The bearing timbers may be so arranged as to tie in the principal walls, and if the building forms a corner, having two or more external walls, they may be laid in opposite directions in the alternate stories.
All parts of timber built into walls should have clear spaces round them for circulation of air.
Timbers passing over several points of support, such as joists over binders, joists or binders over party walls, and similar cases, should be in as long lengths as possible, by which their strength is greatly increased as compared with what it would be if they were cut into short lengths, just sufficient to span the intervals between each pair of supports. (See Part IV.)
Fixing uniformly loaded timbers rigidly at the ends increases their strength by one half, but this can seldom be done in practice. If the ends are built into the wall they have a tendency to strain and destroy the masonry. The want of a free circulation of air causes the timber to decay, and in any case it soon shrinks and becomes loose.
1 S.M.E. Course.
The ground below the floor should be thoroughly drained, and covered with concrete to prevent damp from rising.
These consist of very short pieces arranged in different ways so as to support one another, but a description of them would be more curious than useful.
Ceiling Joists1 are light beams to carry the laths for the plastering of the ceiling. They are fixed to the under side of the bearers of the floor, running at right angles to them; that is, in a Single floor to the bridging joists, in a Double or Framed floor to the binding joists.
They should be 14 inches from centre to centre where double laths are used; if more widely placed than this, the laths are likely to give with the weight of the plaster. With thinner laths the joists must be closer together.
The mode of fixing ceiling joists is generally to notch them and nail them, as shown in Fig. 278.
Sometimes, however, the depth from the ceiling to the surface of floor has to be kept as small as possible, in order to gain space. With this object the ceiling joists may be tenoned in between the bridging joists or binders with chase mortises, formed either as at x or as at y, Fig. 304. This should, however, be avoided as much as possible, for the mortises weaken the bearers.
Another plan is to support the ends of the ceiling joists iff) upon fillets nailed to the bridging joists, as shown in Fig. 305.
1 Sometimes called Raglins in the North of England.
Where ceiling joists are fixed in between bearers, their lower edges are allowed to come a little below the latter, a firring1 (F in Figs.) not wider than the ceiling joist being attached to the bearer below, so as to afford a key for the plaster.
Fig. 304. Ceiling Joists Chase-mortised into Binder.
Fig. 305. Ceiling Joists supported by Fillets fixed to Binder.
This is advisable also, because the bearers are sure to sag, and if the under sides of the ceiling joists were flush with those of the bearers, the ceiling would be curved as in Fig. 306 (the curve in which is of course exaggerated); but by allowing them to be lower, they can be so arranged (see Fig. 307), that, after the bearer has sagged, their lower surfaces may be in a horizontal plane, so as to form a level ceiling.
Fig. 307. Ceiling Joists on Sagged Floor.
In single floors with ceiling joists every fifth, or sixth bridging joist is generally made 2 inches or so deeper than the others, and extends below them to carry the ceiling joists (see Fig. 288).
This, as already explained, is to prevent the passage of sound, by reducing the number of points at which it is conducted through the wood.
Ceiling joists should be fixed slightly higher in the centre of the room (about 3/4 inch in 20 feet), to allow for the inevitable settlement of the floor.
In single floors of small span, the ceiling joists are frequently altogether dispensed with, and the laths nailed to the under side of the bridging joists (see section, Fig. 274).
Ceiling joists are sometimes hung from the bridging joists in a framed floor, such as that mentioned at page 127, by wooden straps, as in Fig. 308. Thus the separation between the floor and ceiling is more complete, and the sound is less readily conducted.
Fig. 308. Ceiling Joists hung by Straps.
The ceiling joists to which the laths are nailed somewhat interrupt the key for the plastering of the ceiling. To remedy this, and to obtain a stiffer ceiling, battens about 1 inch square, and from 12 to 14 inches apart, are sometimes nailed to the under side of the joists, crossing them at right angles. These battens keep the laths at a little distance from the joists, and thus give room for the plaster to be squeezed in behind them and form a "key."