Concrete, Mortar & Screed Mixes

1The Basics of Mix Design

Concrete, mortar, and screed all use the same fundamental ingredients — cement, fine aggregate (sharp sand or soft sand), and water — but the proportions and aggregate types vary significantly depending on the application. Getting the mix wrong leads to cracking, weak joints, or structural failure.

2Concrete Mix Ratios

Concrete is classified by its characteristic compressive strength (C10, C20, C25, C30 etc.). Mix ratios by volume are a useful rule of thumb but for structural work always refer to a specified mix design.

• C10 (1:3:6) — blinding, non-structural fill, and lean-mix sub-bases.
• C20 (1:2:4) — mass concrete foundations for domestic extensions and outbuildings.
• C25 (1:1.5:3) — standard domestic reinforced slab and pads.
• C30 (1:1:2) — driveways, garage floors, and areas subject to vehicle loading.
• C35 / GEN 4 — commercial slabs, exposed external paving.

3Mortar Mix Ratios

Mortar uses soft (building) sand rather than the sharp sand used in concrete. The right mix depends on the brick or block type and the exposure conditions.

• 1:3 (cement:sand) — very strong mortar for engineering bricks and high-exposure chimney stacks. Can be brittle with softer bricks.
• 1:4 — general-purpose mortar for below-DPC brickwork and exposed external walls.
• 1:5 — most common mix for above-DPC external brickwork.
• 1:6 — light-duty mortar for internal blockwork and sheltered locations.
• 1:0.5:4.5 (cement:lime:sand) — lime-modified mortar for traditional and heritage brickwork; more flexible and breathable.

4Floor Screed Mixes

Traditional (bonded) screed is mixed at approximately 1:3 to 1:4.5 (cement:sharp sand) at a water-cement ratio of around 0.45. It should be semi-dry — the mix should hold its shape when squeezed in the fist but not release water. Minimum thickness is 25mm when bonded to a concrete base, 65–75mm when unbonded (on a DPM), and 65mm for floating screed over insulation.

5Render Mixes

External render is typically applied in two or three coats. The base (scratch) coat should be stronger than the top coat — a common combination is a 1:3 or 1:4 mix for the base and a 1:5 or 1:6 mix for the finish. Never apply a strong mix over a weaker one as it will crack away from the substrate. Adding lime (1:1:6) improves workability and reduces cracking.

6Mixing by Hand vs Machine

For small volumes (up to about 50kg), hand mixing on a spot board or in a tub is practical. For larger volumes, a cement mixer is far more consistent and less physically demanding. Rotating drum mixers produce good concrete but can under-mix mortars — pan mixers are more effective for mortar and screed. Always add dry ingredients first, then water gradually, to achieve the correct consistency.

7Ready-Mix & Bagged Products

For large pours (above 1m3), ordering ready-mixed concrete from a batching plant is almost always more economical and consistent than site mixing. Specify your strength class, aggregate size, workability (slump), and any admixtures (retarder, plasticiser, fibre). For smaller jobs, our range of pre-blended bagged products (post-mix concrete, pointing mortar, floor screed) removes the guesswork from mix design.