Floor Screed Types: Sand/Cement, Liquid Anhydrite, Fibre-Reinforced — Depths & Drying Times

Summary

Screed is the layer between the structural floor and the floor finish. Getting it right — correct depth, correct mix, correct curing — determines whether the floor is level, durable, and compatible with the finish above it. Getting it wrong is an expensive mistake: a failed screed means ripping it out and starting again, often after the floor finish is already installed.

The screed market has expanded significantly in recent years. Traditional sand and cement screed — mixed on site or delivered by volumetric lorry — remains common for domestic work. Liquid screeds (primarily calcium sulphate/anhydrite, also known as Gyvlon, Topflow, or Fermacell) have become the dominant choice for new-build and refurbishment projects with underfloor heating, as they flow around heating pipes without air pockets and achieve near-perfect flatness. Fibre-reinforced screeds (with polypropylene or steel fibres added) reduce shrinkage cracking in large pours.

The key variable that catches tradespeople out is moisture. All screed takes much longer to dry than most installers expect, and many floor finish failures are caused by laying too early. The only reliable test is a hygrometer test at the right depth, carried out correctly.

Key Facts

• Sand/cement bonded screed — Minimum 25mm, typically 40mm. Bonded to substrate with slurry coat
• Sand/cement unbonded screed — Minimum 65mm, typically 75mm. Laid on polythene DPM
• Sand/cement floating screed (over insulation) — Minimum 65mm on rigid insulation; 70–75mm recommended
• Liquid anhydrite (floating) — Minimum 35mm, 40mm over UFH. Cannot be bonded to substrate
• Liquid screed over UFH — Minimum 30mm cover over pipe crown (so pipe + 30mm minimum)
• Sand/cement over UFH — Minimum 65mm total depth with pipe embedded
• Fibre-reinforced screed — Same depths as sand/cement but reduces cracking risk in large pours
• Drying rate sand/cement — Approximately 1 day per mm up to 40mm depth, then slower. 75mm takes approximately 60–90 days at 20°C
• Drying rate liquid screed — Faster initial set but similar or slower overall drying. Typically 1mm per day for first 40mm then 0.5mm/day
• Target moisture content — Below 75% RH for most floor finishes. Below 65% RH for wood flooring and parquet
• Calcium sulphate (anhydrite) screed — Must have laitance removed by sanding before tiling/flooring. Incompatible with cementitious adhesives without priming
• Anhydrite and moisture — Calcium sulphate re-absorbs moisture easily. Cannot be used in wet rooms or below DPM level

Quick Reference Table

Detailed Guidance

Sand and Cement Screed

Mix ratio: 1:4 or 1:4.5 cement:sharp sand by volume. Aim for a 'dry-ball' consistency — the screed should hold its shape when squeezed in the hand but not be wet enough to show free water. Too wet causes settlement, shrinkage cracks, and extended drying time. Too dry causes poor compaction and a weak surface.

Bonded screed: Substrate must be clean, sound, and dampened (not soaking). Apply a slurry coat of cement:PVA bonding agent immediately before laying and work the screed onto the wet slurry. Key for screed under 50mm where lifting or curling is a risk.

Unbonded screed: Lay polythene (min 1000 gauge) over the structural floor. Lap joints 200mm minimum and tape. Turn up at walls. The screed floats on the polythene — this is the standard approach for most domestic ground floors with a DPM.

Floating screed (over insulation): Rigid insulation boards (PIR, EPS, or mineral wool) must be full-coverage, butt-jointed, and level. Lay polythene separation layer over insulation. Minimum screed depth 65mm.

Curing: Cover with polythene for 3–7 days to slow drying and prevent surface cracking. Keep away from draughts and direct sunlight for the first 24 hours.

Joints: Bay sizes ideally no more than 40m² or 8m in either direction. Install movement joints (pre-formed metal or compressible foam strip) at doorways and between areas. Screed shrinks as it dries — large unsupported bays will crack.

Liquid Anhydrite Screed

Delivery: Arrives ready-mixed by pump truck. Usually installed by the screed supplier's own operatives. As a flooring contractor or heating engineer, your job is to prepare the substrate and — for UFH — ensure the heating circuit is pressurised to 1 bar during pour to prevent pipes floating.

Surface preparation for liquid screed: Substrate must be primed with an appropriate primer (e.g., Mapei Eco Prim Grip or equivalent). All penetrations and pipe entry points must be sealed.

Floating freely: Liquid screed cannot be bonded. It is always laid on a separation layer (polythene or damp-proof membrane).

UFH commissioning before tiling: After the screed has dried to below 75% RH, the UFH should be commissioned (heated up and cooled down progressively over 7 days minimum) to drive out residual moisture and acclimatise the screed to operating temperatures before the floor finish goes down.

Laitance removal: After drying, liquid anhydrite develops a hard surface layer (laitance) that must be sanded off with a disc sander before applying primer or adhesive. This is critical — adhesive will not bond to laitance. Most tilers underestimate this step.

Incompatible with wet areas: Calcium sulphate absorbs water readily. Never use anhydrite screed in bathrooms, wet rooms, utility rooms with open washing machines, or anywhere with regular liquid spillage. Sand/cement is required in these locations.

Fibre-Reinforced Screed

Polypropylene or steel fibres are added to a standard sand/cement mix (typically 0.9kg/m³ PP fibres). This reduces plastic shrinkage cracking during curing and can reduce the number of movement joints needed in large pours. It does not significantly change strength, drying rate, or application method versus standard sand/cement. Useful for large commercial or industrial floors and for residential floors with awkward shapes where bay control is difficult.

Rapid-Drying Screeds

Various manufacturers produce rapid-drying screed formulations (Ardex K360, F-Ball F77, Instarmac UltraFloor, etc.) that allow floor finishes to be installed within 24–48 hours at 20–30mm depth. These are significantly more expensive than standard screed per bag but save time in commercial fit-out and fast-turnaround refurbishment. Always check the specific product's moisture testing requirements — some rapid-drying products use alternative moisture measurement methods.

Testing for Moisture

Do not trust the calendar. Screeds can retain moisture for far longer than expected due to cold weather, poor ventilation, high humidity, or thick pour. The only reliable method is:

Hygrometer test (slab humidity): A calibrated electronic hygrometer probe is sealed beneath a foil pad on the screed surface for a minimum of 72 hours. The reading must be below the threshold for your floor finish. This method is specified in BS 8201 and BS 8203.

Calcium carbide (Speedy) test: A screed sample is mixed with calcium carbide in a pressure vessel. The pressure is read from a gauge. The reading gives moisture content by weight of the screed. More reliable for thick screed than surface methods. More invasive — destroys the test sample.

Resistance meter (pin meter): Quick but unreliable for screed. Use only as a preliminary indicator. Do not rely on it for sign-off.

Frequently Asked Questions

Can I lay tiles directly on new screed after 3 days?

Only if the screed has been tested and is below 75% RH. In practice, 3 days is not enough for any screed over 20mm depth under normal conditions. Standard sand/cement at 75mm will take 60–90 days. Ignoring this causes tiles to delaminate, adhesive to fail, and in extreme cases, the screed to crack or heave. Get a hygrometer test done.

My screed has cracked. Is it structurally compromised?

Hairline shrinkage cracks (less than 0.5mm wide) in isolated positions are normal and not structurally significant. Fill with appropriate flexible filler before tiling. Bay cracks following movement joints are expected. Wide cracks (over 1mm), cracks through the full depth, or cracks where the two sides are at different levels indicate a more serious issue — investigate the cause before laying the floor finish.

Does the type of screed matter for underfloor heating?

Yes significantly. Liquid anhydrite is preferred for UFH because it flows around pipes with no air pockets, giving excellent thermal contact. Sand/cement requires a minimum 65mm depth, uses more material, and is harder to ensure full pipe coverage. However, anhydrite cannot be used in wet areas, so bathroom UFH must use sand/cement. The UFH commissioning protocol also differs by screed type — always follow the screed manufacturer's commissioning specification.

What depth of screed do I need over existing concrete?

If bonding to the existing concrete (bonded screed), minimum 25mm with a slurry coat. If floating (over insulation or DPM), minimum 65mm. If the substrate is very uneven, adding a self-levelling compound first can reduce the screed volume needed and give a better result.

Regulations & Standards

• BS 8204-1:2003+A1:2009 — Screeds, bases and in-situ floors — Code of practice for concrete bases and screeds to receive in-situ floorings

• BS 8204-7:2003 — Self-levelling screeds

• BS 8201:2011 — Code of practice for installation of flooring from wood and wood-based panels (moisture requirements)

• BS 8203:2017 — Code of practice for installation of resilient floor coverings (moisture requirements)

• Approved Document C — Site preparation and resistance to moisture (ground floor DPM requirements)

• Gyvlon (Fassa Bortolo) Liquid Screed Technical Guide — Liquid anhydrite specification and installation

• Screedmaster Technical Support — Sand/cement screed specification and drying time tables

• RIBA Good Practice Guide: Floors — General screed and flooring design guidance

• lvt installation — LVT moisture requirements over screed

• underfloor heating — Wet UFH pipe and screed depth specification

• underfloor heating electric — Electric mat over screed

• screed depth — Screed depth and volume calculator