Source: New Zealand Building Code, Clause B2 Durability
COP v23.09:Durability; Nzbc-Clause-B2
The objective of this provision is to ensure that a building will continue to satisfy the other objectives of this code throughout its life.
Building materials, components and construction methods shall be sufficiently durable to ensure that the building, without reconstruction or major renovation, satisfies the other functional requirements of the NZBC throughout the life of the building
Building elements must, with only normal maintenance, continue to satisfy the performance requirements of the NZBC for the lesser of the specified intended life of the building, if stated, or:
- The life of the building, being not less than 50 years, if:
- those building elements (including floors, walls, and fixings) provide structural stability to the building; or
- those building elements are difficult to access or replace; or
- failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.
- 15 years if:
- those building elements (including the building envelope, exposed plumbing in the subfloor space, and in-built chimneys and flues) are moderately difficult to access or replace; or
- failure of those building elements to comply with the building code would go undetected during normal use of the building, but would be easily detected during normal maintenance.
- 5 years if:
- the building elements (including services, linings, renewable protective coatings, and fixtures) are easy to access and replace; and
- failure of those building elements to comply with the building code would be easily detected during normal use of the building.
Individual building elements which are components of a building system and are difficult to access or replace must either:
- all have the same durability; or
- be installed in a manner that permits the replacement of building elements of lesser durability without removing building elements that have greater durability and are not specifically designed for removal and replacement.
NZBC Clause B2, Durability, requires fifteen years to perforation for claddings easily accessed for replacement. Fifteen years is also required for internal gutters and downpipes, and five years for external gutters.
NZBC B2 requires 50 years’ durability for flashings that require the removal of cladding above to be replaced, while table 20 of NZBC E2/AS1 only requires 15 years’ durability for such flashings. The COP recommends the higher figure as good trade practice and in many cases, lower life-cycle costing.
Generally, higher durability than the minimum requirements can be achieved by using materials and methods outlined in this COP, with no maintenance of coatings other than washing areas which are not naturally washed by rain. Elements more difficult to replace, or to access for maintenance, should be constructed of more durable material.
Normal Maintenance means work recognised as being necessary to achieve the expected durability of a given building element.
According to B2/AS1, normal maintenance may include:
- Washing down surfaces subject to wind-driven salt spray and contaminants.
- Re-coating protective surfaces.
- Replacing sealant seals and gaskets in joints.
Although roof or wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for flashings. Flashings might be embedded in plaster or behind other building elements, making them hard to replace without removing cladding or other building features such as windows.
Cladding material may be described as hidden, sheltered or exposed. Some flashings may have sections falling into all these categories, in which case the worst case (sheltered) should prevail in material selection.
Good design, correct selection of materials, and good installation and maintenance practices are required to achieve optimum product lifespan.