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. [Illustration: 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. [Illustration: Fig 11]