The Difference Between Concrete and Screed

The Difference Between Concrete and Screed

Concrete and screed are both cement-based materials, but they serve very different purposes in construction. Understanding the differences is important for specifying the right material for each layer of a floor build-up.

Composition

Both materials contain cement, aggregate, and water, but the mix proportions and aggregate sizes differ significantly:

  • Concrete uses coarse aggregate (typically 10-20mm gravel or crushed stone) combined with sand and cement. A typical structural concrete mix might be 1:2:4 (cement:sand:coarse aggregate) by volume
  • Screed uses fine aggregate only (sharp sand, typically 0-4mm) with cement. A standard screed mix is typically 1:3 to 1:4.5 (cement:sand) by volume

The absence of coarse aggregate in screed is what allows it to be finished to a much smoother, flatter surface than concrete.

Purpose and Application

  • Concrete provides the structural floor slab — it carries the building loads and is typically 100-300mm thick depending on the structural design. It is placed by pouring, vibrating, and power floating
  • Screed is a finishing layer applied over the structural concrete (or over insulation/membranes) to provide a smooth, level surface for the final floor finish. Screed thickness typically ranges from 25mm (bonded) to 75mm+ (floating)

Finish Quality

This is one of the most significant differences. Concrete is a structural material — while it can be power-floated to a reasonable finish, achieving a truly flat, smooth surface is difficult due to the coarse aggregate. Screed, with its fine aggregate, can be finished to very tight surface regularity tolerances (SR1, SR2, or SR3 per BS 8204-1), making it suitable as a base for sensitive floor finishes like vinyl, carpet, and ceramic tiles.

Strength

Concrete is typically much stronger than screed in compressive terms. Structural concrete commonly achieves 25-40 N/mm² compressive strength, while standard floor screeds are designed to 20-30 N/mm². However, screed strength is measured differently — using the BRE screed tester for surface hardness rather than cube crushing tests — because its function is different.

Consistency

The two materials have very different workability characteristics:

  • Concrete is a wet, pourable material with a measurable slump. It flows to fill formwork and is compacted by vibration
  • Traditional screed is a semi-dry material — it should hold together when squeezed but not release free water. It is placed and compacted by hand tamping
  • Flowing (liquid) screeds are an exception — calcium sulphate and some polymer-modified cement screeds are self-levelling liquids, but these are specialist products distinct from traditional screeds

Drying and Curing

Both materials require curing (keeping moist for the first 7 days to allow cement hydration), but their drying characteristics differ significantly. Concrete, being thicker and having a higher cement content, generates more heat during hydration. Screed is thinner and dries more quickly in relative terms, though the traditional rule of 1mm per day for the first 40mm and 0.5mm per day thereafter still means a 75mm screed can take 95+ days to dry to the 75% RH level needed for most floor finishes.

When Each is Used

In a typical floor construction, you will find both materials in the build-up: concrete provides the structural slab, and screed provides the finishing layer. However, there are situations where the distinction blurs — granolithic toppings (high-strength wearing screeds) serve a dual purpose, and power-floated concrete floors sometimes eliminate the need for a separate screed layer in industrial settings.

For guidance on selecting the right screed to finish over your concrete slab, read our guide to choosing the right screed or contact our technical team.

Call us on 0118 370 2060 for expert advice on your project. Free delivery on orders over £600 ex-VAT.