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.
Fire place
Fig 12
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 corbelled-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.
Fig 11
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 fire-stopping 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 base-board 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 base-board 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.