A masonry chimney consists of a liner and an outer wall (Figure 145). Masonry chimneys must be built on a concrete footing designed to support the load. The size of the chimney depends on the number, size and arrangement of flues. The outer wall of a masonry chimney must not be less than 75 mm (3 in.) and must consist of solid masonry units. The NBC provides minimum sizes for round and rectangular flues for fireplace chimneys. The size of the flue required depends on the size of the fireplace opening and the chimney height. The rule of thumb is for the area of the flue opening to be at least one-tenth of the area of the fireplace opening but not less than 200 × 200 mm (8 × 8 in.).
There must be at least 12 mm (1⁄2 in.) clearance between chimneys and wood framing for chimneys on exterior walls and 50 mm (2 in.) for interior walls.
The liner must start not less than 200 mm (8 in.) below the lowest flue pipe connection and must extend between 50 and 100 mm (2 and 4 in.) above the chimney cap. It can be clay, concrete, firebrick or metal that meet established standards. Rectangular glazed clay sections 600 mm (24 in.) long are most often used. The liner is installed as the concrete or masonry outer wall is erected. A space of at least 10 mm (7⁄16 in.) that is not filled with mortar is required between the liner and the surrounding masonry. The mortar used for the liner must be suitable for high temperatures.
If there are two or more flues in a chimney, they should be separated from one another by at least 75 mm (3 in.) of solid masonry or concrete, or 90 mm (31⁄2 in.) of fire brick where fire-brick liners are used (see Figure 145).
The top of masonry chimneys should be capped to keep water away from masonry joints. The top of the cap should be sloped away from the flue lining and extend beyond the chimney wall at least 25 mm (1 in.) to form a drip edge. A drip notch should be provided around the underside of the cap.
Factory-built flues do not require a foundation (see Figure 147). The flue sections are comprised of inner and outer stainless steel liners separated by insulation. Provide clearance between the outer wall of the flue and the framing and provide lateral support with clips attached to the framing. Use only factory-built flues that have been tested and approved for use in Canada.
Source : Canada Mortgage and Housing Corporation (CMHC)