Screed Depth Calculator: Sand/Cement vs Liquid Screeds, Drying Times and Moisture Testing

Quick Answer: Standard floating sand/cement screed minimum depth is 65–75mm. Over underfloor heating pipes, minimum total depth is 65–75mm with minimum 30mm cover above the pipe crown. Liquid anhydrite (calcium sulphate) screed minimum is 30mm unbonded or 25mm bonded. Sand/cement screeds dry at approximately 1mm per day up to 50mm. Floor finishes must not be laid until the screed reaches ≤75% relative humidity (RH), tested with a calibrated hygrometer, or ≤65% RH for timber flooring.

Summary

Screed depth is one of the most critical parameters in floor construction — get it too thin and the screed cracks, delaminates, or lacks the structural integrity to carry floor loading. Get it too thick and the floor raises excessively, doors won't clear, and valuable thermal mass is wasted (or worse, UFH response time becomes impractical).

Specifying the correct minimum depth requires knowing: (1) the screed type (sand/cement or liquid anhydrite); (2) the installation method (bonded, unbonded, floating); (3) whether underfloor heating pipes are present; (4) the subfloor type. All these factors interact.

Equally important is understanding drying time. The most common cause of floor finish failure in the UK — debonded tiles, cupped timber, bubbled vinyl — is a floor finish laid before the screed has dried sufficiently. Sand/cement screeds dry slowly and the drying rate slows significantly above 50mm depth. Moisture testing with a calibrated hygrometer to BS 8201 or BS 8203 standards is not optional — it is the only way to verify that a screed is ready for floor finishes.

This article provides the reference depths, a worked example, and drying time guidance for the most common UK domestic floor constructions.

Key Facts

Quick Reference Table

Construction Type Screed Type Minimum Depth Notes
Solid concrete slab, bonded S&C 25mm Slurry coat essential; rare in practice
Solid concrete slab, unbonded S&C 65mm Standard for most domestic
Solid concrete + DPM + insulation S&C floating 65mm Standard insulated ground floor
Solid concrete + DPM + insulation + UFH S&C floating 65mm total, min 30mm above pipe Check pipe OD to set total depth
Beam and block + polythene S&C 75mm Some manufacturers specify 65mm
Beam and block + insulation + UFH S&C 75mm minimum Deeper for adequate pipe cover
Solid concrete, liquid screed floating Anhydrite 35mm 40mm recommended
Solid concrete + insulation + UFH Anhydrite 30–35mm 25mm minimum cover above pipe crown
Existing concrete (refurbishment) S&C or SLC Minimum 3mm (SLC) / 25mm (S&C) Depends on surface preparation

Worked Examples

Example 1: New Ground Floor Extension with UFH

Construction: Concrete ground-bearing slab + DPM + 100mm EPS insulation + UFH pipes (20mm OD PEX pipe in 16mm clips, total pipe height including clip = 30mm) + screed

Required:

Volume of screed required (for 50m² floor):

Example 2: Beam and Block Upper Floor

Construction: Beam and block floor + polythene separation layer + sand/cement screed (no UFH, no insulation)

Minimum depth: 75mm (to provide adequate structural contribution and prevent differential movement between beams and blocks from telegraphing through)

Volume for 35m² bedroom:

Example 3: Liquid Anhydrite Over UFH Retrofit

Construction: Existing concrete slab + DPM (existing) + retrofit UFH pipes (15mm OD) + liquid anhydrite screed

Pipe height above slab: 15mm Required cover above pipe: 25mm minimum Total minimum depth: 15mm + 25mm = 40mm

Volume for 60m² kitchen/living area:

Note: 40mm anhydrite over 15mm pipe gives 25mm cover — this is borderline minimum. 45mm total is safer for this pipe size.

Example 4: Bathroom Wet Room Floor

Construction: Timber suspended floor + 18mm ply overlay + tile backer board + minimum screed with built-in falls (wet room former)

For a wet room, the appropriate approach is NOT screed as a floating layer — use a purpose-made wetroom former (pre-formed with built-in falls) or tile backer boards. If screed is used, a minimum 50mm sand/cement bonded to the ply (with SBR slurry) with built-in falls is acceptable, but a tanking membrane is mandatory.

Additional Reference: Screed Volume Calculator

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For any rectangular floor area:

Volume (m³) = Length (m) × Width (m) × Depth (m)

Example: 5m × 4m room at 65mm depth = 5 × 4 × 0.065 = 1.30m³; add 10% waste = 1.43m³

With waste:

Sand/cement materials per m³ (1:4 mix by volume):

As a ready-mix: Order by m³ as C8 or C10 screed (consult supplier for specification).

Drying Time Reference

Screed Type Depth Minimum Drying Time (20°C, 60% RH ambient)
Sand/cement 50mm ~50 days
Sand/cement 65mm ~75 days (slower above 50mm)
Sand/cement 75mm ~90+ days
Liquid anhydrite 30mm ~20–30 days
Liquid anhydrite 40mm ~28–40 days
Rapid-drying cementitious 40–50mm 24–72 hours (BBA certified)

Drying times extend significantly in cold or humid conditions. Heating the building accelerates drying but must be done gradually to prevent surface cracking.

Moisture Testing: Hygrometer Procedure

  1. Cut test holes 50–75mm deep, 20–25mm diameter at minimum 1 per 50m²
  2. Install plastic test sleeves; seal surface around sleeve with tape
  3. Leave for 72 hours minimum to reach moisture equilibrium
  4. Insert calibrated hygrometer probe; read after 60 minutes equilibration
  5. Proceed only if ≤75% RH (most finishes); ≤65% RH for timber
  6. Record test date, location, and reading for building records

UFH Commissioning Sequence

  1. Allow screed to cure 28 days at ambient temperature (no heating)
  2. Start UFH at 25°C flow temperature for 3 days
  3. Increase 5°C per day to maximum operating temperature
  4. Hold at maximum for 7 days
  5. Reduce to 15–20°C for 3 days before laying flooring
  6. Test RH — proceed only when below threshold

Frequently Asked Questions

Can I lay a 40mm screed over insulation to save cost?

No — 40mm is below the minimum for a floating screed (65mm minimum). A floating screed below 65mm is at high risk of cracking under load as it has insufficient structural depth. The cost saving on thinner screed is outweighed by the risk of failure.

My screed has already been laid at 50mm. Is it usable?

If the screed has cured and is sound (does not sound hollow when tapped, has no cracks), it may perform adequately in practice even if below specification. However, it is below the minimum in BS 8204 and any warranty from the screed installer is void. Have it assessed by a specialist before laying floor finishes.

Does screed depth affect UFH performance?

Yes significantly. Deeper screed over UFH acts as greater thermal mass — it takes longer to heat up (slower response time) but holds heat longer. This can be an advantage in cold climates where the system runs continuously, but is a disadvantage in rooms where you want fast response (bathrooms used briefly in the morning). Liquid anhydrite at 30–40mm has faster response than sand/cement at 65–75mm over the same UFH circuit.

Can I tile immediately after a 65mm sand/cement screed is laid?

No. At approximately 1mm per day, a 65mm screed takes around 65+ days minimum to reach ≤75% RH under normal conditions. Test with a hygrometer and do not lay tiles until the threshold is confirmed. Tiling over a wet screed leads to hollow-sounding tiles, eventual debonding, and tile cracking from residual movement.

What causes a screed to crack?

The most common causes: mix too wet; curing too fast (not covered, building heated prematurely); screed too thin; inadequate expansion joints in large areas; bonded screed on a poorly prepared substrate; and structural movement transmitted through the slab.

Regulations & Standards