This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Chimney flues if built of brick should have walls 8 inches in thickness, unless terra cotta linings are used, when only 4 inches of brick work is required. Except in small houses where an 8 X 8 flue may be used, the nominal size of the smoke flue should be at least 8 X 12 to allow for contractions or offsets. A clean-out door should be placed at the bottom of the flue for removing ashes and soot. A square flue cannot be reckoned at its full area as the corners are of little value. To avoid down drafts the top of the chimney must be carried above the highest point of the roof unless provided with a suitable hood or top.