Underfloor Heating Screed Guide

Quick Answer: Screed depth over underfloor heating pipes is critical: minimum 65mm for traditional sand/cement screed, minimum 30mm for anhydrite (calcium sulphate) liquid screed over 20mm pipes (or 25mm over electric mat). Anhydrite liquid screed is the industry-preferred option for wet UFH — it flows around pipes without air pockets, achieves better thermal conductivity, and can be laid thinner. Both types must be fully commissioned and dried to ≤75% RH before any floor finish is installed.

Summary

The screed specification in an underfloor heating system is not a cosmetic choice — it directly affects thermal performance, drying time, and the long-term stability of the floor finish. A screed that is too thin will crack over the pipes; too thick and it adds unnecessary thermal mass, reducing the system's responsiveness. Getting the depth wrong also invalidates manufacturer warranties on both the pipe system and the floor finish.

In UK domestic construction, underfloor heating screed falls under two British Standards: BS EN 1264 covers the design and performance of water-based UFH systems, and the screed itself is governed by BS 8204. The Building Regulations (Part L — Conservation of fuel and power) require that UFH systems are designed to a specified flow temperature commensurate with the insulation levels in the building; the screed specification directly affects this calculation.

Two screed types dominate the market: traditional sand/cement screed (site-mixed or bagged) and anhydrite liquid screed (calcium sulphate binder, pumped). Understanding the difference — in depth, drying time, finishing requirements, and compatibility with different floor finishes — is essential before specifying which to use.

Key Facts

• BS EN 1264-4 — installation requirements for water-based UFH embedded in floors; includes screed depth requirements
• BS 8204 — Screeds, bases and in-situ floorings; Parts 1 (concrete bases) and 7 (pumpable self-smoothing screeds)
• Sand/cement screed minimum depths — unbonded: 65mm minimum over pipes; bonded: 40mm minimum over pipes (rarely used in UFH)
• Anhydrite liquid screed minimum depths — 30mm over 20mm pipes; 25mm over electric mat; maximum 80mm to avoid excessive weight and drying time
• Pipe cover — minimum 25mm of screed above the top of the pipe in sand/cement; minimum 20mm in anhydrite
• Thermal conductivity — anhydrite screed: typically 1.0–1.2 W/mK; sand/cement screed: typically 1.5–2.0 W/mK (denser = better conductor)
• Drying rate — sand/cement — approximately 1mm per day in good conditions (>15°C, ventilated); a 75mm sand/cement screed takes approximately 75 days to dry
• Drying rate — anhydrite — approximately 1mm per day after commissioning; but anhydrite must be force-dried (heated and ventilated); can achieve 65mm dry in 28 days with proper commissioning
• Commissioning — sand/cement — minimum 21 days cure before applying heat; then a controlled heat-up programme
• Commissioning — anhydrite — minimum 7 days cure; then heat-up protocol per EN 1264-4 (typically: 25°C flow for 3 days, then raise to design temperature for 4+ days)
• RH before floor finish — 75% maximum (BS 8203 for resilient floors; some manufacturers specify 65%); test with calibrated hygrometer at depth, not surface reading
• Laitance on anhydrite — anhydrite forms a weak surface laitance that must be removed before applying any adhesive or floor finish; light sanding, grit blast, or specialist primer
• Expansion joints — required in sand/cement screed at all doorways and at intervals not exceeding 40m², matching the pipe zone layout; anhydrite liquid screed does not require expansion joints within a zone

Quick Reference Table

Detailed Guidance

Choosing Between Screed Types

Choose anhydrite liquid screed when:

• Pipe circuits are complex or close-spaced (liquid flows around pipes without risk of voids)
• The slab is load-bearing and you want to minimise depth (30mm vs 65mm = significant dead load saving on upper floors)
• The programme is tight — anhydrite can be force-dried faster with proper commissioning
• UFH is in a large open-plan area without many doorways (no expansion joints required)

Choose sand/cement screed when:

• The finish floor will be solid hardwood (anhydrite sulphates can react with some wood finishes)
• The job is small enough for site-mixing (anhydrite requires a pump truck)
• Very low ambient temperatures are likely during curing (anhydrite freezes — do not lay below 5°C)
• Budget is constrained — anhydrite material and installation costs are higher

Insulation Under the Screed

UFH performance is determined as much by the insulation below the pipes as by the screed above them. BS EN 1264 specifies minimum insulation values:

• Ground floor: 150mm PIR or equivalent (U-value 0.13 W/m²K in most zones to meet Part L)
• Intermediate floors (heating the floor above): 30mm PIR minimum to prevent downward heat loss
• Insulation must be rated for floor-loading; EPS 200 (expanded polystyrene) or PIR with appropriate compressive strength

Do not use standard loft insulation or soft mineral wool below UFH screed — it will compress under the weight of the screed and create differential loading.

Pipe Layout and Manifold Connection

Before screed is poured, all pipe circuits must be pressure-tested (typically 4 bar static pressure for 24 hours). Record the test result and keep the circuit under pressure during screed pour to prevent pipes floating. The manifold should be in place and all pipe tails labelled.

Standard pipe spacing is 150–200mm for ground floors in well-insulated buildings. At 100mm spacing (closer), heat output is higher but the system is more responsive. At 250mm, heat output falls and a higher flow temperature is needed. Always have the system designed by a competent person to BS EN 1264-2.

Commissioning Protocol

The commissioning heat-up programme must be followed — skipping it is the most common cause of cracking in sand/cement screed:

Sand/cement (after minimum 21-day cure):

1. Heat flow water to 25°C; maintain for 3 days
2. Increase to 35°C; maintain for 3 days
3. Increase to design flow temperature; maintain for 3 days minimum
4. Cool down gradually; do not switch off suddenly
5. Test RH — must be ≤75% before laying floor finish

Anhydrite (after minimum 7-day cure):

1. Maintain building at 15°C for 7 days (ventilation key)
2. Raise flow to 25°C for first 3 days
3. Raise to 45°C maximum for 4 days (forced drying)
4. Return to 25°C; maintain until RH ≤75%
5. Sand or prime laitance before applying any finish

Never lay any floor finish — tiles, LVT, timber, or carpet — before the RH is confirmed at ≤75% (some engineered timber manufacturers require ≤65%). The drying target must be measured at depth, not at the surface. Surface readings are unreliable for internal moisture assessment.

Floor Finish Compatibility With UFH Screed

Frequently Asked Questions

Can I pour anhydrite screed over sand/cement?

No. Anhydrite and Portland cement are chemically incompatible — placing anhydrite screed over a cement base will cause sulphate attack and structural failure of the screed over time. Anhydrite must be poured over a clean concrete slab, compacted hardcore, or appropriately primed surface.

How long will the screed take to dry completely?

Rule of thumb: 1mm per day at 15°C+ with reasonable ventilation. A 65mm sand/cement screed takes approximately 65 days without forced drying. Anhydrite with a proper commissioning heat-up protocol can reduce this to 28–35 days for 65mm depth. Independent of screed type, the only reliable measurement is a calibrated hygrometer at depth — never rely on elapsed time alone.

What happens if I lay tiles before the screed is fully dry?

Residual moisture vapour migrates upward through the adhesive bed and causes adhesive failure, tile debonding, efflorescence on grout joints, and in severe cases, osmotic blistering under resilient floor coverings. The consequences are expensive — full tile removal, screed remediation, relaying. Test first.

Does the screed thickness affect my Part L compliance?

Indirectly. Part L requires UFH systems to be capable of operating at a certain flow temperature given the building's insulation level. A thicker screed has more thermal mass and may require a higher flow temperature to achieve the desired room temperature at design conditions. Your UFH system designer should take the screed specification into account in their hydraulic calculation.

Regulations & Standards

• BS EN 1264-4 — Underfloor heating systems; Part 4: Installation

• BS 8204-1 — Screeds, bases and in-situ floorings; Part 1: Concrete bases and cementitious levelling screeds

• BS 8204-7 — Pumpable self-levelling screeds (covers anhydrite/calcium sulphate screeds)

• Building Regulations Part L (England): Conservation of fuel and power — UFH system efficiency and insulation requirements

• Building Regulations Part C — Site preparation and resistance to moisture; DPM continuity requirements under ground floors

• BS EN 1264 — BSI Group — UFH design and installation standard

• Screedmaster UK: Anhydrite Screed Guide — practical guide to calcium sulphate screed installation and commissioning

• LABC Warranty Technical Manual — warranty requirements for UFH and screed in new build

• floor screed types: sand/cement, anhydrite, and fibre-reinforced — screed selection for non-UFH applications

• wet UFH pipe sizing, manifold layout, and commissioning — the pipework system that sits below the screed

• electric UFH: mat and cable systems, thermostats, and running costs — the alternative to wet UFH, embedded in adhesive or thin screed

• subfloor preparation before laying any floor finish — assessment and preparation once the screed is dry