Mortar Mix Ratios — Bricklaying, Blockwork, Rendering and Pointing

For general bricklaying above DPC, use a 1:1:6 mix (cement:lime:sand) or a 1:5-6 mix with plasticiser (cement:sand + plasticiser). This is BS 5628 Designation (iii) mortar, suitable for most above-ground brickwork including garden walls, house walls, and internal blockwork. Lime or plasticiser makes the mortar more workable and allows slight movement without cracking.

Below DPC, use a stronger 1:0.5:4-4.5 mix (cement:lime:sand) or 1:3-4 mix with plasticiser. This is BS 5628 Designation (ii) mortar. For very exposed locations or engineering brick work, use a 1:0.25:3 mix or 1:3 straight cement:sand — this is Designation (i), the strongest standard mix. Below DPC mortar must resist ground moisture and be durable against frost.

This is a common practice but is not recommended. Washing-up liquid creates air bubbles that weaken the mortar and can reduce its strength by up to 50%. Purpose-made plasticisers (such as Feb Mix or Sika Mortar Plasticiser) are designed to entrain a controlled amount of air without significantly reducing strength. The cost of proper plasticiser is minimal compared to the risk of weak mortar.

Render mixes are similar to mortar but typically richer (more cement) to provide weather resistance and a durable external finish. A standard render scratch coat uses 1:0.5:4-4.5 or 1:4 with plasticiser. The finish coat is slightly weaker (1:1:6 or 1:5-6) so that any cracking occurs in the finish coat rather than the base coat, making it easier to repair. Render should always be weaker than the substrate it is applied to.

For half-brick (stretcher bond) walls using standard metric bricks with 10 mm joints, you need approximately 0.02 m³ of mortar per m² of brickwork. For one-brick (English or Flemish bond) walls, approximately 0.04 m³ per m². For blockwork using 440 x 215 mm blocks, approximately 0.01 m³ per m². These figures assume 10 mm joints and include minimal waste.