Where counter flashing is installed between a built-up roof and a masonry chimney, to limit the probability of an inability to accommodate the differential vertical movement of roofs, masonry walls and chimneys, which could lead to the displacement of or damage to cant strips or roofing membranes, which could lead to precipitation or meltwater ingress, which could lead to damage to chimney liners from freeze-thaw stresses, which could lead to the premature failure of chimney liners, which could lead to carbon monoxide gas leaking into living space, which could lead to the acute poisoning or asphyxiation of persons.