205. As plastering consists in the application of a plastic material called mortar, composed of various substances, and which is spread over the surfaces of the walls and ceilings in thin layers, the surface to be covered should present every facility for the perfect adhesion and retention of the material which is to be applied. Generally speaking, there are only two kinds of bases that are coated with plaster, and built up with subsequent layers to form a mass of sufficient thickness to withstand pressure, and to retain a true and uniform surface. The first may be called a lithic, or stone, base, and the second a grille base. In the former case, the plaster is laid directly on the face of the naked wall, which may be of either stone or brick; in the latter, it is spread over a grille-like arrangement of wood or metal strips, the edges of which are sufficiently separated to allow the mortar to pass through the spacing between them, and fold over on the inner face, thus forming a key, or continuous clinch, which will effectually hold the mortar in position.

206. Lathing is the general term applied to the grille base above mentioned, and generally consists of wood in non-fireproof buildings, like ordinary frame dwellings, and of metal in fireproof structures. Although not absolutely necessary on brick and stone walls - as plaster adheres well to such surfaces - it is usually better to have some kind of lathing on exterior walls built of masonry; by this means the plaster slabs are isolated from the wall, and a clear air space intervenes. This insures a continually dry surface, which would otherwise be liable to dampness, from the condensation of the heated air of the room on the cold surface of the walls, or from moisture penetrating the body of the wall during a period of wet weather.

Lathing on exterior brick or stone walls is usually attached to vertical strips, 1 inch thick by 2 inches wide, called furring, which have been fixed to the walls by the carpenter; these strips are set at 12-inch or 16-inch centers, according to the grade of the work. In the case of walls in frame buildings, the lath is attached directly to the studs, or vertical posts, which form the framing of the walls. The ceiling lath may be nailed directly to the under edges of the joists, or it may be attached to cross-furring - a series of strips, similar to those used on the walls - arranged at right angles to the joists and fixed at 12-inch centers; by the latter method better results are obtained, as the warping of the joists does not affect the lath.