Where the shingle roof slopes upwards from a masonry chimney, to limit the probability of an insufficient upwards extension of flashing, which could lead to precipitation or meltwater ingress, which could lead to damage to masonry or 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.