Hollow tile arches of whatever type should be set in a good Rosendale or Portland cement mortar on plank centring, slightly cambered. The best centring for flat arches is that in which the planks run at right angles to the beams and rest on 2x6 sound lumber centre pieces, placed midway between the beams and extending parallel with them. These centre pieces are supported by T-bolts from like centre pieces above, crossing the beams. The planks on which the tiles are laid should be 2-inch plank, dressed on one side to a uniform thickness and laid close together. If the soffit tile is a separate piece it should first be laid directly under the beam on the planking; if a projecting skew-back is used, then the skewbacks must first be set, after which the centring is tightened by screwing down the nuts on the T-bolts until the soffit tile, or skewbacks, are hard against the beams and the planking has a crown not exceeding ¼ of an inch in spans of 6 feet.
This system gives what is very essential - a firm and steady centre on which to construct the flat tile work. The tiles should be shoved in place with close joints and keys should fit close. The centres should remain from twelve to thirty-six hours, according to condition of weather, depth of tiling and mortar used. When centres are "struck " the ceiling should be straight, even, free from open joints, crevices and cracks, ready to receive plastering.
Wherever openings are required through the floor they may be made by punching a hole through the blocks; or, if the side-method arch is used, a single block may be omitted. Small holes may afterward be plugged up with mortar and broken pieces of tile.
The variations in width of spans between beams is provided for by supplying tiles of different sizes, both for interiors and keys, whereby a variety of combinations can be secured. A great variety of skew-backs are also provided for fitting different sizes of beams.
Tie-Rods. - All forms of flat or segmental tile arches require that the beams supporting them shall be bolted together with tie-rods to take up the thrust of the arch. These tie-rods are usually ¾ inch in diameter and spaced from 5 to 7 feet apart. They should be secured to the web of the beam near the bottom flanges and drawn tightly in place by nut and thread.
The laying of flat construction in winter weather without roof protection should not be practiced in climates where frequent severe rain and snow storms are followed by hard freezing and thawing, as the mortar joints are liable to be weakened or ruptured, resulting in more or less deflection of the arches. When it is intended to plaster on the under side of the arches the architect should see that the smoke and soot from the boiler used for the hoisting plant are not allowed to strike the arches, as neither can be removed, and they are sure to stain the plaster. For the same reason the architect should see that only clean water is used for mixing the mortar, and that it is not allowed to flow over the arches.
Many architects have had trouble, where flat tile arches have been used, from stains and excrescence appearing on the plastered ceiling after the latter had become dry. Such stains cannot always be concealed, even by oil paint, and the only way in which they may be avoided is by observing the above precautions and not plastering until the arches are well dried out. A coating of Duresco applied to the bottom of the arches before plastering has been recommended as a safe precaution against stains.