The walls of rooms above ground-line are protected from damp by the following methods : -
1. The use of the Hygean Rock composition (as before mentioned).
2. The cement rendering.
3. Blue brick facing (also as previously explained; though the latter is not very bright and pleasant-looking to the eye).
4. Building the walls hollow with the "thick skin," as it is called, either inside or outside; the two being bonded together (a) by the use of galvanised iron wall-ties (vide fig. 189); (b) by the patent vitrified bonding bricks, as shown in fig. 190. The outer skin (about 4 1/2 inches or a half-brick thick) is built up - with a 2-inch cavity, ventilated at top and bottom for the circulation of the air, which is a non-conductor - simultaneously with the inner or thicker portion, about five bonding bricks or ties being used to each superficial yard - about twelve inches apart in height, and from two to three feet horizontally. Thin strips should be used during construction, in order to keep the mortar from felling inside the cavity; or sand courses may be left at the bottom, to allow of its being cleaned out when finished; though this latter method is not so good, from a practical point of view, for the stability of the work.
It is, however, to be noted also that hollow walls are a source of trouble, wherever openings occur, especially at the heads; the jambs being, easily got over, as shown at fig. 191. The heads, or lintels over the heads, in hollow walls are unprotected against all possibility of wet falling on to their upper surfaces. The best way to guard against this source of weakness, in good work, is to lay thin sheet lead (formed into a gutter) over them, between the joints (vide fig. 192), and having a fall both ways from the centre of the lintel, so that the wet is carried and thrown off them by the gutter, which should project two or three inches beyond the end of the lintel.
It is better to have the thin skin at the outside, and the thicker (or main) walls inside; inasmuch as the penetration of damp is all the sooner stopped by the circulation of the air in the cavity, and there is then a thicker dry wall inside. This, in fact, is the strongest, best, most advantageous and economical system; as, with the thin skin inside, all joints, roof, and other bearing timbers can only get a bearing of 4 j inches, on a half-brick wall, which is not sufficient, as the 4 1/2 wall is not strong enough to carry the whole weight of the building; hence all such bearings have to be of increased length to go into the outer wall, which, though it is the thicker and stronger, is also more exposed to damp.
Bridging over the cavity at these points of support may be recommended as a remedy, to avoid that increased expense; but, obviously, that would be to do away with all or most of the benefit, for the attainment whereof the hollow wall had been employed; seeing that it would break the non-conductor, and the damp would be transmitted from the outside to the inside wall by the bridge.
5. The external face of the walls may be slated or tiled, to wood laths nailed to bond timbers on pads built in as the work proceeds. If slates be used this method gives the building too much of the appearance of a roof, and creates a dull and uniform monotony of colour. When tiles are used, which may be of a different and lighter colour, and varied in form, they can be treated ornamentally, which cannot be done with slates; and to obviate the use of wood battens and plugs thin Wright's breezefixing blocks are built in, alternately with every brick course, having a slight projection on which the nib of the tile rests, while the tile itself is nailed to the blocks which are the "gauge" from centre to centre, two courses of blocks and one of bricks replacing two ordinary brick courses.
6. The outside of barns and other unimportant buildings are often coated with tar to keep out the damp.
Of the means used to stop damp where it has penetrated the walls of a building, Szerelmey's solution is one of the best, a coating or two of the solution (either applied internally or externally) often completely stopping it and rendering the wall water- and damp-proof.
Blunders petrifying liquid is also a good remedy, and can be applied in various colours, which, having a glassy surface, can be readily washed. The walls can also be covered with Willesden paper, which is said to be water- and rot-proof; or they may be papered with laminated lead or tinfoil papers before the decorative work is applied. If the walls are very damp, it may be advisable to plaster the walls in Portland cement instead of the ordinary materials, and finish them in Parian for paint; or in other cases the external walls are battened and lathed as hereinafter explained.