Sentence 9.26.4.6.(1)

Intent 1:

Where the intersection is between a built-up roof and a chimney, to limit the probability of:

• cracking, tearing or shrinking of built-up roof membranes (when returned up walls at a sharp angle),
• interruption of roofing membranes at junctions with walls or chimneys, or
• water temporarily ponding and eventually overtopping cant strips.

This is to limit the probability of water ingress into chimneys, 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.