Sentence 9.26.4.6.(2)

Intent 1:

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, 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.

Intent 2:

Where counter flashing is installed between a built-up roof and a masonry chimney, 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)].