Figure 9.—Cross section of chimney showing the proper arrangement for three flues. The division wall should be well bonded with the side walls by staggering the joints of successive courses. Note the studs are kept 2 inches away from the brickwork for reasons explained on [page 14].
When two or more flues are used in unlined chimneys, they must be separated by well-bonded withes 8 niches thick. An attractive and effective method of separating unlined flues in colonial times is shown in [figure 10].
Chimneys extending above the roof are exposed to the wind and may sway enough during a gale to open up the mortar joints at the roof line. Openings in a flue at this point are especially dangerous because sparks from the flue may come in contact with the woodwork of the roof. It is therefore good practice to make the upper walls 8 inches thick ([fig. 11]) by starting to offset the bricks just below the intersection with the roof.
The brickwork around all fireplaces and flues should be laid with cement mortar, as it is more resistant than lime mortar to the action of heat and flue gases. A good mortar to use in setting flue linings and all chimney masonry, except firebrick, consists of 1 part portland cement, 1 part hydrated lime, and 6 parts clean sand, measured by volume. Slacked-lime putty may be used in place of hydrated lime; firebrick is best laid in fire-clay.
Figure 10.—This Williamsburg chimney shows the pains taken to make the chimney attractive. The three flues are arranged as a T with well-bonded withes between them. Often four flues were used in the form of a cross.