buildings, factories, schools, and large residential flats are now constructed of fire-resisting materials. There are two descriptions of flooring, single and double.
Single flooring (fig. 23) consists of one row of wood joists resting on a wall or partition at each end without any intermediate support, and receiving the floor boards on the upper surface and the ceiling on the underside. Joists Single flooring. should never be less than 2 in. thick, or they are liable to split when the floor brads are driven in; the thickness varies from 2 to 4 in. and the depth from 5 to 11 in. (see By-laws, below), the distance between each joist is usually 12 in. in the clear, but greater strength is obtained in a floor by having deep joists and placing them closer together. These floors are made firm and prevented from buckling by the use of strutting as mentioned hereafter.
The efficiency of single flooring is materially affected by the necessity which constantly occurs in practice of trimming round fireplaces and flues, and round well holes such as lifts, staircases, &c. Trimming is a method of supporting the end of a joist by tenoning it into timber crossing it; the timber so tenoned is called the trimmer joist, and the timber morticed for the tenon of the trimmer is called the trimming joist, while the intermediate timbers tenoned into the trimmer are known as the trimmed joists. This system has to be resorted to when it is impossible to get a bearing on the wall.
A trimmer requires for the most part to be carried or supported at one or both ends by the trimming joists, and both the trimmer and the trimming joists are necessarily made stouter than if they had to bear no more than their own share of the stress. In the usual practice the trimmer and trimming joists are 1 in. thicker than the common joists, but there are special regulations and by-laws set out in the various districts and boroughs (see By-laws, below) to which attention must be given.
The principal objection to single flooring is that the sound passes through from floor to floor, so that, in some cases, conversation in one room can almost be understood in another. To stop the sound from passing through floors the remedy is to pug them (fig. 24). This consists in using rough boarding resting on fillets nailed to the sides of the joists about half-way up the depth of the joists, and then filling in on top of the boarding with slag wool usually 3 in. thick. Also to further prevent sound from passing through floors the flooring should be tongued and the ceiling should have a good thick floating coat, in poor work the stuff on ceilings is very stinted. In days gone by, ceiling joists were put at right angles to the floor joists, but this took up head room and was costly, and the arrangement is obsolete.
Double flooring (fig. 25) consists of single fir joists trimmed into steel girders; in earlier times a double floor consisted of fir joists called binding, bridging and ceiling joists, but these are very little used now and the single fir Double flooring.
joists and steel girders have taken their place. Steel girders span from wall to wall, and on their flanges are bolted wood plates to receive the ends of the single joists which are notched over plates and run at right angles to the