To limit the probability that, on severe overheating [i.e. during a chimney fire], the expansion of the liners will put stress on the masonry surrounding the flue liners, which could lead to the failure of surrounding masonry or the displacement of liner sections, 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.