The floors of wooden buildings are usually constructed of 2-inch planks called "joists," which are set on edge and spaced either 12 or 16 inches apart from centres. These joists are supported, in the first floor, by the sills at the outer ends and by a wooden girder or brick wall near the middle, the girder or wall being generally placed directly under the "bearing partitions" in the first story.
The attic floor joists may either rest directly on the wall plate, as shown in Fig. 18, or on a false girt, as shown in Fig. 20, according to the design. The inner ends of the joists are supported by the second story partitions.
In a one-and-a-half-story house the attic joists are supported at their outer ends by being spiked to the sides of the rafters.
Sizing and Crowning, - The first step toward framing the floors is the sizing and crowning of the joists. For spans of 16 feet or over all floor joists should be crowned or the top dressed to the arc of a circle, with a rise of ¼ of an inch in the centre for every 16 feet of span. This must be done by hand with a hatchet and plane. The ends of the joists are then sized, so that the distance from the bearing to the top of the joist will be the same in each beam, thus insuring an even surface on top. Ordinary timbers often vary ¼ to ½ an inch in size, and if they were not sized at the ends their tops would not be in the same plane. The object of the crowning is to offset the inevitable deflection or sagging of the joist, and thus secure a level floor.
In the Eastern States the bottom of the floor joists is almost always cross-furred with 1½-inch strips, so that any irregularity in the depth of the joists is easily overcome. In many of the Western States, however, this is very seldom, if ever, done, and all the joists have to be dressed to a uniform width, if they do not come so. A little irregularity in a ceiling can be overcome in plastering.
The spacing of the floor beams should not exceed 16 inches from centres, and, where the ceilings are not furred, it is better to space them 12 inches from centres.