Question 1

What is the difference between dense and lightweight blocks?

Accepted Answer

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.

Question 2

What block strength do I need?

Accepted Answer

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.

Question 3

What are Thermalite and Celcon blocks?

Accepted Answer

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.

Question 4

Can I use aerated blocks below ground?

Accepted Answer

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.

Type	Strength (N/mm²)	Density (kg/m³)	Weight (100 mm, kg)	Lambda (W/mK)
Dense concrete	7.3 / 10.4 / 22.5	1800–2100	~20	1.13–1.46
Lightweight aggregate	3.6 / 7.3 / 10.4	1000–1400	~12–15	0.27–0.50
Aerated (AAC) — standard	2.9 / 3.6	460–600	~7–9	0.11–0.15
Aerated (AAC) — high strength	7.3	700–800	~10	0.17–0.19
Trenchblock (AAC foundation)	3.6 / 7.3	460–700	~16 (300 mm wide)	0.11–0.17

Face Size (mm)	Thicknesses (mm)	Blocks per m²
440 x 215	75, 100, 140, 150, 190, 200, 215	~10
440 x 140 (Trenchblock)	255, 275, 300, 305, 355	~15

Application	Recommended Block	Min. Strength (N/mm²)
Below ground (foundations)	Dense concrete or trenchblock	7.3
External wall — inner leaf	Lightweight aggregate or aerated	3.6
External wall — single leaf	Dense or lightweight aggregate	7.3
Internal partitions	Aerated blocks (75 mm or 100 mm)	2.9
Party walls (separating walls)	Dense concrete (215 mm)	7.3

UK Block Types & Specifications

Common UK Block Types

Standard UK Block Sizes

Where to Use Each Block Type

Related Calculators

Frequently Asked Questions

Related Calculators

Related Calculators

UK Block Sizes

Fixings Guide by Wall Type

UK Insulation Lambda Values

Movement Joint Spacing

Was this calculator helpful?