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.
This is to limit the probability of precipitation or meltwater ingress into or through roofs, masonry walls or chimneys, which could lead to deterioration, which could lead to the structural failure of roofs, masonry walls or chimneys, or elements protected by such assemblies, which could lead to harm to persons.