Single Floors. This species of floor consists of a series of timbers termed "joists," or " flooring joists" aa, as fig. 194, the ends of which rest on the walls b b, and run in a direction at right angles to these. In better class work the ends of the joists rest upon, and are framed into, or secured to, "wall plates," as cc, fig. 194, these being set into and rest upon the walls of brick or stone. In ground floors, in the best work, the joists rest upon wall plates supported by piers of brick or stone, these being carried up from the ground to the level of the under side of flooring joists. The object of these piers is to preserve the soundness of the timber, by leaving it exposed on all sides; timber being found to decay much more rapidly when built into walls and surrounded by brick, or stone and mortar, than when left freely exposed to the air. In the upper floors of buildings the joists are built into the walls, as in fig. 194. At right angles to the " flooring joists " a a, the " flooring boards " d d are placed, resting on the upper side of the joists. Such is a " single floor " as employed on the ground floor of a building; but in the upper floors, where a ceiling is to be carried by the floor timber, "ceiling joists," as ee, fig. 194, are secured to the lower edges of the joists, running in a direct line at right angles to these. To these "ceiling joists" the laths which support and carry the plaster are secured. The " bearing," or " span," or distance between the walls in which the joists a a, fig. 194, rest, should not exceed twenty-four feet for single floors on the ground level, but as this is, however, too great, we are disposed to place the maximum span at twenty feet. Where a ceiling is to be carried by the floor, as in upper floors, the span should not exceed fifteen feet. The " bearing " of the joists on the wall plates should not be any less than four inches, but, according to the bearing of the joists, may go from this up to nine inches. By the term " bearing," here given, is meant the part, or length, of one of the joists which rest upon the wall or wall plate. The distance or interval between the joists is usually fourteen inches. Where any opening is required in a floor, as a a, fig. 195, the flooring joists bb are jointed into and rest upon what is called a "trimmer " cc, this being jointed into, and being borne by, the " trimming joists" d d, which are stronger than the ordinary joists by one-eighth of an inch for each joist as carried. Where the bearing of the joists is considerable, and the depth therefore increased, they should be strengthened, and lateral movement prevented by what is called " strutting." The simplest form of strut is a flat and thin piece of board, as a a, fig. 196, placed between the joists, the strut bearing at its ends in the faces of the two contiguous joists bb. A more complicated and complete form of strutting is known as " herring-bone strutting," and is also illustrated in fig. 196, and is formed by two pieces, c d, crossing each other, butting at each end on the faces, or inner sides, of the joists ee, and secured thereto by nails. In superior work the struts are slightly notched at the bearings into the joists, as at f. As simple longitudinal struts, as ii, in fig. 196, are sometimes apt to give way laterally, the best plan is to make the edges butt up on one side to triangular fillets nailed to the joists, as shown in fig. 196, at g g. For joists with a "bearing" of from eight to ten feet, one row of strutting will be sufficient, allowing another row for each four feet of increase in length of bearing of the joists.