In exposed situations rain is often driven by the wind quite through a solid wall, especially when the materials of the wall are porous or badly laid. Sometime* the dampness of walls is due to neglect in flushing the joints with mortar, or in the external pointing, but in many cases the moisture is actually driveu through the bricks or stones. In stone walls the moisture usually follows the "through", and not infrequently these can be counted inside a room by the damp patches on the plaster.1 Something will be said in a subsequent section on the means to be adopted in order to improve existing damp walls, but for the present we are concerned with prevention, which is easier than cure.

of course a solid wall can be made impervious by means of a vertical asphalt layer between two skins of brick or stone, as shown in Fig. 26, page 80, and in other way-, as explained in connection with basement walls, but, as a rule, quite as effective protection from damp can be obtained at less cost by forming a simple cavity in the wall. Solid walls, however, have certain advantages; they do not harbour vermin, and for the same quantity of materials they are stronger and cheaper.

Theoretically, a cavity half an inch wide is to all intents and purposes as effective as one a foot wide, but a narrow cavity is so easily bridged by a piece of brick or a chance dropping of mortar, that a width of not less than 2 inches should be allowed; frequently cavities 2 or 3 inches wide are adopted.

The thickness of hollow walls for small villas is often only 11 inches, that is to say, two half-brick skins and a 2-inch cavity, and where cheapness is a primary consideration this is all that can be afforded. At the same time it must be said that a thicker skin on at least one side of the cavity is preferable, and in London and some other places is indeed obligatory. Thus, the London by-law on hollow walls ordains that "when hollow walls are constructed, there shall be a wall on one side of the hollow space of the full thickness preseribed" for solid walls; in other words, the total thickness of a hollow wall must exceed that of a solid wall for a similar building by the width of the cavity and the thin skin on one side of it. Clearly in London hollow walls are somewhat heavily handicapped; consequently they are not often used. Indeed it is in the exposed situations of country and seaside, rather than in the sheltered streets of towns and cities, that hollow walls are most needed.

1 Damp stone shaped patches , however, are not always due to this cause; they may be the result of condensation, the shape of the cold and dense atones being marked on the plaster by patches of damp, while the warmer and more porous mortar-joints leave the plaster apparently dry.

Plate III

HOLLOW BRICK WALLS WITH STONE DRESSINGS.

HOLLOW BRICK WALLS WITH STONE DRESSINGS.

BB. Stoneware bonding - blocks across cavity.

C C. Stoneware ventilating damp-course.

DD. Asphalt damp-course under parapets and gutters.

E. Asphalt on concrete ground-layer.

F. Weathered and throated stone sill.

G. Weathered and throated stone coping.

H. Lead gutter.

I. Lead flat over bay-window.

K. Roof-tiles on horizontal and vertical laths, laid on boardscovered with felt. L. Ordinary laths and plaster. M. Plaster on expanded metal lathing secured to small steel angles fixed to wood joists.

N. Parquet flooring laid on inch tongued and grooved boards. O. Wrought-iron casements. P. Rain-water pipe. Q. Through-stone course closing top of cavity. R R. Air-bricks ventilating the floor.

Where a wall 11 inches thick is not sufficient, additional strength is usually gained by increasing the thickness of the skin on one side only of the cavity, and the question is often asked, Should the thicker skin form the external face of the wall or the internal? The balance of opinion is in favour of the latter alternative, as in this way the greater part of the whole wall is kept dry, and the floors and roof are more firmly supported; "set-offs" (for reducing the thickness of the wall) can also be more easily arranged without breaking the continuity of the cavity, as shown at A in Fig. 48.

Several forms of tubular bricks and concrete blocks have been devised for the purpose of forming hollow walls, but none has met with general acceptance. Ordinary bricks are so cheap and convenient, and prejudice so strong, that there is little possibility of any patented hollow wall being largely adopted.

In the case of stone-and-brick walls, cavities may be formed exactly as in brick walls, an outer skin of stone being substituted for the outer skin of brick, but as ordinary wall-stones vary much on the bed, a somewhat wider cavity ought to be specified. In thicker walls, however, it is customary to build the outer skin of the wall with a lining of brick, the stone and brick being tied together with bond-stones; the inner skin is entirely of brick. A reference to Fig. 48 will explain this method of construction.

In building hollow walls great care must be exercised that the cavity is continuous throughout the circuit and height of the building. In order to prevent the cavity being bridged with droppings of mortar or brick-bats, battens or iron pipes wrapped with haybands, or hay bands alone, should be placed in it, and lifted out when the wall is ready to receive the iron ties or bonding-blocks; the battens or pipes are then laid on the top of these, and the wall carried to the necessary height for the next row of ties, and so on.

Cavities are sometimes formed in concrete walls, as shown in Fig. 49, by inserting in the required position between the temporary shutters a 2-inch or 3-inch plank, tapering slightly in thickness from the top edge to the bottom; the taper facilitates the removal of the plank. When the concrete has hardened sufficiently the plank is withdrawn, and metal ties are then laid across the cavity; on these ties the plank rests during the formation of the next layer.

In order to bind the two skins of a hollow wall together and so strengthen the structure, metal wall-ties, or bonding-blocks of brick or stoneware, are inserted. Sometimes dense bricks of ordinary shape are used, but as moisture is apt to pass along (if not through) these, it is better to adopt special blocks or ties. Iron ties are from 6 to 9 inches long, and may be either cast, as in Fig. 50, or wrought, as in Fig. 51. They should be of such a shape as to prevent water passing over them to the inner portion of the wall. When the bricks are without "frogs", the projections under the ends of the ties must be omitted. The cast-iron ties are sometimes rendered malleable in order to prevent them snapping. All metal ties should be galvanized, or dipped in boiling tar and sanded, before being used; otherwise they may rust and injure or stain the wall. Bond-ing-bricks are usually of semi-vitrified ware. Two good examples are given in Fig. 52. That marked a is so shaped that water cannot pass along it or mortar rest on it. The size is 9 inches by 4 inches by 3 inches. The bonding-brick b is made by J. C. Edwards, and may be had with 2 or 4 inches bearing on the walls, and for cavities 3 or 4 inches wide. The upward slope prevents moisture passing from the outer to the inner skin of brickwork. Blocks of other shapes can be obtained from other makers. For the sake of appearance the bonding-brick is seldom allowed to show on the face of the wall, as it would not match the colour of the ordinary brickwork; where the outer skin is only a half-brick in thickness, the bond extends into it only 2 inches, certainly not an amount calculated to give excessive stability.

Fig. 48  Hollow Wall of Stone and Brick.

Fig. 48 -Hollow Wall of Stone and Brick.

Fig. 40. Hollow Coocrete Wall

Fig. 40.-Hollow Coocrete Wall.

Any shortcoming in this respect, however, can be counterbalanced by the spacing of the bonds. These, whether of iron or stoneware, are usually placed 3 feet apart horizontally, and 9, 12, or 18 inches apart vertically; that is to say, four, three, or two are allowed in each square yard of the wall.

The general construction of hollow brick walls will be more clearly understood by reference to Plate III., which gives plans, sections, and elevation of 11 -inch and 15-inch hollow brick walls with 2-inch cavities and stone dressings.

Fig  50   Cast Iron Wall ties

Fig- 50 - Cast-Iron Wall-ties.

Fig. 51.   Wrought iron Will tie*

Fig. 51. - Wrought-iron Will-tie*.

5 Hollow Walls 10057Fig. 52 Bonding brick for Hollow Walls.

Fig. 52-Bonding brick for Hollow Walls.

The cushion of air in the middle of a hollow wall helps to keep the temperature of the house more equable; but this advantage is lost when the cavity is over-ventilated, a condition which may be caused by porous materials, bad mortar-joints, or by excess of air-grates. Holes for egress of moisture are often provided at the foot of the cavity, but if the top of the cavity be closed, as at Q in Plate III., and the wall be well built, little or no circulation of air can take place.

Where parapets and lead gutters are adopted, an asphalt damp-course should be laid on the wall immediately under them, as at d in the Plate.

In order to prevent rain damaging the woodwork of the window or finding an entrance to the room at this point, a strip of 5 lbs. milled lead about 6 inches longer than the head of the window-frame should be built into the wall immediately over it.

A kind of hollow wall is sometimes formed by fixing upright pieces of wood about a foot apart against the internal face of a wall, and covering these with laths and plaster. The uprights may be merely "grounds", about 2 inches wide and or 1 inch thick, nailed to plugs in the walls, or may be of larger section (3 inches by 1 inches, 3 inches by 2 inches, or more, according to the height of the room), and fixed quite clear of the wall. The latter is the better method, as there is much less liability of the wood decaying. A more durable construction consists in the use of small steel uprights of L or T section, to which reticulated or perforated sheets of metal, known as "metal lathing", are secured with wire, and afterwards covered with plaster in the ordinary way. Undoubtedly each of these three devices will hide the dampness of an external wall, but in two there is a great likelihood of decay, and in all a cavity is formed for dirt and vermin.