Reduction in maximum spans for lumber beams or in maximum supported length of floor joists used to support floors with a concrete topping not more than 51 mm thick, in residential applications or where the live load on any floor does not exceed 1.9 kPa, in wood-frame constructions to which Section 9.23. applies [see Sentence 9.23.1.1.(1) for application of Section 9.23.].

To limit the probability of excessively long spans of beams or joists, which could lead to beams being unable to resist the additional weight of a concrete topping, which could lead to excessive deflection or failure.

Where wood-frame construction supports or is part of an environmental separator, this is to limit the probability of the displacement or failure of required environmental separation elements, which could lead to:

• condensation,
• precipitation ingress,
• excessive heat transfer,
• pollutant ingress, or
• compromised thermal performance of components intended to resist heat transfer.

This is to limit the probability of:

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

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.