To limit the probability that slight variations in liner dimensions and minor misalignments will lead to a mortar bed of less than 6 mm as required by CAN/CSA-A324-M, “Clay Flue Liners,” which could lead to the ingress of condensed water vapour into the cavity between the chimney liner and the surrounding masonry, which could lead to damage to the masonry due to freeze-thaw stresses, which could lead to the collapse of the chimney, which could lead to harm to persons.