The simplest kind of floor is that termed "single-joisted," in which the joists are 12 in. apart, resting on the wall-plates, and carrying the boards above, while, if there be a ceiling, the ceiling laths are nailed on below. Fig. 645 shows the boards a as they rest on the joists b. When ceiling joists are used, the arrangement is as shown in Fig. 646: a, flooring boards; b, joist; c, wall-plate; d, ceiling joists. The scantling of the wall-plate will vary with the length of the bearing of the joists, as follows : -

Up to 10 ft.................3 in. x 3 in.

10 to 20 ft.................4 1/2 in. x 3 in.

20 to 30 ft.................7 in. x 3 in.

The joists should have at least 4 in. of their length resting on the wall-plate and wall, and this may be increased up to 9 in.

When the joists are unusually deep (for greater strength), or far apart (for economy sake), there is a danger that an extra weight on them may cause them to turn over on one side. To obviate this danger, "strutting" is resorted to. In its simpler form this consists of sections of flat thin wood placed edgewise between the joists, as seen in Fig. 647, where the joists a are kept vertical by the struts b. Great force would be required to crush these struts, but there is a risk of their ends slipping. This is sometimes remedied by attaching them at one end to triangular fillets c nailed to the joist. The struts should all be placed in the same line, and the lines may be 2 or 3 ft. apart. A more secure way of strutting is that known as the " herring-bone," illustrated in Fig. 648. It consists of strips of wood a of small scantling (say 2 1/2 in. by 1 in., or 3 in. by 1 1/2 in.), crossing each other, and nailed at the top of one joist b and bottom of the next, maintaining regular lines at a distance of about 4 ft.

Whenever a space has to be left in a floor, to provide for the insertion of a staircase or a flue, the construction has to be modified by the introduction of a "trimmer" for the support of one end of those joists which are prevented from reaching to the wall-plate as before. Fig. 649 shows the arrangement where the hole is required nest the wall: a is a wall, supporting the 2 joists b, while the 3 joists c are cut off to leave the space d. The trimmer e is mortised at both ends into the joists b, and carries the free ends of the joists c, which are mortised into it. As the extra strain from the 3 joists c is thus supported by the 2 joists b, it is necessary that these latter be stronger than the others. They are called the " trimming" joists, and it is usual in ordinary flooring to add 1/8 in. to their thickness (not depth) for every joist trimmed. Fig. 650 illustrates the system adopted when the hole is at a distance from the wall, requiring the intervention of 2 trimmers: a is the wall, b, ordinary joists; c, trimmed joists; d, trimmers; e, trimming joists; f, hole.

The preceding paragraphs refer to "single" floors; but when the strain to be borne is great, as in warehouses and similar structures, "double" floors are adopted, as well as "double framed" floors. In the double floor, Fig. 651, a "binder" or "binding joist" is introduced, having a thickness usually half as great again as that of the joists it supports, bearing about 6 in. on the wall, and situated at intervals of 5-6 ft. apart, centre to centre. In Fig. 651, a are the ordinary joists resting on the binders b, and supporting the flooring boards c above, while the ceiling joists d are attached to the under side of the binders.

The " double framed " floor differs in having "girders" to carry the binders at intervals of about 10 ft. centre to centre. Fig. 652 represents this plan : a, the ordinary joists, carrying the floor-boards b, and resting on the binder c, supported by the girder d; e, ceiling joists. Girders should always be placed so that their ends rest on solid walls, where no window or door below weakens the structure. The weight of the girder is distributed as much as possible by resting its ends on templates of stone or iron. These templates often assume a box-like form, enclosing the sides and end of the girder but not so as to exclude all air.

Floor-boards may be laid "folding," in "straight joint," or "dowelled," the first being the commonest method. In laying boards folding, 4 or 5 boards are put in place without nailing, and the outside ones are then nailed so as to have slightly less space between them than was occupied by the others lying loosely; the others are then forced into position by putting their edges together and thrusting them down. Thus in Fig. 653, of the 5 boards a, b, c, d, e, the 2 outside ones a, e would be first nailed and then the intervening b, c, d would be forced into the space left for them. In this case, the ends of the boards are made to meet where they will fall on a rafter, and as nearly as possible in the centre of its width, as at / on the rafters g. When the floor is laid with straight joints, as in Fig. 654, each board is put down and nailed separately, being thrust up close to the one preceding it by means of the flooring clamp. Thus the joints a of the ends of the boards b fall on the rafters c in straight lines with intervals between.

When the flooring is "dowelled," the boards are laid separately and straight as in Fig. 654, the only difference being that their edges are united by dowels (small pegs of oak or beech) driven into holes bored for their reception, either between or over the joists. Most commonly, flooring boards simply have their edges planed smooth, and are forced into the closest possible contact, when they are held by the nails that fasten them to the joists. But there are cases when a more perfect tight-fitting joint is needed.

Fig. 655 shows the various ways of joining floor-boards : a, plain joint; b, ploughed and tongued; c, rebated; d, e, with a tongue of wood or iron inserted; f, with the tongue resting on the joist; g, h, splayed.

When a floor is finished, it is usual to hide the ends of the boards where they meet the wall by nailing a skirting board round. This may be plain or ornamental. It rests on the floor and rises close against the wall, to which it is fastened by occasional nails passing into wooden bricks, called " grounds," inserted in the wall to take the nails. In superior work, floors are " deadened " or " deafened " by placing a bed of non-conducting material beneath them. To support this bed, strips of wood are nailed to the flooring joists to carry thin "sounding" boards, on which is spread a thick layer of old mortar or plaster, known as "pugging." This is shown in Fig. 656: a, joists; b, flooring boards; c, strips called "firring pieces," bearing the sounding boards d loaded with pugging e.