Which screed should I use for underfloor heating?

Quick Answer: Anhydrite (calcium sulphate) liquid screed is the preferred screed for UK underfloor heating, typically laid at 35-50mm cover over the pipes per BS EN 13813 classification CA-C25-F5. Sand/cement screed to BS 8204-1 can also be used but requires 65-75mm cover and conducts heat less efficiently. Both must be commissioned per BS EN 1264-4 with a controlled heat-up cycle before any floor finish is laid.

Summary

Underfloor heating (UFH) screed is a specialised application: the screed encapsulates the heating pipes and acts as a thermal mass that distributes heat evenly to the floor surface. The screed type, thickness over pipes, and commissioning routine determine whether the system delivers its rated output and avoids cracking or pipe damage in service.

Two screed families dominate UK UFH installations. Calcium sulphate (anhydrite) liquid screed is the default for new build and renovation projects, offering thin sections, fast pour rates, and good thermal conductivity. Traditional sand/cement screeds remain valid where waterproofing, point loading, or chemical exposure rules out anhydrite. Self-levelling cementitious flowing screeds (CT-C30-F5 and similar) offer a middle path for retrofits where load and exposure are factors.

This guide focuses on screed selection for UFH specifically, covering thickness, additives, drying time, commissioning, and the impact of screed choice on subsequent floor finishes. For general screed types and tolerances, refer to the screed-types article in Related Articles below.

Key Facts

• BS EN 13813 — Classifies screed materials by binder and strength; the basis for UFH screed specification
• BS 8204-1 — Code of practice for sand/cement screeds and concrete bases
• BS 8204-7 — Code of practice for pumpable self-smoothing screeds (includes anhydrite)
• BS EN 1264-4 — Water-based embedded surface heating; mandatory commissioning protocol
• Anhydrite screed thickness — 35-50mm cover over UFH pipes (typical 40mm)
• Sand/cement screed thickness — 65-75mm cover over UFH pipes for bonded; 75mm for unbonded
• Thermal conductivity — anhydrite typically 2.0-2.5 W/mK; sand/cement 1.2-1.6 W/mK
• Anhydrite max pour area — up to 1000m² without joints; sand/cement requires bay joints at 40-50m²
• Pipe spacing — 100-200mm centres depending on heat output requirement
• Min temperature for laying — 5°C ambient for both screed types; never below 3°C
• Drying rate (anhydrite) — 1mm/day up to 40mm, then 0.5mm/day for additional depth at 20°C 50% RH
• Drying rate (sand/cement) — 1mm/day up to 50mm, then 0.5mm/day with no force-drying for first 7 days
• Force-drying — UFH commissioning can accelerate drying after 7 days (anhydrite) or 28 days (sand/cement)
• Surface laitance (anhydrite) — chalky film forms during cure; must be mechanically removed before flooring
• Joint requirement — perimeter expansion joints required against all walls and columns
• Commissioning heat-up — start at 25°C flow, increase by 5°C/day to maximum, hold 3 days, drop by 5°C/day
• Moisture limit before flooring — 75% RH (or 0.5% by weight CM for anhydrite, 2% CM for sand/cement)
• Reinforcement — typically not required in anhydrite; sand/cement may use polypropylene fibres or A142 mesh
• CE/UKCA marking — Required under BS EN 13813
• Levelling tolerance — SR1 (3mm/2m) anhydrite as-laid; sand/cement typically SR2 (5mm/2m)

Quick Reference Table

Detailed Guidance

Anhydrite (calcium sulphate) liquid screed

Anhydrite is the dominant UFH screed in the UK for residential and small-to-medium commercial work. Delivered by tanker and pumped through hose to the laying point, it self-levels with minimal trowelling, encapsulates pipes completely without voids, and reaches walking traffic in 24-48 hours.

Practical advantages for UFH:

• Thin section — 35mm cover over pipe means lower thermal mass and faster heat-up response
• No voids — liquid form fills around pipes completely, eliminating cold spots and improving heat output by 10-15% compared to dry-rodded sand/cement
• High thermal conductivity — 2.0-2.5 W/mK transfers heat to the surface more efficiently
• Large pour areas — single pours up to 1000m² possible without movement joints
• Fast programme — pour, walk on next day, light traffic within 7 days

Practical constraints:

• Moisture sensitive — degrades on contact with water; not for wet rooms, external use, or where standing water may occur
• Laitance — a chalky film forms on the surface as it cures; must be removed by mechanical sanding (60-80 grit) before any floor finish or smoothing compound
• Drying time — calendar drying is 1mm/day to 40mm, then 0.5mm/day. A 50mm screed laid in February is unlikely to be ready for flooring before late May without force-drying
• Force-drying — anhydrite tolerates UFH commissioning at lower temperatures than sand/cement and can be force-dried after 7 days
• Floor finish compatibility — works with all common finishes (LVT, timber, tile, carpet) but laitance removal is essential before bonding any finish

Sand/cement (CT) screed

Traditional sand/cement screed is still specified where anhydrite's limitations matter. The typical mix is 1:3 to 1:4 cement to sharp sand with a polypropylene fibre dosing or A142 mesh reinforcement on a slip layer.

Practical advantages:

• Water-tolerant — does not degrade if wetted, suitable for wet rooms, plant rooms, garages
• Point load resistance — higher compressive strength than anhydrite, suits commercial use with wheeled traffic
• Familiar to all flooring trades — every fitter has handled it, no special preparation required
• DPM-tolerant — accepts most surface DPM systems if required

Practical constraints:

• Thicker section — 65-75mm cover over pipes adds weight and reduces UFH responsiveness
• Lower thermal conductivity — 1.2-1.6 W/mK, so UFH output is lower for the same flow temperature
• Slow drying — 1mm/day to 50mm, then 0.5mm/day, with no force-drying allowed for first 28 days
• Movement joints — required at 40-50m² intervals, must be detailed through any subsequent floor finish
• Cracking risk — shrinkage cracks common at re-entrant corners and pipe penetrations; mesh or fibres mandatory

Cementitious flowing screeds

A growing category of modified cementitious flowing screeds (e.g. CT-C30-F6 to CT-C40-F7) offers the laying speed of anhydrite with the water tolerance of sand/cement. These products use Portland cement with shrinkage-compensating admixtures and pump-grade fluidifiers.

Practical positioning:

• Better than anhydrite where occasional wetting may occur (laundries, plant rooms)
• Better than sand/cement where laying speed matters (fast-track retrofits)
• More expensive than either alternative
• Thinner section than sand/cement (typically 40mm cover) for similar performance
• Compatible with all common floor finishes including resilient flooring

Commissioning the UFH before flooring

BS EN 1264-4 sets the commissioning protocol that must be followed regardless of screed type. The purpose is to ensure the screed is fully cured, dried, and stress-relieved before a sensitive floor finish is bonded.

The standard cycle:

1. Initial cure period — anhydrite minimum 7 days; sand/cement minimum 28 days from pour
2. Start-up — flow temperature at 25°C, hold for 3 days
3. Daily increase — raise by 5°C per day to maximum design flow temperature (typically 35-45°C)
4. Hold at maximum — minimum 3 days, ideally 7 days for residential
5. Cool-down — drop by 5°C per day back to ambient
6. Moisture verification — test at multiple locations after cool-down; must be ≤75% RH before flooring

The commissioning record must be kept and signed by the heating contractor. Most floor finish manufacturers require a copy as a warranty condition.

Laitance removal (anhydrite specific)

The single most common cause of flooring failure over anhydrite screed is failure to remove the surface laitance. The laitance is a layer of fine particles brought to the surface during pouring. It is weak, friable, and will not bond to adhesives or smoothing compounds.

Process:

1. Verify screed is below 75% RH and fully cured
2. Vacuum thoroughly to remove dust
3. Sand with a 60-80 grit floor sander (or planetary diamond grinder for larger areas)
4. Vacuum again
5. Test bond: tape a piece of LVT down with masking tape, leave 24 hours, lift — if dust comes up, sand again
6. Prime with the smoothing compound manufacturer's recommended anhydrite primer
7. Apply smoothing compound to required depth

Skipping any step risks the final floor lifting in patches within weeks.

Pipe layout, fixings and screed depth

Pipe layout affects screed performance. Common patterns:

• Serpentine — single continuous loop, simple but cold end at the return
• Spiral (counterflow) — supply and return interleaved, even surface temperature
• Bifilar — modified serpentine with closer spacing at perimeter

Fix pipes to the insulation (PIR or EPS) with staples, clips, or rails at 300-500mm intervals. Pipes must not lift during the screed pour. Use pressure-test the system to 6 bar minimum and leave under pressure during the pour to immediately identify any pipe damage.

Edge insulation (typically 10mm closed-cell foam) is fitted vertically against all walls and columns to create the perimeter expansion joint and prevent heat loss to the walls.

Frequently Asked Questions

Can I lay UFH pipes directly on a sand blinding without insulation?

No. UK Building Regulations Approved Document L requires minimum insulation under heated floors to prevent downward heat loss. Typical specification is 100-150mm PIR for ground floors, 25-50mm for intermediate floors over a heated space.

How long before I can walk on liquid screed?

Anhydrite: 24-48 hours for foot traffic, 7 days for light tools and equipment. Loading the screed before 7 days risks cracking. Heavy loads should wait until 28 days.

Why can't I force-dry sand/cement screed for the first 28 days?

Portland cement requires water for hydration. Force-drying before primary cure removes the water needed for the cement to develop full strength. Force-drying too early gives a weak, dusty screed that will fail in service.

Can I use UFH with a timber floor over screed?

Yes, but with constraints. Solid timber is rarely recommended above UFH due to shrinkage; engineered wood with a multi-ply backing is the safer choice. Maximum surface temperature at the timber/screed interface is typically 27°C — confirm with the timber manufacturer.

What happens if I flood test before commissioning?

Anhydrite screed will degrade immediately on contact with water. Sand/cement is tolerant. Run UFH pressure tests in air or with the system filled and pressurised before screeding, not water-flood tested after.

Regulations & Standards

• BS EN 13813:2002 — Screed material and floor screeds. Material requirements. Classification basis for all UFH screeds.

• BS 8204-1:2003+A2:2011 — Code of practice for concrete bases and cementitious levelling screeds to receive flooring.

• BS 8204-7:2003+A1:2008 — Code of practice for pumpable self-smoothing screeds, including anhydrite.

• BS EN 1264-4:2009 — Water-based surface embedded heating and cooling systems. Installation. Mandatory commissioning protocol.

• BS EN 1264-2:2008+A1:2012 — Water-based surface heating. Determination of thermal output by calculation and test.

• Building Regulations Approved Document L — Conservation of fuel and power. Insulation requirements under heated floors.

• Building Regulations Approved Document E — Resistance to passage of sound. Acoustic implications of screed mass.

• Construction Products Regulations 2013 — UKCA/CE marking required under BS EN 13813.

• BS 5325:2001 — Installation of textile floor coverings. Reference for moisture testing.

• COSHH 2002 — Cement burns risk during placement; PPE and skin protection required.

• BSI Standards Catalogue — BS EN 13813 — Classification standard for screed materials

• BSI Standards Catalogue — BS EN 1264-4 — Commissioning standard

• Heating and Hotwater Industry Council — UK guidance on UFH installation

• Building Regulations Approved Document L — Insulation requirements

• Calcium Sulphate Floor Screed Manufacturers Group (CSFSM) — Industry guidance on anhydrite screed

• NHBC Standards — Domestic construction standards for screed and UFH

• screed types — Sand/cement, anhydrite, and other screed types in general

• subfloor preparation guide — Substrate prep before screeding

• floor levelling compounds — Smoothing compound over UFH screed

• lvt installation — LVT over UFH screed considerations

• underfloor heating tiles — Tiling over heated screed