In stone walls, the material should be bonded just the same, and for the same object, as in brick walls - i.e., as regards breaking joint both vertically and horizontally; though this, for obvious reasons, cannot be done in rubble walls. Consequently such walls are weaker, and of necessity have to be thicker to be of the same strength as brick and other uniformly bonded walls. Moreover, rubble walls - the stones from which they are built having neither square beds nor joints require a greater quantity of mortar to bed each irregular-shaped piece of stone, and as a result, there is more and unequal settlement in them on the drying of the mortar. This is a defect which cannot but have prejudicial results when rubble walls are faced with brick or stone, set with quare beds and joints, and, of course, less mortar.
The facings or linings (whether of brick or stone) to rubble stone walls may be thoroughly bonded, as in fig. 200, which is a section of a brick-lined rubble wall; but this has little advantage as regards their strength as a whole.
On an average, no less than three bonders should be used to each superficial yard of stone facing to rubble walls, though the necessary number of such bonders varies from five in thin-coursed facing, such as shown in figs. 206, 207, 208, 209, and 310, to only one in large block-in-course work, as fig. 212. These bonders may either be what are called through?, which go from face to back of the wall, or three-quarters bonders - the latter, I think, being preferable to the former on account of the unequal settling, which would cause the through bonders to split in two, especially in double-faced walls (as fig. 201), when they would take the form of lintels, supporting the part of the wall above, and forming no uniform mass as it should be. Another disadvantage is that "through" bonders of porous stone conduct damp and moisture into a building.
The courses of stone-facing, in addition to special bonders, as previously explained, should not be all of one depth on the bed, where it is possible to have them of depths similar to brickwork - i.e., with two or three courses of about 4 1/2 inches or more on bed to one of about 9 or 12 inches, and so on, in like principle and proportion.
Section Fig. 209.
Section Fig. 205.
The different kinds of stone walling or stone facing are - Random rubble, illustrated by figs. 202 and 203, in elevation and section; from which it will be seen that it is stronger and more solid when not faced, though weak under any circumstances.
Random rubble built up to courses, as in figs. 204 and 205, is little better than the last, though levelled up at every foot or two of height.
Squared or snecked uncoursed rubble, as figs. 206 and 207, is far superior in every way to either of the preceding, being built of stones which have had their beds and joints squared or snecked, and consequently require less mortar, with less moisture to dry up and cause settlements. In snecked or squared rubble, to ensure the best work, the jumpers, or large blockers rising above their neighbours, should not be more than 10 inches high, and proportionate to the size of the adjoining courses.
Squared or snecked rubble built up to courses is illustrated by figs. 208 and 209, where it will be seen that this kind, like "rubble built up to courses," though of stones irregular in size, can be built up and levelled as a horizontal course every 12 to 18 inches instead of every course, or not st all.
Squared or snecked coursed rubble, which it will be seen demands that each rise of stone shall form a horizontal course of its own, is illustrated by fig. 210.
Ashlar, as shown in fig. an, is the finest of all facing, being built on a regular system, and with finely chiselled, perfectly horizontal beds and vertical joints, so that, when set in the work, the joints show no more than 1/8th of an inch; and good ashlar is never supposed to rise more than 9 or 10 inches in a course; and, in limestone or soft stones, each stone hould be no more in length than three times its height, or in depth 1 1/2 times to twice its height - i.e., on the bed. Sandstone will take about half as much again - i.e., 4 1/2 times its height for its length, and from 2 to 3 times for its depth.
Block in course, which really is large ashlar, or that which exceeds the limit in height, rising from 10 to 16 inches each course, is illustrated by fig. 212.