191. The timbers which support the flooring boards and ceiling of a room underneath are called, in carpentry, the naked flooring. There are several kinds of naked flooring, but they may be all comprised under the three following heads, viz. single-joisted floors, double floors, and framed floors.

1st. Single-joisted floors. A single-joisted floor consists of only one series of joists, as shown in Fig. 56, which is a section across the joists.

Fig. 56.

Sometimes every third or fourth joist is made deeper, and the ceiling joists are fixed to the deep joists, crossing them at right angles. This is an improvement where there is not space for a double floor. Fig. 57 shows a section of this kind of floor. The depth is increased very little, and sounds will not pass so freely as in a single-joisted floor; besides, the ceiling will stand better. The ceiling joists, a, a, are notched and nailed to the deep joists, b, b, b.

Fig. 57.

192. 2ndly. Double floors. A double floor consists of three tiers of joists, viz. binding joists, bridging joists, and ceiling joists; the binding joists are the chief support of the floor, and the bridging joists are notched upon the upper side of them; the ceiling joists are either notched to the under side, or framed between with chased mortises; the best method, however, is to notch them. Fig. 58 shows a section of a double floor across the binding joists, 6, 6, 6. The bridging joists, d, d, are notched over, and the ceiling joists, a, a, are notched under the binding joists.

Fig. 58.

193. 3rdly. Framed floors differ from double floors only in having the binding joists framed into large pieces of timber, called girders. Fig. 59 shows a section across the girders of a framed floor, where b, b,. b, are the binding joists.

Fig. 59.

Single joisting makes a much stronger floor, with the same quantity of timber, than a double or framed floor, and may be constructed with equal facility for the same extent of bearing; but the ceilings are more liable to cracks and irregularities; consequently, single-joisted floors of long bearings should only be used in inferior buildings.

When it is desirable to have a perfect ceiling, a double floor should be used; but when the bearing is long, a framed floor becomes the most convenient. The following experiment was made by Professor Robison on the comparative strength of framed and single-joisted floors.

194. Two models were made 18 inches square; one consisted of single joists, the other framed with girders, binding joists, bridging and ceiling joists; the single joists of one contained the same quantity of timber as the girders alone of the other. They were placed in a wooden trunk 18 inches square within, having a strong projection on the inside for the floors to rest on; and small shot was gradually poured over.

The single-joisted floor broke down with 487 pounds, the framed floor with 327 pounds.* The difference probably would not be so great on a large scale, because the girders would not be weakened so much by mortises. This is only one of many cases where apparent strength has turned out to be real weakness; and it shows how necessary it is to distinguish the parts which really support a load from those which only appear to do so.

One cwt. per superficial foot is an ample allowance for the probable load on an ordinary dwelling-house floor, exclusive of the weight of the floor itself. And 2 cwt. per superficial foot is sufficient in most cases for warehouse and factory floors.

Mr. Page, the engineer of the Chelsea Suspension Bridge, found the weight of a crowd of men closely packed to bo 84 lbs. per superficial foot.†