Interior and exterior surfaces of mortar joints of clay, concrete or firebrick chimney liners for rectangular masonry or concrete chimneys that are not more than 12 m in height and that serve fireplaces or appliances having a combined total rated heat output of 120 kW or less, in buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9].

This also applies to rectangular masonry or concrete chimneys, in buildings to which Part 6 applies [see Sentence 1.3.3.2.(1) of Division A for application of Part 6], as stated in Sentence 6.3.1.2.(1).

To limit the probability that, under conditions of high temperature, protrusions into the flue will lead to turbulence and an inefficient flow of flue gases, which could lead to the deposition and ignition of particulates and creosote, which could lead to the ignition of combustible building components, which could lead to harm to persons.

To limit the probability that extruded mortar protruding into the required clearance between the liner and the surrounding masonry [see Sentence 9.21.3.8.(1)] will lead to excessive heat transfer from chimney liners to combustible building components, which could lead to the ignition of combustible building components, which could lead to harm to persons.