This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Built-Up Lintels. It is sometimes necessary to use a stone lintel 10 or 12 feet long, which is difficult to obtain in a single piece. In such a case, the lintel may be made in sections. At least three stones should be used, and the joints should be cut as shown at a, Fig. 40. When cut in this manner the stones are apparently self-supporting. The end pieces may be built into the wall for a considerable length, so as to act as cantilevers supporting the middle section. If such long lintels are used, it is better to carry them on I beams, as shown in Figs. 38 and 39.
Slip sills are made just the width of the opening, and are not built into the walls, but put in place after the frame is set. Slip sills are cheaper, but do not look as well as lug sills; besides, there are exposed vertical joints at the ends, into which water will penetrate. Any settlement of the masonry is not likely to break a slip sill, and hence they are often used in the lower parts of heavy buildings.
Lug sills have flat ends, or lugs, built into the wall. They should not enter the walls over 4 inches, and should be bedded on mortar only, at the ends, while being set. If settlement occurs, and a sill is bedded solid, it would probably be fractured at the jamb line, as the pier or side walls would be very likely to settle more than the wall under the opening. The joints under the sills should be filled when the finished walls are cleaned down.
79. All sills should have a bevel, or wash, of about 1 inch per foot, extending to the back of the reveal, as shown in Fig. 41. They sometimes have a straight, beveled surface the full length of the sill, the bricks being cut to fit the stone. This, however, is not good practice, as such construction permits water, running down the jamb, to enter the joint between the brick and stone; the sloping upper face also forms an insecure bearing for the wall resting on it. In Fig. 41 is shown the proper method of cutting the surfaces; a indicates the flat end of the lug sill, carrying the brickwork reveal c; b shows the bevel, or wash; and d, the drip.