UK Block Types & Specifications

Dense concrete blocks (typically 7.3 N/mm² or 10.4 N/mm²) weigh about 20 kg each and are used below ground, for structural walls and where high strength is needed. Lightweight aggregate blocks (typically 3.6 N/mm² or 7.3 N/mm²) weigh about 12-15 kg and offer better thermal insulation. Aerated blocks (like Thermalite or Celcon) weigh only 7-9 kg and have the best thermal performance, but lower compressive strength.

For standard domestic construction: 7.3 N/mm² dense or lightweight blocks for external walls, 3.6 N/mm² lightweight or aerated blocks for inner leaves and non-load-bearing partitions. Below ground requires 7.3 N/mm² minimum, typically dense blocks. For load-bearing walls over two storeys, a structural engineer should specify the block strength based on the actual loads.

Thermalite (Forterra) and Celcon (H+H) are the two main UK brands of autoclaved aerated concrete (AAC) blocks. They are made from cement, lime, sand, pulverised fuel ash and aluminium powder, which creates millions of tiny air cells. This gives excellent thermal insulation (lambda values from 0.09 to 0.15 W/mK) and very light weight. They are easy to cut but require specific fixings for heavy items.

Generally no. Standard aerated blocks are not suitable below DPC or in wet conditions. Some manufacturers produce specific foundation-grade aerated blocks (e.g. Thermalite Trenchblock) designed for below-ground use in trenches with concrete backfill. These have higher compressive strength and sulphate resistance. Always check manufacturer recommendations.