Floor Joists. The methods of resting the floor joists upon the sills and girders are shown in Fig. 29: a is the method frequently used upon cheap buildings; b, upon the best class of buildings; and c illustrates a method sometimes used where it is necessary to avoid the height above the sill at a, and where it is not desirable to take the time to cut mortises. The joists need be no longer than to reach the stud, as at d, though if they extend to the outside of the stud, they may be spiked to the side of each stud they come against, thus giving additional strength.

Fig. 20. - Sizing Floor Joists, to fit Sills and Girders.

As there is so much difference in the size of timbers, it is necessary, in order to make the floor joists line up straight on top, always to size the floor joists to 1" narrower than the timber, as shown in Fig. 29. This sizing should be done from the top edge of the joist, which in every case should be the crowning or rounding edge, so that when the floor is loaded, the deflection will tend to straighten the joists.

A floor joist should be nailed against the outside wall, to give a nailing for the floor boards and the ceiling laths.

Two floor joists, with a 2" space between them, should be placed under the partitions, or a double floor joist may be used instead, if a piece of 1" X 3" is nailed to each side of it, to give a nailing for the flooring and the ceiling laths.

Floor joists in almost every case should be placed 16" to centers. As laths are cut 48" long, this distance gives four nailings to each lath and makes cutting unnecessary. If the space to be filled is not a multiple of 16", the variation should all come at one side of the room, so that the laths will not have to be cut more than necessary. In heavy work the floor joists are often placed 12" to centers. If floor beams are used as trimmers, to carry the header of a flight of stairs, they should be doubled, unless otherwise supported. Headers and trimmers carrying but one or two tail beams will do if a single thickness is used.

If a floor joist is sprung sideways, it must be held straight by "battens" or bridging until the flooring is nailed, or there will be enough deflection when the weight is placed upon it to crack the plastering of the ceiling below.

Floor joists over 12' in length should have, in the center, a row of bridging (see Fig. 30), which imparts sufficient stiffness to require three times the load to cause the same deflection as without, and which insures that the joists will never buckle. Rows of bridging should not be more than 8' apart; for a small house 1" X 3" material will generally answer, but for a large building, nothing less than a 2" X 3" should be used. To make bridging, it is the usual custom to saw one end of a long piece at the correct angle, and place it as shown at a, or the height of the beveled end above the bottom edge of the floor joist. With a cutting-off saw, cut the other end of the bridging, carrying the saw by the side of the floor joist as at b. This cuts the piece to the desired length and angle without any further measuring, or the use of any other tool, and at the same time gives the angle for the lower end of the next piece. Some workmen have a miter box with a bridging cut made in it, and saw the bridging for an entire floor before nailing. The uneven spaces will have to be cut separately.