Thin boards made of plaster, and reeds or fibre, have also been quite extensively used, not exactly as a lath, but as a ground for the second and third coats of plaster. They are made in slabs about 5/8 inch thick, 16 inches wide and 4 feet long. The Mackite boards are made ¾- inch and 1 inch thick for ordinary work. The under surface of the boards should be grooved or left rough to receive the plastering.
The materials of which the boards are made consist chiefly of plaster of Paris and some sort of fibre. The Mackite boards also have hollow reeds imbedded in them. The boards can be sawed into any size or shape and nailed directly to the under side of the joist, or to studding or furring. They are rapidly put on and require no scratch coat, and with some styles of boards a white or finished coat is all that is necessary.
Actual fire tests appear to show that fire does not harm the plaster board more than the terra cotta tile, and on account of their lightness, and the ease with which they can be cut, they are sometimes preferred to tile or terra cotta for suspended ceilings under iron beams.
Owing to the saving of plaster, the low cost of the boards and the ease with which they are put up, plaster boards probably offer the cheapest fireproof ceiling yet devised.
In using plaster boards, or any of the patented laths, the architect or builder should follow the directions of the manufacturers as to the manner of putting up, etc., as there are often important precautions which might otherwise be overlooked.
There are, however, many places where it is particularly desirable, especially in buildings having ordinary wood floors and partitions. Such places are the under side of stairs in public buildings, the ceilings in audience and assembly rooms, under side of galleries, ceilings of boiler and furnace rooms, etc.
Metal lathing should also be used on wood partitions, on both sides of hot air pipes. Where there are slots in brick walls for plumbing, hot air or steam pipes, they should be covered with metal lath, unless the walls are furred or the recesses cased with boards.
Metal lath should also be used at the junction of wood partitions and brick walls, when the walls are not furred, and particularly when the partition is parallel and flush with the wall.
By using a strip of wire cloth or expanded metal, lapped 12 inches on the wall and partition, a crack at the juncture of the two will be avoided, and at only a very slight additional expense.
It very often happens in outside brick walls that the arched wooden lintels over the windows come partly above the casing, and if the wall is plastered directly onto the brick the plastering generally cracks over, or will not stick to the lintel. This can be avoided by covering the lintel with a strip of metal lath, lapped 6 or more inches on the brickwork.
In general, wherever solid timber has to be plastered, without room for furring and lathing, it should be covered with metal lath, which should also be lapped well on to the adjoining partition or wall.