This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The upper floors of ordinary houses are usually formed with wood joists covered above with floor-boards and beneath with wood laths and plaster. The joists must be trimmed (see Fig. 70, page 130) around all fireplaces, and for stair-wells and other openings. It is better for the ends of the joists to rest on set-offs formed in the walls, as shown at a in Fig. 48, page 107, but where this cannot be done the joists must touch the walls only on their lower surfaces, so that a clear space is left around the ends for circulation of air. This reduces the risk of rot. Special joist-boxes of cast-iron or stoneware can be obtained for building into walls to receive the ends of joists, and are more to be depended upon than a method which leaves so much to the skill and care of the bricklayer.
Sometimes in order to deaden the sound, a counter-floor is farmed between the joists with rough hoards nailed to fillets which have previously been nailed to the sides of the joists; the space above the counter-floor is then filled with sawdust or lime-pugging (both dangerous in respect of rot), or with silicate-cotton.
Sometimes silicate-cotton slabs are nailed under the joists and fillets nailed below, to which the lath-and-plaster ceiling is attached in the ordinary way; or silicate-cotton slabs, plastered on one side and strengthened by wire-netting embedded in the mass, are screwed to the joists to form the rough ceiling. Plate III. shows a floor with ceiling-joists spiked under the floor-joists at right angles to them, thus allowing continuous circulation of air between the timbers; this space affords a simple, although not very cleanly, method of ventilating the room below, if openings are formed in the ceiling and air-grates built into the externa] walls.
Wood floors, however, of the kinds described are undoubtedly combustible, besides containing numerous inaccessible places for the accumulation of fine dust and vermin. They may with advantage be superseded by solid wood floors, formed with ordinary floor-joists fixed close together and spiked to each other, and either planed level on the top or covered with thin floor-boards. Such a floor is practically fire-proof, and has most of the advantages of ordinary wood floors without the disadvantages; there is, however, considerable danger of decay unless the wood is of good quality and properly seasoned.
Parquet flooring is frequently laid on the top of ordinary wood floors, and may be either fixed or removable Thin oak boards are also used, secret-nailed as a rule, and generally polished.
Fire-resisting floors of many kinds have been largely used in recent years, but chiefly in business-premises and public buildings. In houses, somewhat strange to say, the attention has not been given to the subject which its importanee warrants. As I have already pointed out, fire-resisting construction is a matter of materials and also of their arrangement. If we avoid combustible materials, such as wood, a step has been taken in the right direction, but care must also be exercised that the incombustible materials used in their stead are at least moderately fire-resisting. Undoubtedly good clay, burnt at a high temperature (whether in the form of bricks or terra-cotta), furnishes one of the best fire-resisting materials. Concrete, composed of one part of Portland cement and not man than four parts of a suitable aggregate (such as broken bricks, properly-bomt clay, and coke-breeze), is of considerable merit, but bad concrete, especially with stone aggregates, is most treacherous in a fire, and may collapse entirely at an early stage. Iron and steel also, although not com-bustible, are not by any means tire-proof, but bend and twist under great heat, and, if not protected, may give way and so lead to the destruction of the building. from the facts just stated, two rules may be laid down for guidance in designing fire-resisting floors: -
Fig. 88- - Wood floor and Counterfloo with Lath-and plaster ceilling.
1. Concrete alone is dangerous, and must therefore be protected, or prevented from total or extensive collapse.
2. Iron and steel must be entirely surrounded by an adequate thickness of fire-proof material, or in some other way protected from the action of fire.
Rule 1 throws out of court the large-span floors of concrete alone, but not concrete floors containing iron or steel joists or tees at short distances, or a mesh work of rods or bars or expanded metal, with the metal protected beneath and above by fine concrete, plaster, etc.
Rule 2, however, is more stringent, and demands greater protection of the metal from fire than is attained in ordinary fire-resisting floors. Several patented floors, however, have been designed in accordance with it. In Fawcett's floor, illustrated in Fig. 89, the protection of the metal is obtained by means of terracotta tubular lintels, the ends of which are grooved to clip the flanges of the joists in such a manner that the joists are entirely protected beneath by the flat bottoms of the lintels; they are protected albove by coke-breeze concrete, of which the bulk of the floor is formed. The lintels are grooved beneath to afford a good key for plaster, this furnishing an additional protection to the metal. It will be noticed that a continuous circulation of air can be maintained through the lintels and beneath the joists. The joists are of steel, fixed 2 feet from centre to centre.
Section through A B.
Section through C l>.
Fig. 89 - Fawcett's Fire-proof Floor.
Willis & Astley's and Pickings floors are of a somewhat similar kind, although perhaps a little more complex.
In Banks's floor the metal is protected simply by an air-space, and a fire-resisting ceiling consisting of "helical" metal lathing and plaster. It is illustrated in Fig. 90, and has the merits of simplicity and economy, while it also affords opportunity for ventilation and is not as noisy as a solid floor.
Fig. 90 - Section of Bank's Fire-resisting Floor.
In many upper rooms, where much water is used, - as in bath-rooms, cistern-room-, water-closets, housemaids' closets, lavatories, etc, - a floor of other material than wood is desirable, not so much for its fire-resisting qualities as for its imperviousness and freedom from decay. In such cases, a simple slab of concrete may be used for spans up to about 8 or 10 feet. Thinner floors of good concrete are more trustworthy than thick floors of inferior concrete; it is therefore better to use concrete composed of one part cement and not more than three parts of some hard aggregate (say granite or syenite), broken to pass through a screen with meshes 1¼ inch square. A thickness of 4 inches I have found to be sufficient for spans up to 6 feet, and 6 inches will suffice for 10-feet spans. For work executed by unskilled nun in the ordinary way, a considerable margin of safety must always be allowed, but when the floors are constructed by specialists, the thickness may safely be reduced; flat granolithic floors have been constructed L6 feet by 9 feet, and only 3 inches thick, while arched floors of the same material, 21 feet 6 inches span and 29½ inches thick at the springing and 3 inches thick at the crown (the rise of the arch being only 26½ inches), are said to have been loaded with 8 cwts. per square foot without injury.
Combined steel and concrete floors are now largely used, the steel being in the form of expanded metal, rods, bars, tees, or joists. I have used the steel in the form of a meshwork, interlacing bars and rods, for spans up to 14 feet, but the extra labour required in fixing this probably counterbalances any saving which is effected in the quantity of metal. In America, square twisted rods are often employed, the twist preventing the rods being drawn through the concrete when subjected to the and stress. Steel tees, flat side down, may be used with economy to increase the strength of concrete floors. When the metal is in the form of tension members only, it is particularly important that the wood staging should be perfectly rigid; otherwise it will sag under the weight of the wet concrete. As a general rule, however, the metal in concrete floors is in the form of joists, fixed from 2 to 4 feet apart, and varying in size according to the spacing, the span, and the load. Joists strong enough to bear the weight of the concrete and of the load on the floor, are generally used; this is somewhat wasteful of metal, but the resulting floor will generally, notwithstanding this, be cheaper than any of the special floors, - cheaper even than a floor of granolithic concrete alone. Such at least is my own experience. The following table gives the sixes and weights of steel joists which may be used in floors of this description in houses, the joists being fixed 2 feet from centre to centre
Size of Joists.
Weight per ft
3½ x 1½
4 x l¾
4 x 3
4¾ x l¾
5½ x 2
The wood staging for concrete floors should be fixed at least half an inch below the steel joists, and the space between the boarding and the joists should be thoroughly filled with cement-mortar (1 to 2) immediately before the concrete is deposited. The staging should remain in position as long as possible; where joists are not used, the period should be not less than six weeks. Concrete floors are strongest when laid over the walls and subsequently built upon, but where this cannot be done, corbel-courses or chases must be formed to receive them. Newly-deposited concrete must be protected from traffic, drought, and frost. Coke-breeze is a common aggregate for floor-concrete, on account of its lightness; it is, however, weak, and may with advantage be superseded by broken brisk.
The surfaces of concrete upper floors may be finished in one of the ways described in the former part of this chapter, or with wood fillets to which ordinary floor-l)oards are nailed. Sometimes coke-breeze fillets or composition blocks are used to receive the boarding, as wood fillets are somewhat apt to decay.