This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In the process of constructing a wall, the mason or bricklayer first lays the "footings" on the foundation platform. The footing is an enlarged portion of the wall fur the purpose of distributing the weight over the foundation: it is properly a portion of the wall and not of the foundation, although it is not always easy to draw the line between them. When the pressures pass down through the centre of the wall, the footings may project equally on each side; when otherwise, the footings should be so arranged that the line of pressure shall pass nearly through the centre of them into the foundation. The size of footings and the mode of forming the increase to the thickness of the wall must depend on the circumstances and the material. For ordinary buildings, Tredgold recommends that the extreme breadth of the footing, when the subsoil is clay or sand, be double the thickness of the wall; if on gravel or chalk subsoil, that its breadth be to that of the wall as 3 to 2.
Supposing the whole pressure per lineal foot on the wall to be equally distributed over the breadth of footing a b, Fig. 1397, then the reaction of the subsoil on the part b c will be equivalent to that proportion of the whole pressure, acting upwards and tending to break the projecting part b c about the section c d, which section must be strong enough to resist that transverse strain; in brickwork it is usual to make the projection of a footing for light buildings 1/4 of a brick in every course, and for heavy buildings 1/4 of a brick in every 2 courses. In stonework the proportional projection for a given height of course may be greater, according to the relative transverse length of the stone. The footings should always be made of large stones or of picked bricks, laid in very good mortar, and well bonded, with the object of distributing the pressure as uniformly as possible over the foundations. The foundation platform should, if feasible, be in one horizontal plane, and the footings should be equal in height throughout the main walls of a building, in order to avoid, as much as may be, irregularity of settlement from unequal heights of wall.
The " damp course," as it is commonly called, is a course of some impervious material to prevent the damp rising from the ground through the masonry into the body of the wall. It is generally placed immediately above the footings, if these project above ground; but the damp course should be, if possible, 1 ft. above the ground. It generally consists of 2 or 3 courses of hard-burnt bricks laid in hydraulic mortar. A highly-burnt glazed hollow brick is made for the purpose, the perforations being horizontal, so that a current of air passes through the wall at that point. Perforated bricks are liable to crack under pressure.