This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Care must be taken that no vertical joint in any course comes over a joint in the course below, but the stones should break joints or overlap, preferably to an extent of from once to onceand-a-half the height of the course, so that each stone will be supported by at least two stones of the course below, and will in turn support at least two stones of the course above. This will not only distribute the pressure of the weight above, but will tie the wall together in its length, by means of the friction of the stones where they overlap. For the same reason, the thickness of the stones should vary, so as to make a bond in and out of the wall, as in Fig. 141, and at least one stone in every ten square feet of wall should be the full thickness. The strongest bond in a wall of cut ashlar is one in which each course contains a header and stretcher alternately, the outer end of each header coming on the middle of the stretcher. (Fig. 142.) In broken ashlar work the bond should be carefully preserved, and, in the case of broken ashlar used with a brick backing, it will be convenient to use stones of a thickness of four, eight, and twelve inches, alternating, so that the bond may be obtained through and through without much cutting of the bricks. In the backing of ashlar, the joints of the brickwork should be made as thin as possible, and cement should be used to prevent shrinkage of the joints, which will necessarily be more numerous than the joints of the ashlar facing. Brick backing should not be less than eight inches thick, and, if the facing is in courses exceeding a foot in height, each large piece should be tied to the brick by iron clamps in the proportion of about every three feet in the length of the wall, and two feet in the height. It will be of advantage in this class of work if the horizontal joints are not allowed to run to a great length. Changing the level of the courses every four or five feet will make a good looking wall. Broken ashlar is usually prepared at the building site, but it will he a saving if the stones are cut to the required heights in the yard, leaving only an end joint to be cut at the building.
Fig. 138. Broken Ashlar.
Fig. 139. Hollow and Slack Joints.
Fig. 141. Bond Stone.
Fig. 142. Bonded Ashlar.