This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Brickwork carried out from the face of a wall in projecting courses for the purpose of supporting a load such as that brought by a wall-plate, as shown in Fig. 145, or the end of a beam, is called Corbelling. All the projecting bricks should be laid as headers, and should not project more than a quarter of a brick beyond the course below, although, where extra strength is required, this projection should be reduced to one-eighth of a brick. Where wall-plates have to be carried, the greatest projection should not be less than 4 1/2
inches, and the topcourse of the corbel should bedoubled. It is always best to build corbels in cement mortar.