Wood floors are, practically speaking, made up of a skeleton framing, called "naked flooring," surrounded and supported by a brick, stone, wood, or iron enclosure, and covered by boards of different kinds, to form a close and level platform above an open space.
The floor-boards are of various and more or less intricate sections, but one and all could be used at the same time as a covering to each of the different kinds of naked flooring, without any real detriment to the strength of the floor as a whole, provided the various sections be each of a sufficient and the same thickness.
Naked floors are really of three classes, though there may be different ways of carrying out the principles which distinguish one class from another. The simplest and most common class are called single floors, consisting of common joists only placed 12 or 13 inches apart, or 15 to 16 inches centre to centre, forming a bridge from wall to wall to support the boarded floor of the room above and the ceiling of the room beneath. It is this that gives the name of bridging joists to those joists which in all classes carry the floor-boards. The other two kinds of naked flooring are called double and framed floors, each of which will be fully described in the course of these notes later on.
Single floors are suitable for all spans up to 16 feet, though they must be strutted after the span exceeds 8 feet. They are the simplest, cheapest, and strongest kind of naked floor, consisting, as before noted, only of the bridging or common joists, spanning from wall to wall, and carrying the flooring above and ceiling beneath. These advantages are, however, accompanied by the following disadvantages, which sometimes, in special cases, predominate: -
1. The ceilings are apt to crack, on account of the sagging of the bridging joists, when in long lengths.
2. Wood being a conductor of sound, these joists, having no intermediate space between them, and carrying both the floor of the one room and the ceiling of the other, conduct the noises from one room to the other.
3. The joists, going from wall to wall, and having no other supports, distribute the load of the floor equally along the whole length of two of the enclosing walls, whether the wall is one solid mass or full of openings, in which case either the joists have to be carried by plates or lintels built in over the doors and windows in the wall beneath, or they have to be "trimmed" round them. Both methods cause weak points in the construction of a building.
The joists of the ground floors of buildings, when not above cellars, need not be so strong as for other floors, because dwarf sleeper walls, which reduce the spans, can be built on the ground to carry the wall plates which support and distribute the weights and pressure of the bridging joists.
Fig. 266, in plan and sections, illustrates a ground floor supported by simple naked flooring, consisting of bridging joists, B J, carrying the floorboards from wall to wall, with intermediate dwarf walls to lessen the span and carry the wall plates, W P, to which the bridging joists are secured. The dwarf or sleeper walls both render the floor more rigid, and effect a saving of timber.
A damp-proof course must always be put under the wall-plates, to prevent the damp arising; and it is better that these ground-floor wall-plates should be of oak.
It should be noted that the wall-plntes, W P, on the outside main walls, are not built into the main wall, but supported on sleeper or dwarf walls; inasmuch as the building-in of all wall-plates into the carcase walls of a building is held to be objectionable, on account of the dampness of the walls soon causing them to rot. It is the best plan to put the joists on two rows of hoop-iron, so as to distribute the weight to the extent required in the walls, or to support the wall-plates on corbels or over-sailing courses projecting from the main walls, care being taken to anchor the ends of the joists to the walls by means of iron, so that the enclosing walls are tied together by the joists.
Where fireplaces occur in ground-floor rooms it is usual to build fender walls round them, as X on section A B, fig. 266, to carry the wall-plates, and thus save trimming, which will be explained later on.
When single naked floors are used above other rooms it is obvious that the only support the bridging joists can have is the enclosing walls; and consequently it is necessary that the joists must be stronger, in proportion to the distance between the walls which have to carry them.
Fig. 267 illustrates, in plan and section, a single floor, carrying a boarded floor above and ceiling beneath.
It will be seen therefrom that the bridging joists span from wall to wall, a distance of 12 feet, and carry the floor above and the ceiling below.
The ends of the joists rest on the wall-plates, W P, which are either built in the walls, as shown on the right-hand side of section C D, or rest on two courses of brick oversailing, as on the left hand of the same.
When the cornices below allow of the wall-plates being put on projections, and not built in the walls, that is the best, as the joists can be nailed to them; but when this cannot be done, it is better - as wall-plates built in soon decay - to lay the ends of the joists on two rows of hoop-iron, instead of on the wall-plates shown on the right-hand side of section C D.