Polished Concrete Floors: Design and Specification Guide

Introduction

Polished concrete floors have become a highly valued finish in high-quality residential extensions in north London — particularly in open-plan kitchen-dining extensions where the seamless, monolithic quality of a poured and finished concrete slab creates a distinctly contemporary aesthetic that contrasts effectively with the period character of the original house. A polished concrete floor is also highly practical: extremely durable, compatible with underfloor heating, easy to clean, and requiring no grout lines or joints that collect dirt. This guide explains how polished concrete floors are designed, poured and finished for residential use.

What Is a Polished Concrete Floor?

A polished concrete floor is a concrete slab that has been ground, honed and polished to a smooth, dense surface, then sealed to produce a functional and aesthetic floor finish. The process is distinct from concrete overlay systems (thin toppings applied to existing slabs) and from resin-bound aggregate finishes — polished concrete uses the concrete slab itself as the finished surface.

The final appearance is determined by the depth of grinding:

• Cream or salt and pepper finish: Light surface grinding (1–2mm depth) exposes the fine aggregate sand and cement matrix. Produces a consistent, speckled appearance with minimal aggregate exposure.
• Medium aggregate exposure: Deeper grinding (3–5mm) exposes fine pea gravel aggregate. The aggregate type, colour and distribution become the dominant visual element.
• Full aggregate exposure: Deep grinding (5–10mm+) exposes larger aggregate, revealing the full character of the mix. Allows highly decorative effects using selected aggregates.

Concrete Mix Design

The appearance of a polished concrete floor is largely determined by the design of the concrete mix itself. Key variables include:

• Cement type and colour: Standard grey Portland cement (OPC) produces a grey slab. White Portland cement or a blend produces a lighter, warmer finish. Pigments can be added to the mix to produce coloured concrete — terracotta, charcoal, warm buff or other tones.
• Aggregate: The coarse aggregate — typically pea gravel, limestone chips, granite, quartz or recycled glass — determines the appearance at medium and full exposure grinds. Aggregate should be specified by the concrete supplier to the designer's requirements: type, size, colour and distribution.
• Water-cement ratio: A low water-cement ratio (0.40–0.45) produces a denser, harder concrete that polishes more easily and produces a better surface finish. Consistency of mix is important to achieve consistent results across the slab.
• Admixtures: Plasticisers and retarders can improve workability and control the setting time, particularly important in large pours to avoid cold joints (visible lines where pours meet).

Slab Design and Construction

A polished concrete floor in a rear extension is typically a ground-bearing slab (supported directly on the ground) or a suspended slab (spanning between supports, used in basement extensions or where ground conditions are poor). The slab must be:

• Minimum 100mm thick for ground-bearing domestic applications; 150mm+ for suspended slab applications
• Cast on a damp-proof membrane on a well-prepared, compacted sub-base
• Reinforced with fabric reinforcement (A142, A193 mesh) or fibre reinforcement to control shrinkage cracking
• Finished to a flat, level surface — flatness tolerance is critical for polished concrete; a floor level tolerance of ±3mm over 3m (SR2 or better) is required

Crack control is the key challenge in polished concrete. Concrete shrinks as it cures, and without adequate reinforcement and joint design, unsightly cracks will appear. Saw-cut control joints at appropriate intervals (typically 4–6m in each direction for domestic slabs) allow controlled cracking at the joint lines rather than random cracking across the slab. In an open-plan extension where control joints are unacceptable aesthetically, fibre reinforcement and a very low water-cement ratio reduce shrinkage cracking risk, but cannot eliminate it entirely.

Underfloor Heating Compatibility

Polished concrete is an excellent medium for wet underfloor heating (UFH) — the high thermal mass of the concrete slab stores heat effectively, and the conductivity of concrete is significantly higher than timber or screed. UFH pipes are embedded in the structural slab during pour, typically in the upper third of the slab depth. Design of the UFH system must account for the thermal mass of the concrete — a concrete slab takes longer to heat up from cold than a thin screed, making the system less responsive but more stable in temperature once at equilibrium. This characteristic suits a property that is heated continuously or with gradual setback rather than rapid daily cycling.

The Polishing Process

The polishing process takes place after the concrete has cured for a minimum of 28 days (full strength) and typically after the wet trades (plastering, tiling) in the extension are complete. The process involves:

1. Initial grinding: Coarse diamond tooling removes the surface laitance and achieves the target depth of aggregate exposure
2. Progressive honing: Successively finer diamond grits (typically 100, 200, 400 grit) smooth the surface and remove the scratches from coarser grinding
3. Densifier application: A lithium silicate or sodium silicate densifier hardens the surface by reacting with free calcium hydroxide in the concrete to form additional calcium silicate hydrate
4. Polishing: Fine diamond pads (800, 1,500, 3,000 grit) produce the final sheen level — from a low matte finish to a high-gloss mirror finish
5. Sealing: A penetrating sealer, topical acrylic sealer or polyurethane coating protects the surface from staining and abrasion

Maintenance

A polished and sealed concrete floor requires little maintenance beyond regular sweeping and occasional mopping with a neutral pH cleaner. Acidic cleaners must be avoided — they attack both the concrete surface and the sealer. Re-sealing is required every 5–10 years depending on the sealer type and traffic levels. Scratches from furniture or heavy objects may require localised re-polishing.

Costs

Conclusion

A polished concrete floor is a sophisticated finish that rewards careful design, mix specification and construction. The aesthetic quality of the finished floor is largely determined by decisions made at the concrete mix design and pour stage — once the concrete is cast, the palette is fixed. For architects designing high-quality rear extensions in north London, polished concrete is an increasingly standard specification alongside stone and engineered timber floors, providing a contemporary finish that performs exceptionally well in open-plan ground-floor extensions with underfloor heating.