For this purpose the laths are nailed to the underside of the ceiling joists (see Figs. 194 and 228, Part I.), (or in many cases to the bridging joists; see Fig. 192, Part I.), which should, if necessary, be brought into a horizontal plane by adding slips of wood called "firrings."
The laths are fixed parallel to one another, and 3/8| inch apart so that the intervals afford a key for the plaster. Every lath is secured by nails, one being driven through the lath wherever it crosses a joist or batten. The moist plaster passes between the laths, forming protuberances at the back - these harden and form what is known as the "key" which prevents the plaster from falling away from the laths and keeps it in position. Care should be taken that the ends of the laths do not overlap one another, and that they are attached to as small a surface of timber as possible, so that the key may not be interrupted.
If the joists are of wood, a narrow fillet may be nailed along the under side of each to receive the laths, so as to interfere with the key of the plaster as little as possible.
The laths should be laid in "bays," so as to break joint in portions 3 feet wide (see Fig. 369).
The thickest laths should be used for ceilings, and for very important work they should be nailed with zinc nails, so that there may be no danger of their oxidising, and the rust showing on the surface.
Battened Walls1 are so called because wooden battens about 2 to 2½ inches wide, and from 5/8 to 1 inch thick, are fixed vertically at central intervals of about 12 inches, to receive the laths.
The battens are nailed to wood plugs in the wall, except where flues occur, in which case they should be secured by iron holdfasts.
The laths are nailed as above described. They should be fully ¾ inch clear of the inside of the wall, and about 3/8 inch apart - thus affording a key which is sufficient to support the plaster in its vertical position.
Walls likely to be damp should be battened, as the clear airspace between the masonry and the lathing insures the plastered surface being constantly dry; but battened walls harbour vermin, the woodwork is subject to decay, and is injurious in case of fire.
1 Sc. Strapped walls.
Fig. 369. Elevation.
Fig. 370. Sectional Plan.
Figs. 369, 370 show a sectional plan and an elevation of a portion of a battened wall, with some of the plaster removed, in order to show the laths 11 and battens b b below.
The laths are shown as breaking joint in bays. This is not absolutely necessary for walls, but is often done in vertical work as well as for ceilings.
Counter-lathing is necessary when plaster has to be applied close to a flat surface, such as that of a large beam. In such a case laths are nailed on to the surface of the beam about a foot apart, and across these is nailed the lathing to receive the plastering.
This second layer of laths is termed counter-lathing. Being clear of the surfaces of the beam, a key is afforded, and the plaster adheres to the first layer of laths, which it would not do if they were nailed on to the beam itself.