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.
[Illustration: 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.