Arches, if too large to be cut with solid stones, may be built up where they are required to show both sides. (Fig. 148.) In the setting of stone arches great care is necessary to preserve the perfect form of the arch. The joints should always be of equal thickness throughout and the mortar kept back from the face. This is of greater necessity than in the horizontal courses, as the joints of the arch are under increased pressure. The backing of arches should be laid in cement, and well tied to the stones of the arch by clamps. Where two arches come together, the first stone, called the "skewback," should be in one piece for the two arches (Fig. 149), for if each stone were cut to the shape of its arch there would be left a small wedge-shaped stone, A, which, if separate, might crowd the arch stones in. The same thing should be done where an arch comes near to a corner as at B.

Flat arches are often used, but, while they are architecturally pleasing, they are liable to constructional weakness, and if they cannot be given a good height, they should be cut as a lintel with false joints on the face. If the opening is not wide, a flat arch may be cut in three pieces, the key with its side pieces being separate from the other two pieces, as in Fig. 150. In this case, and also in the case of B in Fig. 144, the center should be set about a quarter-inch higher than the jambs, to allow for settlement.

Centering

For the construction of arches, whether of stone or other material, wood centers will be required. These should be strongly made and should be left in position until the mortar in the joints has become hard. Centers for small arches are usually made of plank, with two ribs set apart to the thickness required, and connected for a bearing surface by strips of 7/8 X 2-inch stick nailed to the tops of the ribs. A center of this sort is supported by wooden posts from the sill or floor below. If the arch is of a large span, the pieces will be more in number, in order to use planks of ordinary width, and the center will need additional support and ties as shown by Fig. 151. These centers should be wedged up in setting, to permit of easy adjustment or removal.

Fig. 145. Steel Support of Lintel.

Fig. 145. Steel Support of Lintel.

Fig. 146. Bearing of Stone Lintel.

Fig. 146. Bearing of Stone Lintel.