Joists more than 10 feet long should be strutted at intervals of about 6 to 8 feet, to make them stiff and to prevent them from turning over sideways. The struts also add greatly to the strength of the floor, by causing the pressure on the joists to be transmitted from one to the other.
Herring-Bone Strutting1 consists of small pieces from 2 inches to 3 inches wide and 1 inch thick inserted diagonally and crossing one another between the joists, as shown at ss in Fig. 288. They must not be split in nailing them; the holes for the nails must be bored; or two small saw cuts made in each end of the struts to receive them.
Fig. 288. Section of Single Floor, showing Pugging, Sound-Boarding, and Herringbone Struts.
Wooden keys are occasionally used instead of struts. They are mortised through the joists with very small tenons, which must, however, weaken the joists to a certain extent, and they are therefore objectionable as well as being expensive.
Of the above forms the herring-bone struts are the best, as they do not cut into the joists, and they transmit the pressures upon them in proper directions.
Strutting, to be really effective, should be in straight lines along the floor, so that each strut may abut directly upon those adjacent to it.
Tension rods are sometimes passed through the joists at right angles to their lengths so as to bind them together, compressing the struts; this adds greatly to the stiffness of the floor.
1 Sc. Dwangs.
Puggingl is plaster (coarse stuff), mortar and chopped straw, or other mixtures laid upon boards fitted in between the joists of a floor to prevent the passage of sound or smell from the room below. It has the drawback of making the floor very liable to rot by preventing the circulation of air.
The " sound-hoarding "2 sb (see Fig. 288) to carry the pugging, p, is supported on fillets, /, nailed along the sides of the bridging joists, bj, about half-way down.
The fillets are sometimes rectangular in section, about 1 inch by 1 1/4 inch, but are better if cut diagonally out of a piece 2 inches by 1 1/4 inch (see Fig. 288), as they then have a larger surface for nailing.
Dry moss, or a mixture of lime mortar, earth, and smiths' ashes - are sometimes used instead of the plaster; also slag felt, slag wool, turf, plasterers' rubbish, sawdust, tan, dried moss; but all materials likely to decompose are objectionable.
Slips of cork or list along the upper edges of the joists upon which the boards are nailed, are recommended by Tredgold as a means for reducing the passage of sound. Felt or felt-paper over the boards and under the carpet have been used for the same purpose.
Trimming.3 - It often happens that on account of flues, fireplaces, or from other causes, it is inadvisable to let the ends of the joists rest on particular parts of the walls, and it is necessary that they should be trimmed.
The arrangement of the trimming varies according as the joists are at right angles to or parallel to the wall in which the flue or fireplace occurs.
In the former case (see Fig. 290) the joists are stopped short of the portion of wall to be avoided, and tusk-tenoned into a cross beam T, called a trimmer.
This trimmer is tusk-tenoned at the ends, and framed in between the two nearest bridging joists bearing on the wall, on each side of the portion to be avoided.
The joists, tj, carrying the trimmer, are called "trimming joists." As they have to carry more weight than the other bridging joists, they are made wider.
To the width of the common joists add 1/8 of an inch for every joist carried by the trimmer, and that will give the width of the trimming joists.
1 Sc. Deafening. 2 Sc. Deafening-boarding, or sometimes Pug-boarding.
3 Sc. Bridling.
The trimmer should be calculated by the same rules as binders (see p. 124). This rule refers to the ordinary case in which the joists are all of the same depth, as in Fig. 274. When the trimming joists are deeper than the others, they need not be so wide in proportion.
Figs. 289, 290 show five joists trimmed to avoid a fireplace. A small "trimmer arch"1 is turned from the wall to the trimmer to carry the hearthstone. The length of this arch may with advantage be equal to the full width of the chimney breast, and it should, in any case, be at least 2 7 inches longer than the width of the opening of the fireplace, so as to support the hearthstone, which is 18 inches longer than the width of the opening, and to leave room besides for a cradling piece 4 1/2 inches wide at each end, to support the oak border and the ends of the floor boards.
Fig. 290. Section on XY. Plan and Section of Chimney Breast and Hearth showing Trimming.
N.B. The ceiling joists are omitted in plan to prevent confusion.
1 Sc. Bridle.
In some cases a filling-in piece is fixed between the trimmer and the wall to support the ceiling joists under the arch. This construction is, for some reasons, objectionable, for it requires a corbel or plate in the wall to support the end of the filling-in piece; and in the illustration given (Fig. 290), it is also unnecessary, for the ceiling joists can be fixed to the trimming joists as shown, and require no support between them. If, however, there are no ceiling joists, the filling-in pieces are necessary to support the laths for the plaster of the ceiling.
When the hearth to be supported is wide and the depth of the floor is not sufficient to afford room for the rise of an arch such as that in Fig. 290, then the trimmer arch may be continued past the crown, as shown in Fig. 291, springing on one side from the chimney breast and on the other from a splayed fillet nailed against the trimmer or trimming joist to form a skewback.
When the joists are parallel to the wall in which a fireplace occurs, the trimmer arch is turned against the first continuous joist (in this case called the "trimming joist "), and short trimmers, T, are inserted to carry the trimmed joists between that joist and the wall, in the same way as shown in the trimming for the fireplace in Fig. 272.
In some cases iron pipes are substituted for the timber trimmers T; each rests at one end on the wall, and passes through holes in the short bridging joists which it supports, its other end being supported by passing through a hole in the trimming joists tj.
A layer of 3 or 4 inches of Portland cement concrete, supported by wooden fillets extending from the hearth to the trimmer, is sometimes used instead of the trimmer arch. Curved tiles have also been used for the same purpose.
The thrust of the trimmer arch is sometimes counteracted by iron rods built into the wall, as shown in clotted lines. They are more useful when the joists are parallel to the fireplace, in which case the trimming joist against which the arch abuts requires support against bending laterally; the rods, however, are seldom used.
Fig. 276 shows two joists trimmed to avoid the flue at A.
In Fig. 272 is shown the trimming necessary for a trap-door in the floor, and in Fig. 279 a trimming for a lift.
Openings for stairs are trimmed in a similar manner (see R, Fig. 272).
Floor Boards are laid in several different ways.
Plain jointed) - The boards are simply laid side by side, as close as possible (see Fig. 292), a nail or generally two being driven through the boards into each joist.
The inevitable shrinkage of the boards, as at A, will cause openings through this description of floor.
Fig. 292. Plain Jointed Floor.