In the absence of iron cramps or bonding bricks, hollow walls'may be built with ordinary bricks placed on edge, after being dipped in boiling tar to make them as non-absorbent as possible. Every course is composed of alternate headers and stretchers, so arranged that each header comes immediately over the centre of a stretcher in the course below. The wall thus formed consists of two portions, each 3 inches thick, separated by a 3-inch space.
Another plan is to lay the bricks as in ordinary English bond, leaving a space of about 2½ inches between the stretchers in the front and back. This makes the wall (4½ + 2 ½ + 4½ ) = 11½ inches thick, and the headers are therefore too short to reach from face to back ; the deficiency is made up by inserting bats at the ends of the headers.
These and other plans adopted for building hollow walls with ordinary bricks are defective in strength as compared with the walls constructed with special bonds or cramps, and, moreover, the common bricks being porous, conduct moisture to the interior of the wall and defeat the object aimed at in making it hollow.
Where the lintels of doors and windows occur in a hollow wall with a 4 ½-inch exterior portion, the following arrangement may be adopted to prevent the wet which may enter the air-space from dropping upon the window or door frame.
Just above the window or door head a piece of sheet lead is built in on the inner side of the 4½-inch exterior wall. This lead may be 4¾ inches wide, 2 inches being built into the 4½-inch wall, 1¾- inch projecting into the air-space, and the remaining inch turned up so as to form a sort of gutter, which should be carried about 2 inches farther than the ends of the lintel each way, so as to lead the water clear of the door or window frame.