Floor Screed: Complete UK Guide

What is floor screed?

Floor screed is a cementitious material applied in a thin layer over a concrete slab. It creates the final smooth, level surface you walk on or lay flooring onto. The concrete slab provides structural strength. The screed provides the finish.

Screed sits on top of insulation, damp-proof membranes, and underfloor heating pipes. It's trowelled flat by hand (traditional screed) or poured and left to self-level (flowing screed). Once cured, it's hard-wearing enough for direct foot traffic or ready for tiles, vinyl, carpet or wood.

The term "screed" is sometimes used to describe the straight edge (a screed board) used to level the material. In floor construction, "screed" means the layer itself.

Types of floor screed

Traditional sand-cement screed

The most common type in UK domestic builds. Mixed on site from sharp sand and cement, then spread by hand and trowelled smooth. Typical thickness 50-75mm. Cures slowly (50+ days). Low material cost but labour-intensive. Good for small areas and DIY. Strong and durable when properly cured.

Flowing liquid screed (anhydrite or calcium sulphate)

Poured from a pump truck and self-levels without trowelling. Much faster to lay (100m² in a day vs 20m² by hand). Thinner layers possible (30-40mm). Faster drying (faster than sand-cement at thin depths). More expensive material cost but lower labour. Popular for large sites and underfloor heating installs.

Rapid-drying screed

Modified sand-cement with additives to speed curing. Walk-on in 24-48 hours, flooring after 7-14 days instead of 50+ days. More expensive. Used when programme time is tight. Available as bagged product or ready-mix.

Polymer-modified screed

Sand-cement with polymer additives for improved adhesion, flexibility and crack resistance. Used for bonded screeds (thin layers stuck directly to concrete) or areas with slight movement. Costlier than standard but performs better in demanding conditions.

Screed thickness requirements

Thickness depends on the screed type and whether it's bonded, unbonded or floating.

• Bonded screed: 25-40mm minimum. Stuck directly to concrete with no membrane between. Needs good adhesion.
• Unbonded screed: 50mm minimum. Laid over a damp-proof membrane. The membrane breaks the bond, so the screed must be thick enough to resist cracking.
• Floating screed: 65-75mm minimum (75mm if walking directly on screed). Laid over insulation. The insulation compresses slightly, so extra thickness prevents cracking.

For underfloor heating, floating screed is usual. The pipes sit on insulation, screed goes over the top. 65mm is typical (measured from top of pipe).

Mix ratios for traditional screed

Standard mix: 1 part cement to 3 or 4 parts sharp sand by volume. The exact ratio depends on required strength and workability.

• 1:3 mix: Stronger, less workable. Used for heavy-duty floors or thinner bonded screeds. Harder to trowel smooth.
• 1:4 mix: Easier to work, slightly weaker. Standard for most domestic floating screeds. Good balance of strength and workability.

Use sharp sand (also called grit sand or concreting sand), not building sand. Sharp sand has angular grains that lock together for strength. Building sand is too fine and weak.

Add water gradually until you reach a semi-dry consistency. The mix should hold together when squeezed but not be wet or sloppy. Too wet causes shrinkage cracks. Too dry makes trowelling difficult.

How to lay traditional screed

1. Prepare the base: Concrete slab must be clean, dry and sound. Lay insulation (if floating screed) or DPM (if unbonded). Tape all joints in the membrane.

2. Set level pegs: Mark the finished screed level around the room. Install timber battens or screed rails at the correct height as guides.

3. Mix the screed: Use a forced-action mixer for consistent results. Mix to semi-dry consistency (holds shape when squeezed).

4. Spread and compact: Tip the mix onto the floor between the battens. Spread it roughly level, then compact by tamping with a beam. Finish with a float or power trowel.

5. Cure properly: Cover with polythene after 24 hours to prevent too-rapid drying. Keep covered for 7 days. Walk-on after 48 hours, but don't load it or lay flooring yet.

Drying times

Screed dries from the surface down. Moisture trapped in the lower layers takes weeks to evaporate.

Traditional sand-cement screed

Rule of thumb: 1 day per mm for the first 50mm, then slower. A 50mm screed is dry enough for most flooring after 50 days. A 75mm screed takes 75-90 days. In winter or poorly ventilated rooms, add 30-50% to these times.

Flowing screed

Faster drying at thin depths due to lower water content and faster evaporation rates. A 40mm flowing screed can be ready for flooring in 21-28 days with good ventilation. Check manufacturer data for specific products.

Rapid-drying screed

Walk-on in 24-48 hours, flooring after 7-14 days. Much faster programme but higher material cost.

Always test moisture content before laying impermeable flooring (vinyl, laminate, resin). Use a hygrometer or calcium chloride test. Most vinyl requires less than 75% RH. Laying too early traps moisture and causes adhesive failure or bubbling.

Common problems

Cracking

Caused by shrinkage as the screed dries, or movement in the base. Prevent by using the correct mix (not too wet), proper thickness for the screed type, and movement joints in large areas. Hairline cracks are common and not structurally serious, but wide cracks (3mm+) indicate a problem.

Curling

Edges of the screed lift up. Happens when the top dries faster than the bottom, creating differential shrinkage. Prevent by proper curing (cover with polythene) and adequate thickness. Curling is more common in thin bonded screeds or over-wet mixes.

Delamination

The screed separates from the base. Only an issue for bonded screeds. Prevent by ensuring the concrete is clean, applying a bonding agent, and laying the screed while the bonding agent is still tacky.

Dusting

Surface powders when walked on. Caused by a weak mix (too much sand, too little cement), or trowelling when there's surface water (brings weak material to the top). Prevent by correct mix ratios and proper trowelling technique.

Cost

• Traditional screed (materials only): £3-5 per m² at 50mm for DIY mix
• Traditional screed (installed): £15-25 per m²
• Flowing screed (installed): £20-30 per m²
• Rapid-drying screed (installed): £25-35 per m²

Flowing and rapid screeds cost more in materials but save labour time. For large areas (100m²+), flowing screed is often cheaper overall despite higher m² cost because it's laid so much faster.

Related guides

• Self Levelling Compound Guide
• Concrete Screed Mix Ratios
• Liquid Screed Explained
• Self Leveling Concrete