In buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9], minimum thickness and maximum height of above-ground, unreinforced and nominally reinforced masonry cavity walls, loadbearing masonry cavity walls of buildings of more than one storey in regions where the spectral response acceleration S_a(0.2) is greater than 0.55 that are reinforced in accordance with Sentence 9.20.1.2.(1), and loadbearing masonry cavity walls of 3-storey buildings in regions where the spectral response acceleration S_a(0.2) is greater than 0.35 and less than or equal to 0.55 that are reinforced in accordance with Sentence 9.20.1.2.(2):

• where the wall height above the foundation wall does not exceed 11 m, and
• in which the roof or floor system above the first storey is not of concrete construction.

Exceptions: 

To limit the probability of insufficient strength and stability to resist expected vertical and lateral loads.

This is to limit the probability of:

• compromised structural integrity of masonry construction,
• where masonry construction is part of an environmental separator, the displacement or failure of required environmental separation elements, which could lead to deterioration,
• in assemblies exposed to moisture or the exterior, damage and deterioration, or
• an inability to resist expected loads, which could lead to:
  • the excessive movement or deformation of walls, or
  • the excessive deflection or vibration of floors.

This is to limit the probability of:

• the space being unsuitable for its intended use,
• compromised operation of windows or doors, or
• damage to the building.