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BCAB #1080 - Chimney Corbelling, Section 9.21.

Last updated on March 24, 2016

July 21, 1989

BCAB #1080

Re: Chimney Corbelling, Section 9.21.

With reference to your letter of June 20, 1989 regarding the connection of a solid fuel-burning appliance to an exterior masonry chimney, through a combustible wall.

To deal with your specific questions, in sequence:

  1. The breeching, or flue pipe, does not have to be lined in accordance with Articles 9.21.3.1. and 9.21.3.2., these refer to chimneys, not flue pipes. We would point out that this should not be confused with the thimble required under Article 9.21.5.5., through which the actual flue pipe must pass. The preamble to your questions mentions a galvanized sheet metal lining, but while a thimble could be of galvanized steel, this metal is specifically precluded for flue pipes by Article 9.21.5.1.

  2. Article 9.21.3.5. again applies to a chimney, not a flue pipe.

  3. You refer to a 50 mm clearance, which we assume to be representative of different clearances required, as in Article 9.21.6.1. This clearance is air space, and should not be filled with insulation or any other material.

  4. Any corbelling would need to comply with Subsection 9.20.12. For this purpose, as explained later, we would not accept the suggested support by a steel bar.

While the above answers to the specific questions raised will be of assistance, we feel that a fuller understanding of the situation could be beneficial, so the following may also be of value:

Basically you have a condition where the actual masonry wall of the chimney is projected through a combustible wall. If this was achieved merely by increasing the chimney wall thickness, there would be no problem, it would be similar to the connection of a stove to the breast of a masonry fireplace. Alternatively there is no theoretical reason why a sufficient mass of masonry could not be corbelled out to provide adequately for safe penetration of the combustible wall, although we question the feasibility; as an example, the total projection of a corbel must not exceed one third of the wall thickness, effectively ruling out such an application for the suggested situation shown in your diagrams, which would necessitate a much thicker chimney wall.

If the corbelling proved to be practically achievable, there is little in the Code to provide guidance for masonry thickness around the penetration. In the absence of such guidance, or testing, the penetration could only be dealt with as indicated in Article 9.21.5.8. for the passage of a flue pipe without protection; this would necessitate 450 mm of masonry. A protected penetration, either as shown in the Code, or by using a listed connector, would probably be more practicable.

We disagreed with the use of steel bar in our answer to question #4 because as shown in your diagram it would be of no value, you indicated two pieces of bar projecting from the chimney wall, basically as cantilever brackets. We consider this to be unacceptable and dangerous. Structurally it would be possible for engineered steel supports to be used to support a sufficient mass of masonry, but for practical reasons it seems extremely unlikely that such a situation would ever arise.

J.C. Currie, Chair