Chimneys should be built with concrete footings that extend below the frost line and down to the level of the footing for the main foundation walls. When chimneys are set alongside outside walls or inside bearing walls the footings should be bonded with the wall footing. The footing should be at least 12 inches deep and twice as wide as the chimney. The foundation of the footing must extend at least six inches from the outer edges of the chimney.
All fireplace chimneys should be built of solid brick, spaced at least two inches from the wood framing of the floors and roof, and must have no openings except those required for the fireplace. Chimneys must be built straight from the source to the outlet, without any bends, if at all possible. When two sources feed one chimney, they each must have separate flues. The two flues must be finished to heights at least six inches apart. If they are the same height, smoke and fumes from one flue could be sucked down the other flue. To provide for proper draft the chimney should extend at least two feet higher than any portion of the roof within 10 feet of the chimney and at least three feet above the highest point of the roof from which it extrudes.
Any chimney wall less than 8 inches thick must have a flue lining to prevent extreme heat from burning the masonry work. Round flues are more effective than rectangular ones, since gases typically travel in a spiral motion. Since the corners of rectangular flues contain dead space, they must be planned larger to obtain the same results. Flue linings should extend four inches above the chimney. Flue linings are generally no less than one-twelfth the face area of the fireplace opening. When the fireplace is planned higher than it is wide the flue can be one-tenth that of the opening. See typical rectangular flue sizes outlined on our Table of Fireplace Dimensions.
Every chimney should have a concrete cap that extends approximately two inches beyond the brickwork of the chimney and placed within two inches of the top of the lining. The cap which is either pre-cast or poured in place, is made with a slope and with a drip notch on the underside. Its purpose is to shed water from the chimney and to direct wind currents so that rain or snow cannot blow down the stack. The cap should never be poured against the liner. A one-half inch gap filled with a flexible and water proof compound should be left to accommodate expansion caused by heat.
In addition, flashing must be installed where the chimney passes through the roof and a clean-out door should be built near the bottom of the chimney.
Bon Tool Co. has made a serious effort to provide exact and current information, but does not guarantee the accuracy of the data. Bon Tool Co. cannot assume responsibility for the application of information nor guarantee results.
