chisel draft 11 or 2 inches wide is usually cut at each exterior corner. In the best work, as fine cut-stone buildings, all projecting courses, as window sills, water tables, cornices, etc., have grooves, or "drips" cut in the under surface a little way back from the face, so as to cause rain-water to drop from the outer edge instead of running down over the face of the wall and disfiguring it.
The bond is the arrangement or overlapping of the stones to tie the wall together longitudinally and transversely, and is of great importance to the strength of the wall. No joint of any course should be directly above a joint in the course below; but the stones should overlap, or break joint, from one to one and one half times the depth of the course, both along the face of the wall and also from the front to the back. The effect is that each stone is supported by at least two stones of the course below, and assists in supporting at least two stones of the course above. The object is twofold: first, to distribute the pressure, so that inequalities of load on the upper part of the structure (or of resistance at the foundation) may be transmitted to and spread over an increasing area of bed in proceeding downwards (or upwards); and second, to tie the building together, i.e., to give it a sort of tenacity, both lengthwise and from face to back, by means of the friction of the stones where they overlap.
The strongest bond is that in which each course at the face of the structure contains a header and a stretcher alternately, the outer end of each header resting on the middle of a stretcher of the course below, so that rather more than one third of the area of the face con sists of ends of headers. This proportion may be deviated from when circumstances require it, but in every case it is advisable that the ends of headers should not form less than one fourth of the whole area of the face of the structure. A header should be over the middle of the stretcher in the course below. In a thin wall a header should extend entirely through the wall.
A trick of masons is to use "blind headers," or short stones that look like headers on the outside but do not go deeper into the wall than the adjacent stretchers. When a course has been put on top of these short headers, they are completely covered up; and, if not suspected, the fraud will never be discovered unless the weakness of the wall reveals it.
Where very great resistance to displacement of the masonry is required (as in the upper courses of bridge piers, or over openings, or where new masonry is joined to old, or where there is danger of unequal settlement), the bond is strengthened by dowels or by cramp irons (I 547) of, say, ?}-inch round iron set with cement mortar.
Ashlar is usually backed with rubble masonry (I 574), which in such cases is specified as coursed rubble. Special care should be taken to secure a good bond between the rubble back.
ing and the ashlar facing. Two stretchers of the ashlar facing having the same width should not be placed one immediately above the other. The proportion and the length of the headers in the rubble backing should be the same as in the ashlar facing. The "tails" of the headers, or the parts which extend into the rubble backing, may be left rough at the back and sides; but their upper and lower beds should be dressed to the general plane of the bed of the course. These "tails" may taper slightly in breadth, but should not taper in depth.
The backing should be carried up at the same time with the face work, and in courses of the same depth; and the bed of each course should be carefully built to the same plane with that of the ashlar facing. The rear face of the backing should be lined to a fair surface.
laying masonry of any character, whether with lime or cement mortar, the exposed edges of the joints will naturally be deficient in density and hardness. The mortar in the joints near the surface is especially subject to dislodgment, since the contraction and expansion of the masonry is liable either to separate the stone from the masonry or to crack the mortar in the joint, thus permitting the entrance of rain-water, which upon freezing forces the mortar from the joints. Therefore, it is usual, after the masonry is laid, to refill the joints as compactly as possible, to the depth of at least an inch, with mortar prepared especially for this purpose. This operation is called pointing.