This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Random Work. Broken ashlar is often arranged as shown in Fig. 28, the courses being 18 to 24 inches high. This is called random-coursed work, for the reason that the stones have broken, or random, joints, and do not break joint directly over one another, as in coursed ashlar. Stonework of this kind is used very often for piers, and makes a strong wall, as all the stones bond well and the perpendicular joints are well broken.
Laying Out Ashlar. If ashlar in regular courses and sizes is to be used, drawings should be made showing each different-sized stone, the heights of the courses, and other necessary details. The plans for public and office buildings, etc., usually show every stone, unless broken ashlar is used, in which case it is only necessary to show the quoins and jambs on the drawings, together with enough of the ashlar to indicate the character of the work desired. It is almost impossible to carefully follow a drawing showing all the stones laid as broken ashlar.
Thickness Of Ashlar. All kinds of ashlar are usually backed with stones not as carefully finished as those on the face. When the ashlar does not exceed 12 inches in height, the thickness varies from 4 to 8 inches, usually the latter. A much better bond is formed when one stone is 6 or 8 inches thick, and the other 4 inches, than when there is a uniform width of, say, 6 inches. When ashlar is laid in low and high courses, the lower one should be at least 8 inches, and the higher one 4 inches, thick; each stone in the courses, when more than 18 inches high, should have an iron anchor extending through the wall.
Backing. Both stone and brick are used for backing, but brick is, in most cases, the cheapest, and, hence, most extensively used. It has, in addition, the advantage that in dry climates the plastering may be applied directly to the brickwork; while if stone backing is used, it usually has to be plugged and furred for the lathing. When brick is used for ashlar backing, the joints should be as thin as possible; if lime mortar is used, it is best to add some cement, to prevent shrinkage of the joints. No ashlar wall should have a brick backing less than 8 inches thick. When a hard, laminated stone, with flat, parallel beds, is obtainable, it should be used, as it is considered to be a stronger backing than brick. Irregular rubble walls should not be used for anything higher than 2 or 3 story dwellings, unless the walls are made at least one-fourth thicker than when brick backing is used. All backing, whether of brick or stone, should be carried up at the same time, and built in courses of the same thickness as the ashlar. This is shown at a in (a) and (b), Fig. 29.
When the courses are not over 12 inches high, they are usually bonded sufficiently to the backing by making the stones of various thicknesses, and by having one through bond stone to every 10 square feet of wall, as shown at b in (a) and (b), Fig. 29.
63. Very often ashlar is only from 2 to 4 inches thick, especially with marble, and some sandstones. In such cases, each piece of ashlar should be tied to the backing by at least one iron clamp, or anchor, similar to that shown in Fig. 30; while if the stones are more than 3 feet long, two anchors are generally used. All iron clamps or anchors should be either galvanized or dipped in hot tar or asphalt, to prevent the formation of rust on them.
Belt courses should also be laid about every 6 feet in height, extending 8 inches or more into the wall, to give support to the ashlar. When a wall is faced with thin ashlar, the effective bearing strength is only that given by the thickness of the brick or stone backing, the facing not being relied on for that purpose.