External masonry walls

6.1.8Concrete blocks

Concrete blocks shall be capable of supporting intended loads, have appropriate thermal resistance and be resistant to the adverse effects of climate. Issues to be taken into account include:

  1. intended loads
  2. freeze/thaw and sulfate attack
  3. other characteristics.

Intended loads

Concrete blocks should:

  • comply with BS EN 771-3 & 4 and PD 6697 and be used in accordance with BS EN 1996-2 and PD6697
  • be of sufficient compressive strength for the application
  • be used in accordance with the manufacturer’s recommendations.

The maximum load-bearing capacity of the wall should not be exceeded. Other factors may dictate the strength of blocks required in certain circumstances, e.g. sulfate resistance may require blocks of greater strength.

For one and two storey homes, blocks with a minimum compressive strength of 2.9N/mm2 could be used.

For three storey homes or those with storey heights over 2.7m, 7.3N/mm2 blocks are required for certain parts of the structure, unless structural design shows that strengths lower than 7.3N/mm2 are suitable.

Freeze/thaw and sulfate attack

Concrete blocks used in the outer leaf without protective cladding or render should:

  • have a compressive strength >7.3N/mm2 or have a density of at least 1,500kg/m3
  • be made with dense aggregate to BS EN 12620, or
  • be aircrete concrete blocks having had their suitability confirmed by the manufacturer.

Where the level of sulfates in the ground, at the level where blockwork is to used, is DS-2 or above their suitability for use should be confirmed by the manufacturer. Where this is permissible, the mortar should be sulfate-resisting with a mix suitable for the level of sulfates in the ground.

Other characteristics

Concrete blocks may have been specified according to their specific characteristics.


Tolerances should be declared in accordance with the relevant product standard