Both stone and brick are used for the backing of ashlar. Brick is more largely used for this purpose than stone, because in most cases it is the cheapest, and it possesses the further advantage that the plaster may in dry climates be applied directly to the brick, while the stone backing generally has to be plugged and stripped for lathing. If brick is used for backing the joints should be made as thin as possible, and it is desirable to use some cement in the mortar to prevent shrinkage in the joints. The backing, if of brick, should never be less than 8 inches in thickness. If a hard laminated stone, with perfectly flat and parallel beds, can be obtained for backing, it makes a stronger job than brick, but irregular rubble blocks are not suitable for anything but dwelling house walls, unless the walls are made one-fourth thicker than with brick backing. The backing, whether of brick or stone, should be carried up at the same time with the ashlar, and, if of stone, should be built in courses of the same thickness as the ashlar, as shown in B, Fig. 117.
Bonding. - Ashlar not exceeding 12 inches in height is usually bonded sufficiently to the backing by making the stones of different thicknesses, as in Fig. 117, and by having one through stone to every 10 square feet of wall.
Where the ashlar is only about 2 inches or 4 inches thick, as is generally the case with marble, and often with sandstones, each piece of ashlar should be tied to the backing by an iron clamp, about 1/8 of an inch thick and 1 or 1¼ inches wide, with the ends turned at right angles, as shown in Fig. 114. The anchors should be made of just the right length for the longer end to turn up just on the inside of the wall. Every stone should have one clamp, and if over 3 feet long two clamps should be used. There should also be belt courses about every 6 feet, extending 8 inches or more into the wall, to give support to the ashlar.
The effective thickness of a wall faced with thin ashlar is only equal to the thickness of the backing. When iron clamps are used for tying the ashlar they should be either galvanized or dipped into hot tar to prevent being destroyed by rust.