In chimneys containing three or more flues, building codes generally require that each group of two flues be separated from the other single flue or group of two flues by brick divisions or withes not less than 3¾ inches wide (fig. 9) . Where two flues are grouped without divisions, joints in the linings of adjacent flues are safer if staggered at least 7 inches, and particular care should be taken to have all joints filled with mortar. Individual flues are advisable for fireplaces and heating furnaces or boilers. Table 2.--_Dimensions of commonly used standard commercial flue lining_ Rectangular linings[B] Outside Cross-sectional area Wall dimensions --------------------- thickness (inches) Inside Outside ----------- ---------- -------- ---------- Square Square inches feet Inches 4½ by 8½ 23.6 0.26 5/8 4½ by 13 38.2 .41 5/8 7½ by 7½ 39.1 .39 5/8 8½ by 8½ 52.6 .50 5/8 8½ by 13 80.5 .78 3/4 8½ by 18 109.7 1.10 7/8 13 by 13 126.6 1.20 7/8 13 by 18 182.8 1.70 7/8 18 by 18 248.1 2.30 1-1/8 20 by 20 297.6 2.60 1-3/8 Round linings[C] Inside Cross-sectional area Wall diameter --------------------- thickness (inches) Inside Outside ----------- ---------- -------- ---------- Square Square inches feet Inches 6 28.3 0.29 5/8 8 50.3 .49 3/4 10 78.5 .75 7/8 12 113.0 1.07 1 15 176.7 1.62 1-1/8 18 254.4 2.29 1-1/4 20 314.1 2.82 1-3/8 22 380.1 3.48 1-5/8 24 452.3 4.05 1-5/8 27 572.5 5.20 2 [B] All rectangular flue lining is 2 feet long. [C] Round flue lining, 6 to 24 inches in diameter, is 2 feet long; that 27 to 36 inches in diameter is 2½ or 3 feet long. [Illustration: 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.