SECTION III.

OF THE CONSTRUCTION OF FLOORS.

133. — The timbers which support the flooring boards and ceiling of a room are called,in carpentry, the naked flooring. There are different kinds of naked flooring, but they maybe all comprised under the three following denominations; viz. single joisted floors, doublefloors, and framed floors.

First, Single joisted floors. A single joisted floor consists of only one series of joists.Plate IV. fig. 38, shews a section across the joists of a single joisted floor.

Sometimes every third or fourth joist is made deeper, and the ceiling joists fixed to thedeep joists, and crossing them at right angles. This is an improvement in a situationwhere there is not space for a double floor. Fig. 39 shews a section of a floor of this kind.It increases the depth of the floor very little, and will not allow sounds to pass so freelyas a single joisted floor, and the ceilings will stand better. Tire ceiling joists, a, a, arenotched to the deep joists b, 6, b, and nailed.

134. —Secondly, Double floors. A double floor consists of three tiers of joists; that is,binding joists, bridging joists, and ceiling joists : the binding joists are the chief supportof the floor, and the bridging joists are notched upon the upper side of them: the ceilingjoists are either notched to the under side, or framed between with chased mortises; thebest method is to notch them. Fig. 40 shews a section of a double floor across the bind-ing joists b, b, b. The bridging joists d, d, are notched over, and the ceiling joists a, a, arenotched under the binding joists.

135. —Thirdly, Framed floors differ from double floors only in having the binding joistsframed into large pieces of timber, called girders. Fig. 41 shews a section across thegirders of a framed floor; where b, b, b, are the binding joists.

Single joisting makes a much stronger floor, with the same quantity of timber, than adouble or framed floor, and may be constructed with equal ease to the same extent of bear-ing ; but the ceilings are more subject to cracks and irregularities; consequently singlejoisted floors of long bearings can be used only in inferior buildings.

\\ hen it is desirable to have a perfect ceiling, a double floor is used; and when the bear-ing is long, a framed floor becomes the most convenient. The following experiment wasmade on the comparative strength of framed and single joisted floors by Professor Robison.