Floors may be considered under the following headings : (a) Ordinary floors ; (b) Fire-resisting floors; (c) Reinforced concrete floors.
(a] Ordinary Floors. Ordinary floors are constructed with wooden joists, placed about one foot apart, and covered with floor boarding on the upper side, and with lath-and-plaster ceiling on the under side, as shown in picture 30. This form of floor is unhealthy because of the accumulation of dirt which collects in the spaces beneath the boarding, for every time such a floor is washed it means the addition of more filth to that which has already passed through the open joints of the floor boards, and found a resting place on the plastered ceiling beneath (picture 30). Such floors when used should be covered with grooved - and - tongued or ploughed - and - tongued boarding, which will, to a certain extent, prevent dust and dirty water from falling between the boards. Picture 31 shows a floor in which the usual lath-and-plaster ceiling is omitted. The timber joists are visible from the room below, and give a certain homely effect, as seen in pictures 229 and 230, while obviating the insanitary drawbacks of the ordinary floor. Two thicknesses of boarding with a layer of non-conducting material between them are placed on the top of the joists.
Picture 30. A Timber Floor With Herring Bone Strutting.
Picture 31. Hygienic Floor.
A rough guide for the depth of the joists placed a foot apart and between two and three inches in thickness is :
Span in feet divided by two + 1 in. = depth of joist in inches.
Where the bearing exceeds eight feet, the joists should have one row of herring-bone strutting as shown in picture 30, and when the span exceeds twelve feet there should be two rows.
Double floors in which binders are introduced are occasionally used over large rooms, and in such cases separate ceiling joists are sometimes used, as in picture 32.
A Double Floor.
Steel joists are now generally employed in lieu of the wooden binders, and the joists then rest on plates bolted to the webs of the girder.
In order to prevent dry-rot in wooden floors ventilation is always necessary where there is a space between the floor boards and ceiling (picture 30), also with ground floors having a space between the floor boards and ground (picture 25), and floors have constantly to be relaid in old buildings because of the neglect of this precaution. The ventilation is effected by means of perforated iron gratings as shown in picture 25, or air-bricks built into the outer walls and so arranged that cross-ventilation is produced.
Dry-rot almost invariably arises where dampness, stagnation of air and warmth are in combination, and the most extraordinary instances occur of the destructive effects of this disease in timber. When once thoroughly started its ravages are remarkable and it will spread across brick and stonework, and, in fact, almost any material, in order to attack adjacent timber. Directly it is discovered it should be ruthlessly dealt with, all affected parts cut away and all adjoining materials thoroughly scraped and treated with corrosive sublimate or other strong disinfectant.
(b) Fire-resisting Floors. Fire-resisting floors formed upon hygienic principles should be used where possible, so that dust and vermin may not be harboured in cracks and crevices.
Ordinary wooden joists placed side by side form a fire-resisting floor (picture 33). In this case the floor boarding is nailed direct to the upper surface of the joists, which are left exposed on the under side with a V-joint formed between them as shown or with dovetailed grooves formed so that plastering may have an effective key.
A Fire Resisting Timber Floor.
There are many kinds of steel and concrete fire-resisting floors. One of the best, which costs little more than the ordinary combustible wooden floor, consists of steel joists placed about two feet apart, and the space between filled up with six or seven inches of concrete (picture 34). The concrete projects about one inch below the bottom flange of the girders, so as to protect them from fire and to enable the ceiling below to be plastered. Steel rods are often used as shown, in order to increase the tensile strength of the floor.
Picture 34, A Steel and Concrete Fire-Resisting Floor.
The floor boards can be nailed direct on to the concrete or may rest on fillets of dovetail section about three inches by two inches, laid flat and fixed thereto. The space thus formed is useful for gas and water pipes, but it is objectionable as it forms a convenient place for dust and dirt. Care must be taken that the concrete is thoroughly dry if the boards are nailed direct to it, otherwise dry-rot may result.
(c) Reinforced Concrete Floors. Where steel bars are used in conjunction with concrete floors, the latter are said to be "reinforced", and one method is shown (picture 35) where steel bars are placed near the under side of the floor in order take the tensional strain where it is greatest. There are many forms of patent bars and stirrups for such floors now on the market.
Picture 35. A Reinforced Concrete Floor.
Solid wood blocks (picture 36) may be laid direct to the concrete, but floor coverings are dealt with more fully later.
Picture 36. A Wood Block Floor.