This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
This is used for rough work, such as foundations, backing, etc., and although frequently consist-ing of common field stone, quarried stone should be used where possible, as better bonding and bedding can be secured.
At (a), Fig. 8, is shown a common- or random-coursed rubble wall, in which the stones are bonded every three or four feet, as at a. The angles are laid with large well-shaped stone, the long sides alternating; in the body of the wall the stones are set irregularly, the interstices in the heart of the wall being filled with spalls and mortar.
At (6) is shown cobweb rubble, which is used for suburban work. The quoins, or corner stones, are hammer-dressed on top and bottom, but may be rock-faced. All the joints should be hammer-dressed and no spalls should show on the face, while the joints should not be thicker than 1/2 in. This class of work is more expensive than common rubble.
At (c) is shown a rubble wall with brick quoins. In this work all the horizontal joints have hammer-dressed level beds. This makes a good wall and can be built cheaply when the stone used splits readily.
At (d) is shown regular-coursed rubble. In this work continuous horizontal joints are run at intervals of 15 to 18 in. in the height, as at abc and def. No attention need be paid to uniformity of height in the different courses, but the beds should be made as nearly parallel as possible.
When the outside facing of a wall is of cut stone, it is called ashlar, regardless of the manner in which the stone is finished.
Regular-coursed ashlar, shown at (e), Fig. 8, has pieces uniform in height and the courses continuous. Stones about 12 in. high and from 18 to 24 in. long are the cheapest, both as to first cost and in expense of handling. The illustration shows the stone bush-hammered with tooled draft lines. A good effect is produced by making the courses of two differ ent heights, as shown at (f), the courses a being from 10 to 18 in. high and composed of "facers," while the courses b are from 5 to 7 1/2 in. high and constitute binding courses. The latter should be at least 4 in. wider than the thickest stones in the facing courses. Random, or broken-range, ashlar consists of blocks truly squared but of different sizes, forming a broken range or course. As this masonry is in itself irregular, it is well adapted to buildings of irregular plan or of irregular sky line. Random ashlar is finished in various ways. The simplest, though not the least effective, finish is the plain rock face, shown in (g). The block must first be cut true and square all around, forming beds and vertical joints, after which straight lines are drawn around the edges, about 2 in. from the face of the block. A wide pitching chisel is then used along these lines, knocking off the surplus material. The smallest stone used should not be less than 4 in. in height, nor the largest greater than 16 in. The bond, or the lap of one stone over another, should be at least 6 in. for the smaller stones and 8 in. for the larger. The length of any block should not be less than l 1/2, nor more than 4. times its height. The hardest kinds of rock are best suited for masonry of this sort, which is, perhaps, the most common kind of ashlar used in modern building.