In view of what was said in the first part of this chapter, the construction of a chimney by approved methods is also a safeguard against fire. It can be considered a rule that every chimney should be lined with a terra-cotta flue, that every chimney should be an independent structure of its own, with walls thick enough for stability, capable of standing upon their own foundations and not hung from any part of the structure, that all woodwork of the building should be framed far enough from the chimney to make no contact with it, and, finally, that all the smoke-pipes which enter into the flues should be proof against leakage of flames and heat of such intensity as to cause combustion.
In the past this need of lining the flues of a chimney with terra-cotta flue tiles was not considered important, but to-day it is a well-recognized fact that no chimney is safe without this protective lining. There are many instances where chimneys are built without this lining and show no fire dangers, but the action of flue gases is slow and sure, and the mortar is attacked gradually, with the resulting disintegration of the brickwork, through which the flames eventually find their way to the surrounding wood timbers. It is found that even where terra-cotta flue linings are used the hot gases from the burning of natural gas as a fuel break down their resistance and they crumble, so that in such cases the flue linings should be made of fire-clays. From practical experience the minimum thickness allowable for any of these flue linings should be 1 inch, and the joints should not be made with collars.
When setting these linings they should be laid in cement mortar, not in lime mortar, for this disintegrates under the action of gases from burning wood. The joints should be struck smooth on the inside, and the space between the lining and the brickwork filled in solid with mortar. Wherever two flue linings are run within the same chimney space, the joints should be staggered or offset at least 6 inches. Two linings, however, in one chimney space should be the maximum number permitted. Where more are required, each group of two should be separated by brick walls of at least 4 inches, which are well bonded into the outside walls of the chimney. This is in order to give stability to the chimney and also prevent any fires in one flue spreading to others. The thickness of outside walls of the chimney around the flues should not be less than 4 inches if built of brick or reinforced concrete, but if built of stone they should be 8 inches. Wherever there is no flue lining of terracotta, such as in the smoke-chamber, the thickness of the masonry from the interior to the exterior should never be less than 8 inches.
If chimneys are built of reinforced concrete, the reinforcements should be run in both directions to prevent cracks during the setting of the cement or from temperature stresses. Where concrete blocks are used, reinforcements should run continuously around the blocks, and the shell of the blocks should not be less than 4 inches thick.
Wherever the walls of dwellings are of brick and 12 or more inches thick, they may be used to contain chimney flues. If it is necessary to corbel out the flues from the wall, they should not extend farther than 4 inches from the face of the wall, and the corbelling should not be done with less than five courses of bricks.
Next in importance to the correct lining of flues is the proper construction of the foundation under chimneys. There are often cases where it is necessary to cut off the chimneys below in part or in whole to supply room on the first floor. This should be avoided as much as possible, but if it cannot be done it should be supported by steelwork from the ground up.
Another mistake that is continually made is to cut off the chimney at too low a level and cap it with only a plastering of mortar. All chimneys should be carried at least 3 feet above flat roofs and 2 feet above the ridge of a peak roof and properly capped with stone, terra-cotta, or concrete. If they are not capped, and the bricks improperly tied, the mortar joints will be loosened by the action of the weather and the heat issuing from the chimney, and eventually the bricks will be moved from their position, leaving the top in a dilapidated condition.
This extension of the chimney through the roof leaves a joint which must be covered with flashing to prevent leaking. The usual method of building a tin-covered cricket behind the chimney, and protecting the other sides with tin flashing counter-flashed is very satisfactory; but the practice of corbelling the brickwork out over the roof, in order to cover over the joint, is extremely bad. When a chimney built in this way settles, the cor-belled-out parts catch on the roof, and the whole top of the chimney is lifted off, leaving a crack through which the hot gases pass to the wooden rafters. See illustrations on pages 145 and 170. If there are any fireplaces to be built in the chimney the walls should never be less than 8 inches thick around them. It is best to line them with fire-brick of at least 2 inches in thickness. Hearths should extend in front of the fireplace at least 20 inches to prevent sparks from falling on the wooden floors. These hearths should be supported upon trimmer arches or be constructed of reinforced concrete. It is important to keep the woodwork of any mantel away from the opening at the top at least 12 inches and at the sides at least 8 inches.
In fact, no woodwork should be permitted to come in contact with any part of the chimney. Wooden beams and joists should be kept at least 2 inches from the chimney and at least 4 inches from the back of any fireplace. This space, as was previously stated, should be filled in with firestopping material. Where a chimney is on the line with a wooden stud partition, it is better to plaster directly over the brickwork of the chimney than to carry studs over it on which lath and plaster is constructed. By using metal lath over the brickwork the danger of cracks can be eliminated. Where a baseboard must be carried along this wall in which such a chimney occurs, the plaster should be carried down behind it and then asbestos board should be placed behind the baseboard to prevent too much heat coming in contact with it.
If these precautions are taken in the construction of the chimney and the correct methods of fire-stopping employed, the house of wood can be made less of a fire-trap than it is to-day. None of these devices require much additional expense, and should, on this basis, have a broad appeal.