Moist earth can also be prevented from coming in direct contact with basement walls by means of dry areas. Fig. 155 shows a simple form of dry area much used where rough slabs of stone or slate are available.

Fig. 156 shows another excellent type of dry area, to which the moist earth is prevented from coming in contact with the wall by means of a small retaining wall built at a distance of a few inches from the main wall. Ample means of ventilation should be afforded to these dry areas, open shafts being formed at intervals in the brickwork for the purpose.

Footings Copings Cornices Corbels Damp Proof Cours 349

Fig. 154.

Footings Copings Cornices Corbels Damp Proof Cours 350

Fig. 155.

Footings Copings Cornices Corbels Damp Proof Cours 351

Fig. 156.

Footings Copings Cornices Corbels Damp Proof Cours 352

Fig. 157.

Sometimes dry areas are formed in a similar manner to hollow walls, as in Fig. 157. Ample ventilation should be provided to all dry areas and to wood floors below the ground level. The bonding ties for these hollow areas may be formed of bricks soaked in tar or hot pitch, but a more water-proof wall is obtained by the use of proper vitrified stoneware or iron bonding ties.

A very satisfactory dry area is formed by placing a retaining wall at a suitable distance from the basement wall, thus forming an open area, as in Fig. 158. Great economy is sometimes effected by building a flying buttress of brick or iron between the retaining wall and the main wall, as in Fig. 159.

Footings Copings Cornices Corbels Damp Proof Cours 353

Fig. 158.

4. Damp must be prevented from soaking up through the foundation of walls by placing a horizontal layer of some impervious material at a distance of from 6 inches to 1 foot above the ground level. These impervious materials are sheet lead, slates, tiles, vitrified stoneware slabs, asphalt, or various combinations of these materials.

Sheet lead forms a very satisfactory damp-proof course, and has the advantage of being very easily applied in strips cut to the width of the wall. There is a tendency, however, for lead to squeeze out a little and to adapt itself to the irregularities of the stones or bricks between which it is laid, causing them to spall at the edges. Some object to the use of lead on the ground that it is decomposed by the action of the carbon-dioxide in the atmosphere, but the action is so very slow as to be inconsiderable. In fact, the carbonate of lead which is formed acts as a protective surface on the lead. Lead is largely used in the upper parts of buildings to prevent the downward penetration of damp, as will be explained when dealing with plumber's work.

When slates or tiles are used they should be in two courses, laid to breakjoint and set in mortar composed of Portland cement and sand in equal proportions. This form of damp-proof course is a very cheap one, and is much used in consequence; but it invariably cracks with the slightest irregularity of settlement in the building, thus permitting damp to rise in places.

Footings Copings Cornices Corbels Damp Proof Cours 354

Fig. 159.

Vitrified stoneware slabs form an efficient damp-proof course, and are made in sizes from 1 to 2 3/4 inches thick and 9 inches long, and in widths of 4 1/2, 9, 13 1/2, and 18 inches. They are usually perforated through their widths, and may be used to ventilate the lower floors of a building. They should be laid with their vertical joints open, as shown in Fig. 160, as mortar in this position would assist the upward penetration of moisture. Some of these slabs are made with grooves and tongues on the edges, as the accumulation of dirt in open joints sometimes allows water to pass upwards through them.

Asphalt in layers of from 3/8 to 3/4 inch thick forms, under normal conditions, the most reliable of all damp-proof courses. It is best to use asphalts supplied by firms of established reputation, and when ordering the conditions under which it is to be laid should be fully stated.

The surface to receive melted asphalt should be carefully levelled off as smoothly as possible with mortar to economise materials, and where bricks with frogs in them are used the flat sides should be laid uppermost, and the joints should be thoroughly flushed up with mortar. The asphalt in a molten condition is thoroughly mixed with fine grit and laid on the surface thus formed, and spread into an even layer with a wooden float.

Footings Copings Cornices Corbels Damp Proof Cours 355Footings Copings Cornices Corbels Damp Proof Cours 356

Fig. 160.

A preferable form of damp-proof course now much used is composed of bituminous sheets. It is supplied in strips 4 1/2, 9, 13 1/2, etc. inches wide, and has only to be laid upon the walls at the required level with the joints overlapping. It possesses all the advantages of liquid asphalt, while it is much cheaper and more easily laid.

Similar sheets of felt impregnated with asphalt are also made, but the felt soon decays, leaving insufficient asphalt in the joint to resist moisture.

Hygeian rock in 1/2-inch layers is also very efficient for horizontal as well as vertical damp-proof courses, and compares very favourably in cost with those made with good asphalt.

Willesden paper is largely used as a damp-proof course for temporary buildings, and is supplied in rolls of the required widths, and is laid with great facility.

5. Ground air, particularly when very moist, is most dangerous to the health, and every precaution should be taken to exclude it from dwelling-houses. Fig. 161

Footings Copings Cornices Corbels Damp Proof Cours 357

Fig. 161.

shows a house situated apparently in a most delightful position looking down on to a stream. The foundations of the house are formed on good gravel with a subsoil of impervious clay. The river is subject to flood, the level rising as shown. The result of this is that the air contained in the gravel is forced up, and if precautions are not taken to prevent it, it enters the house, carrying with it moist and foul gases. This is prevented by laying a bed of cement concrete over the whole site from 6 inches to 1 foot in depth, and in an extreme case a layer of asphalt should be used, as in Figs. 151 to 154. This example is given as an extreme and obvious case; but ground air is such an unexpected and insidious foe to health that in all good modern building, except it be on rock or chalk, a layer of 6 inches of cement concrete should always be thus laid.

6. Faulty water pipes, drains, and gutters are usually discovered immediately if the fault is great, but if the fault is small it is not so readily detected, especially in the case of drains. The way to prevent this defect is to have all drains, pipes, and gutters, etc. inspected periodically, and all gutters cleaned out once or twice a year, particularly in the autumn, when the leaves have fallen from the trees.