To limit the probability of inadequate strength or depth of the masonry courses that support floor joists, which could lead to over-stressing of masonry units under concentrated roof or floor loads transmitted through framing elements, which could lead to cracking of cavity walls.
This is to limit the probability of:
For floors and elements supporting floors, this is to limit the probability of excessive movement, deflection or vibration, which could lead to negative effects on the psychological well-being of persons.