This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The arrangement of the joists, etc, of floors and ceilings has already been described under Carpentry, pp. 334-40. Before beginning to lath a ceiling, it is necessary to prove the under surface of the joists by applying a long straight-edge, and to make up for any slight inequalities in them, when the work is not to be of a superior character, by nailing on laths or strips. A framed floor with ceiling joists is tolerably sure to be straight; but the carpenter must previously have tested the lower surfaces of the beams or binders, to ensure their accuracy of level with that of the ceiling joists, unless the ceiling joists have been nailed to the beams. If a ceiling is to be divided into compartments or panels, the projecting or depending portions must be bracketed or cradled down to receive the laths. It is an important point to be attended to in plastering on laths, and in ceilings particularly, that the laths should be attached to as small a surface of timber as possible, because the plastering is not supported by its adhesion or attachment to the wood, but by the keying of the mortar itself, which passes through between the laths, and bends round over them.
If, therefore, the laths are in constantly recurring contact with thick joists and beams, the keying is as constantly intercepted, and the plastering in all such places must depend on the portions between that are properly keyed. Under a single floor, in which the joists are necessarily thick, a narrow fillet should be nailed along the middle under the whole length of them, to receive the laths, and keep them at a sufficient distance from the timber to allow the plastering to key under it; thus, too, the surface may be made more perfectly even, as it is in single floors that inequalities mostly occur.
This being all arranged, the plasterer commences lathing. The laths should be of the stronger sort. Thin, weak laths, if used in a ceiling, are sure to produce inequalities by sagging with, or yielding to, the weight attached to them. One or two weak ones in a ceiling of otherwise strong laths may be the ruin of the best piece of work. Laths should be previously sorted, the weak, the crooked, and knotty, if there be such, being reserved for inferior work, and the best and strongest selected for the work of most importance, so that the workman shall find none to his hand that is not fit to be brought in. Taking a lath that will reach over 3 or 4 openings, the plasterer strikes a nail into it on one of the intermediate joists, at about -J in. from the one before it, and then secures the ends of that, and the one that it meets of the last row with one nail, leaving the other end of the lath he has just set, to be secured in the same manner with the end of that which shall meet it next in continuation.
It is of importance in ceilings to pay attention to the bonding of the work. In lathing or quartering partitions or battened walls, the bonding is not a matter of such material consequence as in a ceiling, because the toothing which the thickness of the lath itself affords to the plastering, is enough to support it vertically; but, nevertheless, the more complete the keying, even in work of this kind, the better, as the toothing above will not always protect it from any exciting cause to fall forward or away from the laths. The thinner or weaker sort of lath is generally considered sufficiently strong for partitions In common lathing, the spaces between the laths should be 1/4 in. If they be made less than that, the clinches will not be strong enough; and if more, they will sag down on the ceiling, and drop off with their own weight on the sides. In no case should the spaces exceed 2/5 in., except when the furring is very thin, like strips of lath nailed on inside sheeting or ceiling. Most lathers break joints at every 6th lath, and some every 10th; but it is better to break joints every 2nd lath. When ordinary laths are used, 1/4 - 2/5 in. thick, the studding, joists, etc, should never be over 16 in. apart - 12 in. would be better.