Installation of saddles at junctions, more than 750 mm wide, of sloping roofs with the upper side of chimneys, on buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9].

Exceptions: 

To limit the probability that the flow of water down a roof's surface will be interrupted by a wide chimney, which could lead to a temporary increase in water depth beyond the depth protected by roofing or by the vertical leg of flashing at the chimney, which could lead to:

• precipitation or meltwater ingress, or
• compromised thermal performance of components intended to provide resistance to heat transfer.

This is to limit the probability of:

• an inadequate control of relative humidity, temperatures of interior spaces or water accumulation,
• the generation of pollutants from biological growth or from materials that become unstable on wetting, or
• deterioration, which could lead to compromised integrity of roofs or assemblies protected by roofs.

This is to limit the probability of:

• negative effects on the air quality of indoor spaces,
• the inadequate thermal comfort of persons, and
• contact with moisture.

This is to limit the probability of harm to persons.

To limit the probability of precipitation or meltwater ingress, which could lead to damage to chimney liners from freeze-thaw stresses, which could lead to flue gases leaking into living space, which could lead to negative effects on the air quality of indoor spaces [see analysis for Sentence 9.21.4.6.(1)], which could lead to harm to persons.