Ties of cast iron, Figs. 8,9, or of wrought iron, Figs. 10, 11, and x and y Fig. 7, dipped when hot in tar, are frequently used instead of bonding bricks, and have the advantage of not being liable to be broken if the wall should settle unequally. On the other hand, they are subject to decay by rust, and to expansion from the same cause, which may injure the wall.
Fig. 7. Scale, ½ inch = 1 foot.
The ties are about 8 inches long, ¾ inch wide, by 1/10 inch thick; they are placed about 3 feet apart, horizontally, and with 9-inch vertical intervals between the rows.
1 Figs. 8 to 11 are from Messrs. Chambers, Monnery, and Co.'s advertisements.
Cast-iron cramps are made about ½ inch wide and 3/16 thick, and somewhat similar in form to the above.
The hollow wall is often arranged to begin on the damp-proof course (see page 6), but it is better to continue the hollow for two or three courses lower, as shown in Fig. 7, so that any wet falling into the cavity may be well below the damp course. When this is done the asphalte course may be continued only across the inner thickness (AB, Fig. 7) of the wall. A covering course of brickwork is placed on the top of the air-space, which should have no communication with the outer air.
Some walls are built entirely of hollow bricks made for the purpose.