This is a property of bricks which can be determined accurately. The compressive strength of bricks is found by crushing 12 of them individually until they fail or crumble. The pressure required to crush them is noted and the average compressive strength of the brick is stated as newtons per mm of surface area required to ultimately crush the brick. The crushing resistance varies from about 3.5N/mm2 for soft facing bricks up to 140N/mm2 for engineering bricks.
The required thickness of an external brick wall is determined primarily by its ability to absorb rainwater to the extent that water does not penetrate to the inside face of the wall. In positions of moderate exposure to wind driven rain a brick wall 215 mm thick may absorb so much water that it penetrates to the inside face.
The bearing strength of a brick wall 215 mm thick is very much greater than the loads a wall will usually carry. The current external wall to small buildings such as houses is built as a cavity wall with a 102.5 mm external leaf of brick, a cavity and an inner leaf of block. The external leaf is sufficiently thick, with the cavity, to prevent penetration of rain to the inside face and more than thick enough to support the loads it carries.
It is for heavily loaded brick piers and walls that the crushing strength of brick is a prime consideration.
The average compressive strength of some bricks commonly used is:
Mild (i.e. soft) stocks 3.5N/mm2
2nd Hard stocks 17.5 N/mm2
Flettons 21 N/mm2
Southwater A 70 N/mm2