Substructure and ground-bearing floors

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Substructure and walls below the DPC shall be capable of supporting their intended loads and, where necessary, be resistant to frost action, sulfates and other harmful or toxic materials. Issues to be taken into account include:

  1. brickwork
  2. blockwork.

Frost damage occurs on saturated masonry exposed to freezing conditions. Bricks, blocks and mortars located 150mm above and below ground level are the most likely to be damaged by frost.

Masonry walls below the DPC should be designed and constructed as described in Chapter 6.1 ‘External Masonry Walls’.

Recommendations for the design strength of bricks, masonry blocks and mortars are given in BS EN 1996-1-1.


Bricks should be of suitable durability, especially in the outer leaf below the DPC, or where they could be frozen when saturated. Bricks used in retaining walls should be suitable for the exposure and climate, as recommended by the manufacturer.

Clay bricks should comply with BS EN 771, which classifies bricks according to their durability designation (F) and to the content of active soluble salts (S).

F0Not freeze/thaw resistant and should not be used externally
Moderately freeze/thaw resistant
F2Freeze/thaw resistant
S1Normal active soluble salts
S2Low active soluble salts

Generally, bricks are designated to F1,S2 or F1,S1. If in doubt as to suitability, bricks of F2,S2 or F2,S1 should be specified, or the manufacturer consulted and written confirmation obtained in relation to:

  • geographical location
  • location in the structure.

Calcium silicate bricks for use below DPC should be at least compressive strength class 20.


Concrete blocks for use below the DPC should meet BS EN 771 and one of the following

  • Minimum density of 1500kg/m3
  • Minimum compressive strength of 7.3N/mm2
  • assessed in accordance with Technical Requirement R3.

Where it is necessary to resist sulfate attack and ensure adequate durability, blocks made with sulfate-resisting cement and/or a higher than normal cement content should be used.

Where there is doubt regarding the suitability of the block, particularly where acids or sulfates occur, written confirmation of its suitability should be obtained from the manufacturer in relation to:

  • geographical location
  • location in the structure.