Anhydrite Liquid Screed: Pros, Pouring and Cost

You’ve read our guide on traditional screed and you’re still unsure: should you go for a hand-ruled screed, or switch to anhydrite liquid screed? This flowing screed, based on calcium sulphate, is poured through a hose, self-levels and effortlessly encases underfloor heating pipes. It can’t be done solo — but understanding how it works, its moisture constraints and its price gives you the knowledge to make the right call and hold an informed conversation with the screeding contractor who’ll pump it.

What is anhydrite liquid screed?

Anhydrite liquid screed (also known as calcium sulphate flowing screed) is a ready-mixed compound made up of:

• Anhydrous calcium sulphate (anhydrite II, produced by calcining gypsum) — the binder
• Fine aggregates (calibrated 0/4 mm sand)
• Water and superplasticiser admixtures

Unlike traditional screed that is ruled off by hand, anhydrite is self-placing: once pumped in, it spreads under gravity on its own, needing just a single pass with a dappling bar. The consistency is that of liquid yoghurt.

Anhydrite screed, liquid cement screed, traditional screed: the three-way comparison

Advantages of anhydrite screed

1. Exceptional flatness

This is the headline argument. Anhydrite screed delivers a flatness tolerance of ±2 mm over a 2 m straight edge with no human intervention. No hand-ruled screed comes close to that standard without a considerable amount of finishing work. For glued solid wood flooring or resin, it is the ideal substrate.

2. Laying output

A team of three lays 400 to 800 sqm/day with a screed pump. You can pour the entire habitable floor area of a house in a single day. Compared to traditional screed (5-10 days for the same area), the programme saving is substantial.

3. Ideal for underfloor heating

Anhydrite wraps perfectly around UFH pipes and encases them without any air pockets. This improves thermal diffusion: at an equivalent water temperature, an anhydrite screed heats a room 15 to 20% more efficiently than a standard cement screed.

4. Low shrinkage, fewer joints

With near-zero shrinkage (0.05 mm/m versus 0.6 mm/m for cement screed), you can pour up to 300 sqm without a movement joint. Practical for open-plan spaces or an entire floor level.

5. Reduced thickness

Over underfloor heating, the minimum depth above the pipes can drop to 3 cm (versus 4 cm for cement screed). Less material means less thermal mass to heat up — lower inertia and faster response.

Disadvantages and limitations

1. Moisture sensitivity: the Achilles heel

Calcium sulphate cannot tolerate sustained moisture. An anhydrite screed exposed to continuous water contact degrades through gypsum formation and loses its mechanical strength. Practical consequences:

• Forbidden in wet rooms (bathroom, shower, kitchen with floor drainage, utility room)
• Forbidden externally (patio, open garage)
• Forbidden over a subfloor that may allow moisture ingress without a perfect vapour barrier

2. Mandatory laitance sanding before floor covering

As it dries, anhydrite forms a laitance skin on the surface (superplasticiser residue that migrates upward). This skin must be sanded off before any floor covering is laid, otherwise adhesives will not bond. Sanding is done with a rotary sander and vacuum, 5 to 7 days after pouring. Budget £2.50 to £5/sqm for this service — sometimes included in the quote, sometimes not.

3. Long drying time

Rule of thumb:

• 4 cm thickness: 4 to 5 weeks drying
• 5 cm: 5 to 7 weeks
• 6 cm and above: 8 weeks or more

Before laying timber or LVT/PVC flooring, residual moisture content must be below 0.5% (measured with a CM carbide moisture test). This is more demanding than for cement screed (below 3%), and the drying period is almost twice as long.

4. Cannot be self-built

Unlike traditional screed, anhydrite requires a screed pump and a calibrated factory mix. You cannot mix anhydrite in a site mixer. It arrives by transit mixer or is produced on site with a mobile batching plant, then pumped through 100 m of hose. You therefore need to engage a specialist screeding contractor. This is often the sticking point for the self-builder — but the time saving and quality improvement more than justify the cost.

Price of anhydrite liquid screed

The price depends on area, region, site access and thickness.

2026 price ranges

Break-even threshold

Below 80 sqm in a single pour area, the truck mobilisation and pump amortisation push the per-sqm price up sharply. Most screeding contractors apply a minimum contract value of £1,200 to £2,000 for any job. For a small 40 sqm extension, traditional screed becomes competitive again.

Decision tree: which screed should you choose?

Substrate preparation

Even though you’re not pouring the screed yourself, you are responsible for preparing the substrate before the screeding contractor arrives. Without this preparation, no decennial warranty on the screed is possible.

1. Clean and fill

• Sweep and vacuum the slab: no debris, no dust
• Fill all holes, chases and conduit penetrations with repair mortar
• Pack expansion joints in the slab with compressible foam backer rod

2. Lay the DPM sheet

Anhydrite screed must be de-bonded from the substrate. Lay a 200 µm polythene DPM sheet across the entire area, with minimum 200 mm overlaps between sheets, carefully taped. The sheet must turn up 100 mm at the base of all walls (trimmed off after the pour).

3. Fit the perimeter edge insulation strip

A resilient PE foam strip (8 to 10 mm) is glued or stapled along all vertical elements: walls, columns, door linings, inspection covers, thresholds. It absorbs thermal expansion of the screed and prevents acoustic cold bridges. Without an edge strip, your screed will crack at corners during the first winter heating season.

4. Fix the underfloor heating pipes

If you are installing UFH under the anhydrite, the PEX or multi-layer pipes must be perfectly secured (staple rails, clips or nubs). They must not float up during the pour — the screed is extremely fluid and pipes will rise if not held down.

5. Set level pegs

The screeding contractor sets level pegs (small adjustable dots) throughout the room, set to the target screed height with a laser level. They serve as reference points during pumping to regulate flow rate.

The pour day: step by step

Step 1: pump arrival and set-up

• The pump truck parks as close to the access point as possible: maximum 100 m of hose from pump to pour area
• Allow 1 to 2 hours for set-up and warm-up
• You need an on-site water supply and a stable surface for the truck

Step 2: pumping and spreading

• The screeder discharges the screed from the end of the hose, working from the far end of the room back towards the exit
• A second operative passes the dappling bar (a 2 m aluminium bar with spikes): crossed movements across the surface to break air bubbles and even out the pour
• The screed self-levels within minutes

Step 3: flow test

The screeder checks consistency with a flow test (EN 13454 standard): a 50 mm high cone filled with screed must form, after lifting, a pat 240 to 260 mm in diameter. Too fluid = risk of segregation. Too stiff = poor levelling.

Step 4: initial set and protection

• For 48 hours, the room must remain closed, with no draughts, no direct sunlight, no heating
• Foot traffic is possible after 24 to 48 hours depending on temperature
• No heavy loads for 7 days

Step 5: ventilation and sanding

• From day 4, ventilate the room to evacuate moisture (windows slightly open during the day)
• Between day 5 and day 7, sand off the laitance with a rotary sander (diamond disc + vacuum)
• After sanding, vacuum and dust thoroughly before laying any floor covering

UFH commissioning protocol under anhydrite

The commissioning (heating-up) protocol is specific to anhydrite. It cannot begin until at least 7 days of drying:

1. Days 1-3: water at 20-25°C, increasing in +5°C steps per day
2. Days 4-7: gradual rise to the maximum service temperature
3. Days 8-10: hold at maximum temperature
4. Days 11-14: step down by -5°C per day

The purpose: to drive out residual moisture trapped beneath the pipes before the floor covering is laid. Without commissioning, moisture will migrate upward later and lift your timber or tile finish.

Floor covering compatibility

Common mistakes to avoid

1. Neglecting the DPM: holes, poorly taped joints, insufficient upstand → gauging water infiltrates the slab and causes cracking
2. Missing the edge strip on just one wall: stress concentration, guaranteed crack at that point
3. Walking on the screed before 24 hours: footprints that cannot be removed
4. Heating the room during drying: accelerates surface drying but traps moisture deeper → star-pattern cracking
5. Laying floor coverings without a moisture reading: the number-one cause of timber/LVT claims
6. Using anhydrite in a wet room: described above — a guaranteed decennial defect for the contractor
7. Forgetting to sand: tile adhesive will not key, delamination within 6 months

Questions to ask your screeding contractor before signing

Before signing the quote, make sure the screeding contractor can answer all of these questions. If they hesitate on any one, move on.

Useful links

• Before pouring the screed, make sure your concrete slab is fully dry and level
• To compare with the manual alternative, re-read our traditional screed guide
• Electrical conduits and plumbing connections must be installed and tested before the screed is poured
• To size the underfloor heating system beneath the screed, refer to the heating and ventilation guides
• Reference bodies: CSTB (NF DTU 26.2 standards), Syndicat National des Chapistes