A variety of wood-block flooring consists of the use of longer pieces of wood, "secret-nailed" to fixing-blocks embedded in the concrete below, or to fillets of coke-breeze concrete. Special fixing-blocks for the purpose, composed of a kind of concrete capable of holding nails, can be obtained in 12-inch lengths, and of dovetailed section. Wood fixing-blocks must not be used, at any rate on ground-floors. The damp-course of asphalt (natural or artificial) must not be omitted from the ground-layer, or the wood above may rot The joints of the boards may be as shown in Fig. 87, where joint No. 3 is that used in the patent "Pavodilos" flooring. These boards may be polished as ordinary block floors.

Parquetry is the most elaborate and the most sanitary kind of wood flooring. It consists of small pieces of hardwoods of various kinds and colours, arranged in patterns (often of considerable intricacy), and carefully dowelled, glued, and screwed together. Sometimes the parquetry is one inch thick, each piece of wood being the full thickness; it is then known as "solid" parquetry. "Plated" parquetry consists of a hardwood surface about ¼ inch thick, fixed to a properly-framed deal backing, or, as in Turpin's patent, to two or three thin laminations or layers of hardwood well glued together; in Mackenzie's system a metal backing is used Panels of plated parquetry, containing about 100 square feet, can be Supplied by the makers ready for fixing. Parquetry, like wood blocks, must not be laid on ground-layers unless an asphalt damp-course has been spread to receive it. Flagged floors may be covered with asphalt and finished with parquetry ¼inch thick. The surface of parquetry may be wax-polished or French-polished, the latter process being a little more expensive.

Fig. 87  Sections of Secret naild Wood block.

Fig. 87- Sections of "Secret-naild" Wood-block.

Floors resting on the solid ground were at one time formed chiefly with flags, tile quarries, or bricks, without any preparation beneath except a layer of ashes. This method of construction is seldom adopted now in towns, as it is cold and 'lamp, and, indeed, in most cases it would be in contravention of the by-laws. A layer of concrete 6 inches in thickness is usually demanded, and this, in the case of tiles and bricks, should be floated level with cement-mortar (1 to 2). The joints of flags, tiles, and bricks should be run with neat cement-grout.

Hitherto we have been dealing with floors resting on the solid ground or on the concrete ground layer. Frequently, however, floors constructed of wood joists and boards are desired for ground-floor rooms; sections of such floors have already been given on pages 72 and 80, and in Plate III. The ends of the joists should not be built into the walls, but should rest on wall-plates laid on set-offs formed in the walls, and should always be above the damp-course. Sleeper-walls are often built at intervals under such floors in order to lessen the bearing of the joists, and consequently their scantling, but such walls are of doubtful advantage in most cases, as they interfere with the circulation of air under the floor, and may therefore induce decay in the wood. The hoards are usually 1 or 1¼ inch thick, and tongued and grooved at the edges to prevent warping, and to guard against crevices extending through the full thickness of the boards in case of shrinkage. Good yellow (or as it is often called, "red") deal is the best wood for both joists and boards, but white deal and spruce are often used in cheaper buildings. The rings in red deal should be clearly marked and of bright colour, and the wood should be free from sap, cracks, large knots, and especially from any trace of fungus or decay. For buildings of the best class, and especially for ball-rooms, oak (or other hardwood) boards are used, either alone or on the top of deal boards, and may be left in the natural state or polished at will.

Dry rot is the great danger to be feared in wood ground-floors. This is a kind of decay caused by the development of a fungus, the Merulius lacrymans, four conditions favouring the growth, namely - stagnant air, dampness, warmth, and absence of light or sunshine. The first two conditions are those with which the architect is most concerned, and these he can generally avoid by adequate ventilation and by impervious ground-layers and walls. The ventilation, however, must be adequate; particularly must every corner of the room have its own ventilator. It is not sufficient if air-grates or ventilating damp-courses are inserted on one side only of a room, or on two sides at right angles to each other; ventilation must be ensured on at least two sides opposite to each other, so that a through current of air may be induced. To secure this, it will often In-necessary to form air-drains with bricks or drain-pipes under adjacent solid floors, or to construct vertical air-Hues in the walls, gathered perhaps to the smoke-flues above. The air-grates below the floor-level must be carefully protected outside, so that they may not be inadvertently blocked. Many cases of dry rot have been developed in consequence of gardeners having covered the air-grates which the architect had carefully provided. care must be taken in selecting the wood for floors, as in many cases traces of white fungoid growth can be seen before the wood is fixed; a consignment containing wood of this sort should be condemned in bulk. Chips of wood left under floors are a soun danger, and impervious floor-coverings, such as linoleum, by preventing the passage of air, increase the risk of rot.

Wet rot is not as insidious or as frequent as dry rot, and the quantity of moisture necessary for its appearance will never be present in a building carried out with reasonable care and skill.