Concrete Screed Floors for London Extensions and Renovations: Types, Costs, and What to Expect

**Sand-cement (traditional) screed**:

Sand-cement screed is the traditional screed — a 1:3 to 1:4.5 mix of cement and sharp sand, mixed on site with water. It is laid over a damp-proof membrane and insulation (for ground floor slabs) or directly onto the structural concrete (for upper floors).

*Typical thickness*: 65–75mm for an unbonded screed (laid over insulation without bonding to the structural slab); 40–50mm for a bonded screed (chemically bonded to the structural slab with SBR primer); 25–40mm for a float-and-set screed on the structural slab (without insulation — typically for upper floors).

*Drying and curing time*: Sand-cement screed dries at approximately 1mm per day in normal UK conditions (16–20°C, adequate ventilation). A 65mm sand-cement screed takes approximately 65 days to be 'move-in dry' (i.e., dry enough to lay the floor finish). For underfloor heating systems, the heating cycle must be run before the floor finish is laid (the heating cycle drives off remaining moisture — a process known as 'commissioning the screed') — this typically begins 28 days after laying and runs for approximately 7–14 days.

• *When to use sand-cement screed*:
• •When the project budget is the primary driver (sand-cement is the cheapest type)
• •When standard floor finishes are specified (tile, LVT, engineered wood)
• •When the floor is not incorporating underfloor heating (the slower drying time of sand-cement is less problematic without UFH)
• •When the overall build programme has sufficient float to accommodate the drying time

**Anhydrite (calcium sulphate / gypsum) screed**:

Anhydrite screed (trade names: Gypsol, Primio, Isocrete K-Screed) is a liquid screed made from calcium sulphate, fine aggregate, and water, supplied as a ready-mixed liquid and pumped into the floor area. It flows into all corners and around UFH pipes without tamping, self-levels to a smooth surface, and can be laid at a minimum 30mm thickness over UFH pipes (compared with 65mm for sand-cement screed).

• *Key advantages over sand-cement*:
• •*Faster installation*: A screed team can lay 1,000–2,000m² of anhydrite screed per day (vs. 100–150m² for sand-cement)
• •*Thinner section*: 30–40mm over UFH pipes vs. 65–75mm for sand-cement — important for floor-to-ceiling height in extensions
• •*Better thermal contact with UFH pipes*: The liquid screed envelops the pipes completely with no voids — better thermal conductivity to the floor surface
• •*Better surface flatness*: Anhydrite screed self-levels to a flatter surface than hand-trowelled sand-cement screed
• •*Faster drying*: Anhydrite screed dries faster than sand-cement in equivalent thicknesses — typically 1.5–2mm per day in good drying conditions

• *Key considerations*:
• •*Cost*: Anhydrite screed is approximately 30–50% more expensive per m² than sand-cement screed (pump hire, faster setting, ready-mixed supply)
• •*Incompatibility with cement adhesives*: Anhydrite screed is calcium sulphate-based; contact with water-containing cement products can cause expansion — floor tiles must be laid with sulphate-resistant adhesives; the screed surface must be sealed with a diluted PVA primer before laying any cement-based adhesive
• •*Not suitable for wet rooms*: Anhydrite screed is not suitable for use in shower enclosures or wet rooms — the calcium sulphate dissolves on prolonged contact with water; a cement-based product should be used in these areas
• •*Surface skin*: Anhydrite screed forms a hard crystalline skin when dried; this skin must be sanded off with a floor sander before laying the floor finish (laitance removal)

• *When to use anhydrite screed*:
• •When UFH is installed (anhydrite is the preferred screed type for UFH systems)
• •When programme is tight (faster installation and faster drying than sand-cement)
• •When the floor is large (over 50m²) — the speed advantage of pump-applied liquid screed becomes more significant
• •When flat floor tolerances are critical (for tile or polished concrete finishes with very low tolerance on flatness)

**Self-levelling compounds (self-levelling screed)**:

• Self-levelling compounds (SLCs — brands: Ardex, Mapei, Laticrete) are cement-based or calcium-sulphate-based products mixed with water to a pourable consistency and used for:
• •Topping an existing concrete slab to correct level differences and surface irregularities (without laying a full screed)
• •Levelling over existing solid floors before laying floor finishes (e.g., levelling over a slightly uneven original Victorian concrete floor in a refurbishment)
• •Thin-section levelling over screeded areas that are not perfectly flat

*Typical use in London renovations*: In a full house refurbishment where the existing ground floor concrete slab is sound but uneven, a 10–20mm SLC topping can level the slab for the floor finish without laying a full screed — saving programme time and cost. Where an existing screed has localised low areas, SLC can be used to 'spot level' rather than laying a full replacement screed.

**How wet UFH integrates with screed**:

Wet underfloor heating (water-filled pipe UFH) is the standard system for heated concrete screed floors in London extensions. The UFH pipe (typically Hep2O barrier pipe, MLCP composite pipe, or similar cross-linked polyethylene PE-X pipe, 16mm or 20mm OD) is installed in pre-designed loops on the insulation surface and clipped or stapled to hold the pipe in the correct position; then the screed is laid over the pipe.

• *Design considerations*:
• •*Pipe spacing*: Standard room zones use 200mm pipe spacing; higher-output areas (poorly insulated walls or unusually cold rooms) use 150mm spacing; bathrooms with tiled floors use 150mm spacing for faster warm-up
• •*Maximum loop length*: Individual loops should not exceed approximately 100m for 16mm pipe or 120m for 20mm pipe (to maintain adequate flow velocity and pressure drop) — larger rooms require multiple loops each connected to the manifold
• •*Manifold location*: The UFH manifold (the flow-and-return header from which each zone is controlled) must be accessible and in a suitable location — typically a utility room or purpose-built UFH manifold cupboard; all pipes connect from the zones to the manifold
• •*Pressure testing before screed*: The UFH pipe must be pressure-tested at 6 bar or 1.5× the system working pressure before the screed is laid — with the pipe at pressure throughout the screed pour to prevent collapse; the test certificate is a required milestone before the screed contractor will proceed

**The screed commissioning process (for UFH screeds)**:

Before laying any floor finish over a UFH screed, the screed must be commissioned:

1. *Initial drying period*: After laying, the screed is left to dry naturally for a minimum of 28 days (sand-cement) or 14 days (anhydrite) — windows should be kept closed for the first 48 hours to prevent surface cracking from rapid drying, then ventilation provided 2. *Commissioning heating cycle*: The heating system is set to 25°C flow temperature for 3–5 days, then increased by 5°C per day up to the maximum design flow temperature (typically 45°C for ASHP or 55°C for gas boiler) and held for 4 days, then reduced back to ambient 3. *Moisture content test*: A calibrated hygrometer or CM (calcium carbide) moisture test is carried out to confirm the screed has reached an acceptable moisture content for the intended floor finish (typically ≤75% RH for timber or LVT finishes; tile on cement adhesive can typically be laid at ≤80% RH) 4. *Surface preparation for anhydrite*: The crystalline surface skin (laitance) is sanded off with a floor sander; any cracked or hollow areas are assessed and repaired; the surface is primed before laying the floor finish

**Common mistakes that cause programme delays**:

• •*Laying screed without pressure-testing the UFH pipe*: If the pipe is damaged during screed laying and the test was not done beforehand, the screed must be broken out and replaced
• •*Trying to accelerate drying with excessive direct heat*: Pointing propane heaters directly at fresh screed causes surface cracking; the correct method is gentle air circulation at a controlled temperature
• •*Not allowing the commissioning cycle before laying floor finishes*: Laying tiles or timber flooring on an incompletely dried screed causes debonding of adhesive and warping of timber — expensive remedial works
• •*Allowing foot traffic too soon*: Sand-cement screed is not structurally mature enough for heavy foot traffic for 7 days; workers walking on it before this causes surface damage that then requires grinding or topping

**Typical screed costs for London (2025)**:

| Screed type and situation | Typical cost per m² (supply and install, exc. insulation) | |---|---| | Sand-cement screed 65mm, no UFH | £15–£25/m² | | Sand-cement screed 65mm, over UFH pipe (pipe supplied by others) | £18–£30/m² | | Anhydrite (liquid) screed 35mm, over UFH pipe (pipe supplied by others) | £22–£40/m² | | Anhydrite (liquid) screed 35mm, no UFH | £18–£32/m² | | Self-levelling compound, 10mm topping | £12–£22/m² |

• *Additional costs*:
• •UFH pipe and manifold supply: typically £18–£35/m² of heated floor area (supplied and installed by UFH subcontractor before the screed is laid)
• •Insulation below screed (typically 75–100mm PIR or 100mm EPS): £8–£20/m² (supply and install)
• •Damp-proof membrane: typically £2–£5/m²
• •Screed commissioning cycle (running heating system): no additional cost if the heating system is operational; contractor monitoring not usually charged separately
• •Laitance removal (anhydrite only): £5–£10/m²

**Programme: how long does screed take?**

For a 30m² extension (a typical kitchen-diner rear extension):

| Activity | Duration | |---|---| | Insulation and DPM preparation | 1 day | | UFH pipe installation | 1 day | | UFH pressure test | 1 day (test period) | | Sand-cement screed laying | 1 day | | Sand-cement drying to 75% RH | 55–70 days (depends on conditions) | | Sand-cement commissioning cycle | 14–21 days (typically overlapping last 2 weeks of drying) | | *Total to floor finish* | 65–80 days | | | | | Anhydrite screed laying | 0.5 day | | Anhydrite drying to 75% RH | 35–50 days (depends on conditions) | | Anhydrite commissioning cycle | 14–21 days | | Laitance removal | 1 day | | *Total to floor finish* | 45–60 days |

**What to look for in a screed contractor**:

• •*Experience with the specific screed type*: Anhydrite screed laying in particular requires specialist equipment (pump and hose) and experience with the material's specific characteristics (flow rate, self-levelling behaviour, drying requirements); not all screed contractors work with both types
• •*BS 8204 compliance*: The British Standard for screeds, bases, and in-situ floorings; a professional screed contractor should work to this standard and be able to demonstrate it
• •*Written mix specification*: For sand-cement screeds, the mix ratio, water/cement ratio, and additive specification should be documented; 'we've always done it this way' without a written specification is a warning sign
• •*Evidence of moisture testing*: A professional screed contractor or finishing contractor will carry out a calibrated moisture content test before releasing the floor for finish laying; if no moisture testing is proposed, push back and insist on it
• •*Suitable ambient conditions*: Screed should not be laid in temperatures below 5°C or above 30°C; in winter, the extension must be adequately enclosed and warmed before the screed is laid