Shallow flat arches of stone, although sometimes pleasing to the eye, are very objectionable constructionally. If a flat arch must be used, to be self-supporting it should be of such height that a segmental arch of proper size can be drawn on its face, as indicated by the dotted lines in Fig. 107. Even then it is desirable to drop the keystone about 1 inch below the soffit line, so as to wedge the voussoirs tightly together. An arch such as is shown in Fig. 107 might be safely used for a span of 5 feet, but with great caution for larger spans. The strength of such an arch may be increased by "joggled" joints, that is, notching one stone into the other, as shown by the dotted lines at a. Such joints, however, are quite expensive.
Very shallow flat arches, such as is shown in Fig. 108, should be cut out of one piece of stone, so as to be in reality a lintel with false joints cut on its face. The ends of the lintel should have a bearing on the wall of 6 inches, as shown by the dotted lines, the face being cut away for about 2 inches in depth and veneered with brick. If this method is too expensive the lintel might be cut in three pieces and supported by a heavy angle bar, as shown in Fig. 95.
Very long lintels are often made in the form of a flat arch (see Section 190), but are, or should be, always supported by steel beams or bars.
Rubble Arches. - Arches are sometimes built of rubble stones. The stones should be long and narrow and roughly dressed to a wedge shape. They should be built in cement mortar, as they depend largely upon the strength of the mortar for their stability.