Roy Blount has written:

'About three bricks shy of a load'

At Least 300 Bricks

Blocks are load bearing, bricks are not.

An expression meaning someone who is 'a bit simple, stupid etc'. (IE Not having a full brain)

There are other similar expressions e.g. 'A few sandwiches short of a picnic'.

The compressive strength of bricks varies depending on the type of bricks and what they are used for. The compressive strength of conventional red bricks is 40 to 65 Kg/cm2. In comparison, flyash bricks have a compressive strength of 70 to 120 Kg/cm2.