A slip sill is a sill that is just the width of the opening, and is not built into the wall.
Lug sills are those which have flat ends, built into the wall, as shown in Fig. 100.
All sills should be cut with a wash of at least ½ inch to 5 inches in depth, and if the ends are to be built into the wall they should be cut as shown in Fig. 100. In some parts of the country the sills are cut with a straight beveled surface the full length of the stone, and where they are built into the wall the bricks are cut to fit the stone. This is not a good method, as the water running down the jamb and striking the sill is apt to enter the joint between the brick and stone, and the slanting surface also offers an insecure bearing for the brick. Slip sills are cheaper than lug sills, but they do not look as well, and there is also danger of the mortar in the end joint being in time washed out.
Slip sills, however, are not likely to be broken by any settlement in the brickwork, and for this reason many architects prefer to use them for the lower openings in heavy buildings and also for very wide openings.
Lug sills should not be built into the jambs more than 4 inches, and should only be bedded at the ends when setting.