement screed will never quite manage. It also conducts heat roughly 43% better (thermal conductivity of 2.0 W/mK vs 1.4 W/mK for sand-cement), so your heating system warms the floor faster and uses less energy doing it. And the surface comes out flat enough for tile, LVT, or engineered wood to go straight on top once it's cured.

The British Standard governing it is BS 8204-7 ("Screeds, bases and in-situ floorings: Pumpable self-smoothing screeds"). NHBC 9.3.4 covers the installation requirements for new-build and warranty work. Between them, they set the minimum depths, bay sizes, and sequencing that your screed specialist should be following.

The main UK products are Gyvlon (Anhydritec), Gypsol (LKAB Minerals), and a handful of cement-based alternatives that are increasingly sold under the same "liquid screed" banner: Cemfloor (Holcim), Longfloor, and Tarmac TopFlow. The cement-based variants behave differently in one important way, covered further down, and if your screeder defaults to one of them it's worth understanding why.

Anhydrite vs sand-cement: when each one wins

Sand-cement screed is cheaper per m², but that's close to the only column it wins in once UFH is involved. The depth, flatness, speed, and thermal performance of anhydrite are all better for an extension with underfloor heating.

The thinner build-up over UFH is what makes anhydrite the default for extensions. Knocking 25-30mm off your floor build-up matters when you're trying to fit the finished floor level of the extension to the existing house without a step. And the flat finish matters because SR1 is what large-format porcelain or engineered wood really needs. A sand-cement screed at SR3 will cost you another layer of self-levelling compound before any floor finish can go down.

The sand-cement case is strongest in two places: wet rooms (which anhydrite can't safely take) and very small areas under 25m² where the mobilisation cost of a mixer truck and pump makes liquid screed uneconomic. See sand-and-cement screed for when the traditional mix is the right call.

Types and variants

"Liquid screed" is a trade category, not a single product. The key split is binder type, and it changes what happens after the pour.

Cement-based liquid screeds (Cemfloor, Longfloor, TopFlow C) pour the same way as anhydrite and give similar flatness, but they don't produce laitance, they tolerate moisture after cure, and they dry faster. The trade-off is slightly higher material cost and, on some products, tighter bay sizing. Experienced UK screeders are increasingly specifying cement-based by default for domestic extensions where the laitance removal and wet-room limits of anhydrite are a hassle.

If your screeder quotes "liquid screed" without telling you which product, ask. The aftercare sequence, adhesive compatibility, and drying programme all depend on the answer.

How to work with it

You won't be pouring this yourself. Liquid screed arrives in a mixer truck, is pumped through 50-100m of hose, and needs three trained people on the pour to manage the flow, levels, and dappling. Every UK source that covers DIY pours of liquid screed says the same thing: don't. The material cost is too high for practice runs, and you only get one shot over UFH pipes.

What you do need to understand is what has to happen before the pour, on the day, and afterwards. Missing any of these steps is how extensions end up with cracked screed or lifted floor finishes.

Before the pour

The base must be ready. That means:

• Insulation laid and fully supported (no voids under the boards or the screed will sink into them and develop hollows)
• DPM installed, all joints lapped and taped, turned up at the perimeter
• UFH pipes clipped down and pressure-tested at a minimum of 6 bar for an hour, then held at working pressure right through the pour
• 10-15mm compressible perimeter expansion strip fitted to every wall, column, upstand, and threshold
• Level datums set from a laser around the room so the pump team knows the target depth
• Bay joints in place for pours over 40m² or longer than 8m in any direction (NHBC 9.3.4 requirement)
• Air temperature between 5°C and 30°C, no rain forecast if there's any chance of wind-blown water getting in

The UFH pressure test is the step homeowners most often skip asking about. If a pipe is nicked when it's clipped down, you won't know until the system is filled. Clipping a live, pressurised system at 6 bar means any damage shows as an immediate pressure drop, fixable before the truck arrives. Clipping an empty system and filling after the pour is how people end up breaking out their entire screed.

On the day

The pour itself takes surprisingly little time. A 50-80m² extension can be poured, levelled, and dappled inside an hour once the truck is set up. The team uses a long aluminium dappling bar (a T-shaped tool that looks like a magnetic sweeper) to work the screed and release air, then walks out leaving a mirror-flat surface.

Protect the freshly poured screed from direct sunlight and draughts for the first 48-72 hours. Don't cover it with polythene, because sealed covers trap condensation which damages the surface. Simply close the windows and doors on the sun side and keep the space shaded.

After the pour

Day 1-2: leave it alone. Foot traffic at 24-48 hours only if absolutely necessary, and only in clean soft-soled footwear.

Day 3 onward: open windows on opposite sides of the room for cross-ventilation. The screed needs air movement to dry, but not draughts while it's still green.

Day 5-7: laitance sanding (anhydrite only; skip this section for cement-based products).

Day 7 minimum: UFH commissioning can begin.

Week 4-8: moisture test when the UFH commissioning cycle completes.

Laitance: the step that ruins most anhydrite floors

Laitance is a thin, dusty layer of fine calcium sulphate particles that floats to the surface as the screed cures. It forms a weak skin, roughly 1-2mm thick, and it's the single biggest reason anhydrite screeds fail under floor finishes.

Three things go wrong if it isn't removed:

1. The surface is too weak to bond anything to. Your tile adhesive or LVT glue sticks to the laitance, and the laitance lifts off the screed underneath.
2. The laitance seals the surface, slowing evaporation. Your screed takes longer to reach moisture test threshold.
3. If cement-based tile adhesive goes on over laitance, the cement reacts with the calcium sulphate to form ettringite crystals at the interface. Ettringite expands as it grows. Tiles lift, grout lines open, and the whole installation fails.

When to sand

Target day 5-7 after the pour. The laitance is soft enough to sand off easily, and the screed below is hard enough to walk on without marking. Outer limit is day 10-14. After that the laitance has bonded hard and you're grinding rather than sanding, which takes far longer and costs more.

Do not wait until the screed is fully dry to sand. By then the laitance is set, and some sanding contractors will refuse the job or double the quote.

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How to sand

The contractor uses an orbital floor sander with integrated dust extraction, fitted with 60-grit sandpaper or a carborundum disc. They sand the whole surface lightly to expose the denser screed below, then vacuum everything. Dust extraction is non-negotiable: calcium sulphate dust contains respirable silica, and DIY sanding without proper extraction is a serious occupational health risk. This is a job for a professional with the right kit.

Laitance sanding is usually quoted as a modest per-m² rate when charged separately, and many screed contractors bundle the first sand into their package (they return at day 7 for the visit). A second sand is sometimes specified right before adhesive goes down, once the floor is fully dry, to refresh the surface key.

How much do you need

Depth is set by the installation type and whatever is beneath the screed. For an extension with UFH on insulation (the normal case), 50mm over the top of the UFH pipe is the standard call. Some thermally optimised products are approved at 41mm cover, but the mainstream specification your screeder will quote to is the standard figure.

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Quantities follow from depth. One m² at 50mm is 0.05m³ of screed. For a typical 60m² kitchen extension at 50mm depth, the order is 3m³. Add 3-5% for depth variation and pump loss, and your screeder will book 3.2m³ from the mixer truck.

Weight is the hidden constraint on upper floors. Liquid screed at 50mm weighs roughly 100-120kg/m². A 60m² floor at that depth is 6-7 tonnes sitting on the joists. For a ground-floor extension poured onto insulation over a concrete slab, the loading is absorbed by the slab and the ground below. For a first-floor extension (a loft conversion pouring screed over a new timber structure, for example), this weight is a structural question for your engineer, not your screeder. Lightweight calcium sulphate variants exist at around 43kg/m² but are rarely stocked off the shelf; they need ordering specifically.

Drying: the programme killer

This is where extension schedules collapse. The drying rate for anhydrite is:

• 1mm/day for the first 40mm of depth
• 0.5mm per day for every millimetre above 40mm

Under standard conditions, which means 20°C and 65% relative humidity in the space, maintained continuously. UK extensions in autumn and winter rarely hit those conditions without active dehumidification.

The maths comes straight from the standard drying curve. A 50mm pour needs 40 days for the first 40mm (at 1mm/day) plus another 20 days for the final 10mm (at 0.5mm/day), giving 60 days total. That's two months of your build programme sitting between screed and floor finish.

Force-drying with the UFH system itself, starting at day 7 minimum, is how most extensions recover a workable programme. The commissioning cycle (next section) roughly halves the drying time. Dehumidifiers placed in the room help further, but they're a supplement, not a replacement, for the UFH cycle.

UFH commissioning protocol (BS EN 1264-4)

The UFH commissioning cycle does two jobs: it drives residual moisture out of the screed using heat, and it thermally conditions the screed before normal operation begins. BS EN 1264-4 defines the sequence.

Pre-conditions

• Screed must be minimum 7 days old before the heat goes on. This is the single non-negotiable rule: turning UFH on earlier is the fastest way to crack a screed through thermal shock.
• Laitance sanded and vacuumed (anhydrite only).
• UFH system filled and pressure-tested at 6 bar minimum.
• Heat source (boiler or heat pump) fully installed and operational.
• Commissioning certificate ready to be completed and signed.

The ramp

1. Day 1-3: Set flow temperature to 25°C. Run for 3 days.
2. Day 4 onward: Raise flow temperature by 5°C per day.
3. Hold: Reach maximum commissioning temperature of 45°C (or up to 50°C if the manufacturer's product data sheet allows). Hold for 7 days.
4. Ramp down: Reduce flow temperature by 5°C per day back to starting temperature.
5. Cool: Switch UFH off completely. Allow the floor to cool for 48 hours.
6. Test: Moisture test using a box hygrometer sealed to the floor surface, with 72-hour equilibrium period.
7. Repeat if needed: If the moisture test fails (above the 75% RH threshold), run an abbreviated cycle and retest.

Total duration from commissioning start to moisture result: roughly 25 days, or three and a half weeks.

A common mistake is jumping the ramp rate. More than 5°C per day means the surface of the screed expands faster than the body below it can accommodate, and you get surface crazing or hairline cracks over the UFH pipe runs. These cracks are impossible to repair without breaking out the screed.

BS EN 1264-4 also sets a maximum floor surface temperature of 29°C in living areas (27°C under tiles) for normal operation. This is an operational limit, not a commissioning limit. During commissioning you run the flow temperature above normal to drive moisture out; the floor surface temperature stays below the operational limit because the screed is still absorbing heat.

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Moisture testing before floor finishes (BS 8203)

You cannot rely on how the floor looks. A screed at 85% RH feels bone dry to the touch and looks indistinguishable from one at 70% RH. The only way to know whether your screed is ready for floor finishes is a proper moisture test.

The UK standard is BS 8203, and the method is the box hygrometer test:

1. With laitance already sanded and vacuumed, position an insulated hygrometer box on the floor surface in a central location, away from doorways and edges.
2. Seal the box to the floor on all four sides with tape. No air gaps.
3. Leave sealed for a minimum of 72 hours so the air inside reaches equilibrium with the moisture in the screed below.
4. Read the relative humidity from the digital hygrometer inside the box.
5. Compare against the threshold for your chosen floor finish.

Thresholds by floor finish

Floor finish	RH threshold	Equivalent moisture content
Ceramic / porcelain tiles	75% RH	0.5% by mass
LVT / luxury vinyl	75% RH	0.5% by mass
Over UFH (any finish)	Tighter: often 0.3% CM or 65% RH per manufacturer	Stricter to prevent thermal moisture migration
Engineered wood	65% RH	Lower threshold (wood movement risk)
Solid timber	65% RH or lower per manufacturer	Strictest
Carpet	75% RH	1.0% by mass (least sensitive)

If the test fails, the options are: wait and retest in 2 weeks; run another abbreviated UFH cycle; or add dehumidifiers to the room. Forcing the floor finish down on a failed moisture test is how you end up with warped engineered wood, lifting LVT edges, or adhesive failure under tiles.

One workaround is available: BAL and a handful of other manufacturers sell liquid DPM systems (surface-applied waterproof membranes) rated for tiling over anhydrite screeds at up to 87% RH. They carry a meaningful per-m² premium over standard primer-and-adhesive prep, and they work, but they're a fast-track solution for programme-critical jobs, not a substitute for proper drying. And they are not a wet-room tanking system. They block moisture migration from below, they don't protect against water ingress from above.

Why not wet rooms

This is the single hardest rule with anhydrite screed and the one that catches out the most kitchen extensions, where the screed runs continuously from the main floor into an adjoining utility room or WC.

The chemistry: anhydrite (CaSO4) reverts partially to gypsum (CaSO4·2H2O) when it contacts sustained moisture, losing strength. When cement-based materials (tile adhesive, mortar, waterproof membranes) meet calcium sulphate in the presence of moisture, ettringite crystals form at the interface. Ettringite expands as it grows. The pressure generated lifts tiles, cracks grout, and bows any floor finish on top.

The practical rules:

• Do not pour anhydrite in wet rooms, walk-in showers, or utility rooms with floor-level washing machines.
• Do not pour anhydrite externally, in porches, or anywhere water can pool.
• Where the kitchen floor runs into a separate wet room, terminate the anhydrite at the threshold with an expansion joint and pour sand-cement or cement-based liquid screed in the wet room.
• For tiling over anhydrite in normal dry areas, use two coats of a manufacturer-specified primer (BAL, Palace, Dunlop all make suitable primers) and a C2 or C2F polymer-modified tile adhesive. Never apply cement-based adhesive directly to anhydrite.

BAL specifically warns against using epoxy waterproof membranes over anhydrite. The epoxy blocks moisture escape and traps any ingress, accelerating ettringite formation. If you need waterproofing over anhydrite, use a proprietary anhydrite-compatible system. Better still, specify the wet room in a cement-based screed in the first place.

Cost and where to buy

Liquid screed is priced per m² supplied and installed. The 2026 UK consensus range for typical residential extensions is £18 – £34. That covers supply of the screed, pump and truck mobilisation, the pour itself, and (usually) the first laitance sand visit.

The total for a typical job is roughly £2,000 – £3,000. Real 2022 project data on an 80m² pour at 65mm depth came in at the upper end of the per-m² range shown above, consistent with current pricing allowing for some material inflation since.

Pricing depends heavily on project size, because mobilisation is a fixed cost (mixer truck, pump, team, fuel) spread over whatever area is poured.

For comparison, sand-and-cement screed (see sand-and-cement screed) runs £11 – £18. That cost saving is the main reason some homeowners are pushed toward traditional screed, but it ignores the secondary costs of UFH performance, self-levelling compound to achieve tiling flatness, and the extra 20-30mm of floor build-up.

What's included in a quote

Ask your screeder to confirm in writing:

• Screed material and depth (product name, not just "liquid screed")
• Pump hire and mixer truck mobilisation (usually included in the all-in rate but sometimes quoted as a separate line item)
• Perimeter expansion strip (sometimes charged as an extra per linear metre)
• Laitance sanding visit (usually included for anhydrite; ask explicitly)
• DPM supply and installation (often included if they're doing the whole build-up)
• Insulation supply and installation (usually extra unless agreed up front)
• Number of pour days and any split-pour charges for large areas

Who to buy from

You don't buy liquid screed from a builders' merchant. You engage a specialist screeding contractor who brings the material with them. Major UK screed contractors include Speed Screed, Capital Screeding, Express Liquid Screeds, and dozens of regional firms. The pump-and-truck combination is what makes the price work, so a firm in your region with short travel distances will usually quote cheaper than a distant national.

Get three quotes. Confirm the product being used (anhydrite vs cement-based). Check references for recent domestic extensions of similar size. Liquid screed is a specialist pour, and a firm with experience doing kitchen extensions will get it right.

Where you'll need this

• Screeding: the pour itself, including pre-pour sequencing, pump day logistics, and post-pour protection
• Underfloor heating: pipe layout, pressure testing, and commissioning protocol that works with the screed
• Flooring: moisture testing, laitance removal, and primer choice before any floor finish goes down
• Tiling: adhesive selection over anhydrite, two-coat primer requirement, and why ready-mix adhesive will fail over calcium sulphate screed

Liquid anhydrite screed appears in almost every extension or renovation where UFH is being installed, not just kitchen extensions. The sequencing rules (7-day minimum before UFH, 5-7 day laitance sand, 75% RH before tile) are the same regardless of project type.

Common mistakes

Skipping laitance sanding. The tiles go down, hold for three months, then lift at the grout lines. The most expensive mistake in the sequence because the rework is total: strip tiles, grind surface properly, retile. Budget for laitance sanding at pour + 5-7 days and don't let anyone skip it.

Tiling or laying floor finish on a failed moisture test. 80% RH looks the same as 75% RH but behaves completely differently under an adhesive. Engineered wood on 80% RH will cup or crown within weeks. LVT on 80% RH will show edge lifting in months. Wait for the test to pass or commission a liquid DPM system.

UFH activated before day 7. The screed is still green and the thermal gradient between surface and body cracks the top layer. Cracks over UFH pipe runs can't be repaired without destroying the screed. Some screeders prefer 10 days for safety margin.

Temperature ramp faster than 5°C per day. Same failure mode as early activation: thermal shock cracking. Follow the BS EN 1264-4 ramp exactly. Don't let anyone tell you the cycle can be compressed to save a week.

No perimeter expansion strip. Anhydrite expands slightly during cure and contracts slightly while drying. Without a compressible foam strip around every wall, column, and upstand at the specified thickness, the screed cracks at the edge or (worse) pushes the wall plaster at the skirting line. The strip is a cheap roll. Not having it costs four figures to rectify.

Bays bigger than the NHBC 9.3.4 limits. Without a mid-bay joint, large pours develop map cracking as they contract during drying. If your kitchen extension is more than 40m² open-plan, the screed needs a saw-cut or compressible joint dividing the bays.

Anhydrite in wet rooms or adjacent to water sources. Ettringite formation lifts tiles. If your kitchen screed runs into a utility with a washing machine, or into a WC with a shower, terminate the anhydrite at the threshold and use sand-cement or cement-based liquid screed in the wet area.

Using standard tile adhesive without primer. Cement-based adhesive directly onto anhydrite, without two coats of manufacturer-specified primer, sets up ettringite formation at the interface. Even a "flexible" adhesive will fail without the primer. Always two coats.

Sealing fresh screed under polythene. Polythene traps moisture on the surface and damages the finish. Ventilate the room by opening windows on cross-axes, don't cover the screed.

Ignoring the weight on upper floors. 100-120kg/m² at 50mm. A 40m² first-floor extension pour is 4-5 tonnes of dead load. Your structural engineer needs to know the screed weight before finalising the joist specification. Lightweight anhydrite variants exist; ask your screeder if the loading is marginal.

Commissioning without a certificate. BS EN 1264-4 requires a written commissioning certificate. Some building control officers will refuse sign-off without one, and most flooring installers will refuse to lay floor finish without seeing it. Have the certificate completed and signed on the day the cycle ends.