Floor screed

What is a floor screed and why does it matter?

A floor screed is a cementitious layer, typically 40–80 mm thick, poured over a structural slab or resilient base to create a smooth, level surface for finished flooring. In modern Slovak residential construction, the screed serves three essential roles: it levels the structural slab; it distributes point loads across a larger area; and it encases underfloor heating pipes. This third role transforms the screed from a mechanical layer into a thermal component, making material choice and curing protocol critical to performance. Defects such as curling, cracking, and failed acoustic isolation are expensive to remedy and require opening the floor.

What are the three structural cases for screeds?

Screeds fall into three categories based on support and structural behaviour.

Bonded screed: Poured directly onto a clean structural slab with no separating layer, acting as a composite. The slab carries all loads, so the screed is thin (30–40 mm). Cheapest and fastest, suitable for ground floors or single-storey rooms without acoustic isolation. Risk: slab movement transmits directly to screed, often causing aligned cracking.

Unbonded screed: Poured onto a thin bituminous or plastic separating sheet on the structural slab. Mechanically separate but not acoustically isolated. Typical on ground floors with underfloor heating, where the separating layer protects thermal insulation from wet screed. Screed thickness is 40–60 mm to bridge voids. Less prone to slab-induced cracking but still vulnerable to drying shrinkage if movement joints are poorly spaced.

Floating screed: Sits on a resilient foam layer (20–50 mm, expanded polystyrene or mineral wool) that decouples it from the structure. Thickness is 40–60 mm, and the screed must span as a slab in its own right. This is the most demanding case structurally but the most effective acoustically, creating a spring-mass system that absorbs impact sound. Standard in multi-storey residential buildings and increasingly common in single-family houses where acoustic comfort matters.

What material families dominate screed practice?

Sand-cement screed: Cement and sand, mixed semi-dry on site and tamped. Cheap and familiar, but shrinks significantly as it dries. Edges and corners dry faster than the core, causing curling and cracking. Requires movement joints at 5 m intervals and slow, controlled drying. Drying to finish-ready typically takes 20–30 days for 50 mm. Poor choice for underfloor heating; heating must be delayed until the screed has begun curing.

Flowing cement-based screed: Factory-mixed, pumpable slurry that self-levels. Contains plasticisers and flow agents. Cures faster than sand-cement (3–4 weeks to finished), shrinks less, and requires control joints at 6–8 m. More expensive but reduces labour and provides superior flatness. Better for underfloor heating, though less ideal than anhydrite. Thermal conductivity around 1.4 W/mK.

Calcium sulfate (anhydrite) screed: Binder based on calcium sulfate, cured by crystallisation rather than water loss. Shrinks roughly half as much as cement-based screeds, tolerates larger joint spacing (8–10 m), and is superior for underfloor heating. Dries faster (7–10 days to suitable moisture), has higher thermal conductivity (1.6–2.0 W/mK), and can be applied thinner (as thin as 20–25 mm). Trade-offs: higher cost, higher moisture sensitivity (unsuitable for kitchens and wet areas unless sealed), and unfamiliar to some contractors.

What are the common failure modes?

Curling and edge cracking: Cement-based screeds dry from top and edges inward; surface shrinks more than core, pulling into tension. Edges curl upward, cracks radiate from corners and restrained edges. Prevention: movement joints at correct spacing (5 m sand-cement, 6–8 m flowing cement), controlled water-cement ratio, and slow drying. Anhydrite screeds curl far less because they cure by crystallisation.

Cracking in large bays: If a bay is too large or poorly shaped, shrinkage stress builds too fast. Cracks form perpendicular to the long axis. Correctly sized and positioned movement joints allow each section to shrink independently, preventing stress concentration.

Soft or dusty surface: Over-watering, standing water, or rushing drying by high heat can leave the surface weak and friable. Cement paste separates from aggregate, shedding material underfoot. Prevention: correct water content, protection from standing water during curing, and gradual drying.

Acoustic short circuit: A floating screed is only effective if truly decoupled. Any rigid contact bypasses the resilient layer and transmits impact noise directly into the frame. Common causes: edge strip cut too short (screed touches plaster), heating pipe or cable clip fixed through the resilient layer, skirting screwed to both wall and screed, or partition wall built on the floating slab instead of the structural deck. Even one rigid path destroys floating performance, typically costing 3–5 decibels.

Why the perimeter edge strip is critical and must never be trimmed?

The perimeter (or edge) strip is a continuous foam band (10–15 mm thick) running around the entire room perimeter between screed and wall, forming part of the overall floor structure buildup. In floating floors, it is essential to acoustic performance. It must run continuously from the structural slab all the way up the wall where the final skirting sits. It must NOT be trimmed away after hardening to make room for the skirting. If trimmed, the skirting is screwed to both wall and screed simultaneously, creating a rigid path that defeats the floating layer. Once covered, this defect is invisible but audible: every footstep transmits through to the structure below.

How do drying and residual moisture affect finishing?

The most common and expensive failure in heated screeds is moisture-related. A screed is walkable after 3–7 days but remains saturated internally. Residual moisture must be measured, not guessed, because drying depends on thickness, temperature, humidity, and ventilation rather than calendar days. A 50 mm sand-cement screed can take 6–8 weeks in cool, damp conditions or 3–4 weeks in warm, well-ventilated conditions. Anhydrite screeds dry faster, often reaching suitable moisture (typically below 3%) in 2–3 weeks.

Before any vapour-tight finishing layer (vinyl, epoxy, sealed sealants) is laid, the screed's residual moisture must be verified by calcium carbide test or equivalent to confirm it is below the finish manufacturer's specification. This is mandatory, not optional. Vapour-tight layers installed over damp screeds trap moisture, which migrates into adhesives and finishes, causing delamination and mould growth.

In screeds with embedded underfloor heating, a controlled commissioning heat-up is required before finishing. The heating system is gradually brought to operating temperature over 3–5 days, then held at full temperature while moisture is vented through open windows and doors. This heat-driven drying accelerates the process and ensures trapped moisture is expelled before the finish is sealed. Skipping or rushing commissioning is a leading cause of floor finish failure in heated screeds.