Roof and wall cladding must structurally comply with the performance requirements of NZBC Clause B1 Structure. Loads may be calculated in accordance with AS/NZS 1170 (Loadings Standard), and these loads are used to develop prescriptive solutions such are found in NZS 3604 Timber Framed Buildings and NASH Standard for Light Steel Framed Buildings.
COP v23.12:Structure; Standards
Designers should be familiar with the current Loadings Code, including amendments. Manufacturers' printed technical literature, using different criteria or test values and superseded Standards can information that is not compatible with the latest requirements.
The Loadings Code identifies four load categories relevant to metal roof and wall cladding.
- Wind actions:
- Wind imposed inwards forces (pressure) where the wind is slowed, and outward pressures (suction) where the wind accelerates. The shape of the structure induces local pressure factors where pressure is concentrated. Internal pressures may also be generated within a building.
- Permanent action:
Dead load is the permanent weight of the roof structure and the permanent part of an imposed load, such as an air conditioning unit.
- Imposed action:
Live loads are variable loads imposed on the building by its occupants and contents, such as a person standing on the roof (point load).
- Induced actions:
Loads such as wind, snow or ice, and ponding rainwater.
When a structure, or part of it, fails to fulfil its expected basic functions, it is said to have reached a limit state; the two limit states are Serviceability Failure and Ultimate Failure. (see 3.7 Modes of Failure.)
NZS 3604 Timber Framed Buildings is an acceptable solution to comply with the NZBC for light timber frame buildings not requiring specific design.
It contains prescriptive dimensions for purlin spacing and fasteners, based on maximum design wind speeds of Low (32 m/s), Medium (37 m/s), High (44 m/s), Very High (50 m/s), or Extra High (55 m/s). The load calculations for NZS3604 were based on a simplified interpretation of AS/NZS 1170. These values can be used for calculation of loads on the cladding of structures designed using NZS 3604.
NZS 3604 includes:
- Timber frame construction.
- Height from lowest ground to the highest point on the roof may not exceed 10 m.
- A snow load may not exceed 1.0 kPa, although Section 15 of NZS 3604 does provide additional criteria for 1.5 kPa and 2.0 kPa snow loads.
NZS 3604 excludes:
- buildings dedicated to the preservation of human life;
- buildings which may host crowds;
- publicly owned buildings containing high value contents; and
- curved roof construction.
Because the buildings covered by this standard are limited in size, wind loads include a local pressure factor of 1.5 kPa over the entire structure, rather than varying factors according to the position on the roof as required by AS/NZS 1170.2:2011.
Nash Standard Part 2 is cited in B1/AS1 as an Acceptable Solution for light steel framed buildings within a scope similar to that of NZS 3604