Blockwork Wall Calculator — Blocks, Mortar & Wall Ties

Standard blocks (440mm × 215mm face) require 10 blocks per square metre of wall with 10mm mortar joints. This applies to both dense concrete blocks and aerated (aircrete) blocks. Add 10% for wastage and cuts. For a 10 m² wall, order 110 blocks. Foundation trench blocks are the same face size. Use our Block Calculator for a full materials list including mortar.

Below ground (foundations, below DPC): use a strong 1:3 cement:sand mortar — this is designation (i), class M12 (the strongest common mix), per BS EN 998-2 / PD 6678. Above ground general use: 1:1:6 cement:lime:sand, designation (iii) class M4, or 1:5–6 with plasticiser. Internal non-loadbearing: 1:2:9, designation (iv) class M2. Note: designation (ii) class M6 is the 1:½:4 mix for general above-ground masonry — not the below-DPC mix. Use sulphate-resisting cement in sulphate-bearing soils. Pre-mixed mortars in silos are increasingly popular for large projects. See our Mortar Calculator for quantities of cement, lime and sand.

Standard wall ties are placed at 900mm horizontal and 450mm vertical centres, giving approximately 2.5 ties per m² of wall. Around openings, ties should be at maximum 300mm centres within 225mm of the reveal. Stainless steel ties (to BS EN 845-1) are required in exposed locations and are recommended everywhere in the UK.

A DPC (damp-proof course) is laid at minimum 150mm above external ground level in the inner and outer leaves. It must be a continuous horizontal barrier of polyethylene or bitumen-based sheet. The DPC laps with the DPM (damp-proof membrane) in the floor slab. Additional DPC is needed at sills and thresholds.

Dense concrete blocks (7.3 N/mm²) are heavy, strong and used below ground, for load-bearing walls and fire walls. Aerated blocks (aircrete, e.g., Thermalite at 3.6 N/mm²) are lightweight, provide better insulation and are used for the inner leaf of cavity walls above ground. Dense blocks are typically grey; aerated blocks are white/cream.