As fires are a source of danger to a building, which must necessarily have some wood floors, it becomes essential that some means should be employed to reduce those risks to a minimum, and with that view trimmer arches were devised for wood floors with ceilings under them. The joists are framed round the chimney breast (as is explained in Chapter VIII (Wood Floors. Naked Floors And Floor-Boards).), and the open space left in front of the fireplace is filled in with brick-arching 4 1/2 inches thick, springing from a skewback cut into the breast of the chimney, above the ceiling line, to he trimmer-joist, as shown in fig. 165, and resting on light centring, which is left in for the purpose of carrying the ceiling-lath underneath.
Another method is shown in fig. 166, a feather-edged wood springer being nailed to the trimmer-joist, and the arch springing from both sides instead of from only one (i.e. the wall), as shown in fig. 165. The spandril above this trimmer-arch is filled in with concrete, and levelled up to receive the tiles, cement, or hearthstones about 2 or 2 1/2 inches thick, as shown on the sections. The plan of the hearth, etc., is as shown in fig. 167, which shows the trimmer-arch under the front hearth to be 14 inches wider on each side of the jambs of the fireplace, and to project 14 inches outward - though 18 inches would be better. Certainly it should project no less than 14 inches, and be no less than 9 inches wider on each side of the fireplace jambs.
Ground-floor fireplaces, in stone or brick-paved compartments, require no special provision for safety from fire; and those with wood floors have their hearths as for the upper floors; but, instead of being supported by trimmer-arches, they are secured by sleeper walls enclosing the usual space filled in with concrete, as explained by fig. 168.
Fireplace openings are generally arched over on a 2 1/2 in. by 3/8 in. wrought-iron chimney-bar, curved to the form required, and with the ends going 9 inches into the breasts, split and turned up and down to keep them in, as seen in fig. 169. At the back of this the jambs are gathered over, by 2 1/4-inch projections, to the size of the flue; and in best work a York-stone slab is generally placed on the top course to act as a ledge to stop down-draughts of wind. This construction is delineated in fig- 170. When circular fireclay flue-pipes are used to line the flues a special-made radiating pot (fig. 171) is used to gather the opening over to the size of the flue.
As to the construction of the flue itself, after the brickwork has thus been gathered in or collected to the flue, it must be impressed on the student that no flues should be made of less dimensions internally than 9 by 14 inches, and they maybe made as large as 18 by 18 inches, though it is essential they should be of the same size exactly throughout. They should, however, not be built straight all the way up, but diverted, in one way or another, as much as possible, as a good, well-constructed flue should not allow daylight at the top to be visible to any one looking up from the fireplace. The necessary arrangement of a breast and stack of several chimneys will be understood from figs. 172, 173, 174, 175, 176, 177, and 178, representing complete plans and sections of a large chimney-breast.
The supplementary fig. 179 is an enlarged detail-section of the breast, etc, on the uppermost floor, and fig. 180 is a section through a cottage, showing how flues are, from separate chimney-breasts, sometimes brought together.