Many factors have contributed to problems with weathertightness in buildings since the mid 1990s, with poor design, project management and building practices all contributing.
Globalisation and the exposure of New Zealanders to international design trends and materials have led to a wider range of housing styles, but many house designs and materials are out of context and totally unsuitable to the specific site conditions. House styles and features designed for dry climate locations have been used in areas of high wind and rainfall.
Houses have also generally become larger, taller and more complex. These buildings are exposed to more wind and rain, which makes cladding selection and detailing particularly important.
More complex designs and details require more skill to design and build accurately, and this raises the risk of failure. Builders also have to work with and understand a wide and varied range of materials and systems and involve a larger number of subtrades in the construction.
Features such as parapets, decks and pergolas that penetrate the cladding, low-slope roofs, membrane roofs, lack of eaves and complex junctions have become more common, even though these features contribute to weathertightness risks and may be inappropriate for wet and windy conditions.
Poor understanding of materials
Since the 1990s, there has been an increase in the range of available building materials. Some newer materials have proven to be not fit for purpose or have been used in the wrong situation or incorrectly.
Failures may occur when:
- the manufacturer has overstated the material’s performance, leading to materials being used inappropriately
- the designer or builder has not followed the manufacturer’s instructions and so has specified or installed the material incorrectly
- the manufacturer has not provided information about the interaction of their material with others.
Designers and builders should understand new materials and systems and always ensure that the manufacturer’s instructions and specifications are followed. If there is any uncertainty, they should seek information from the manufacturer.
Where builders believe some material or detail may not perform adequately, they need to make the designer aware of their concerns in writing.
Direct-fixed monolithic claddings
For a period of time starting in the mid-1990s, thousands of houses were built using direct-fixed monolithic claddings. Examples of this type of cladding include texture-coated fibre-cement, stucco and EIFS (polystyrene sheets that are typically plastered with a reinforced polymer modified cement-based plaster).
With these claddings, weathertightness failures can occur when rainwater gets through the exterior walls, particularly around penetrations (such as around doors and windows) and through junctions in the cladding.
Because there is no drainage cavity behind the cladding, once water has penetrated the cladding, it often cannot drain away or dry within the walls. This has led to the growth of fungi and moulds that damage components in the wall assembly and may produce toxins that are harmful to building occupants.
A growing understanding of basic weathertight design principles led to more houses being constructed with drained and vented cavities from around 2005 on. The importance of cavities also became clearer in the 2005 version of Acceptable Solution E2/AS1, vastly expanded from the earlier version.
Changes in timber treatment
From 1998 to April 2004, homes were commonly constructed with untreated kiln-dried timber framing, which will readily deteriorate if it is regularly wetted. During this period, there was also limited use of H1.1 LOSP (which has no resistance to rot), H1.2 boric, H3.1 LOSP and some H3.2 CCA-treated framing.
In April 2004, Acceptable Solution B2/AS1 adopted an amended version of NZS 3602:2003 Timber and wood-based products for use in building, which required the use of treated timber where there was any risk of water getting into the timber frame.
Complex, modern homes require significant construction documentation. Often, the client is unwilling to pay for the required level of documentation, resulting in designers providing only the minimum required for a building consent. This is not always adequate for construction.
Construction documentation shows the builder what to build. If it is inadequate, there is a high chance that the building will not be built as intended by the homeowner or the designer and that it may be built with faults. Builders should always request more documentation from the designer where they believe what they have provided is not sufficient for construction.
During construction, builders should not make assumptions or decisions on how to build something that is not adequately detailed; the builder may be liable should these details ultimately fail. Design decisions need to be made by the designer.
Some weathertightness failures have arisen because of inadequate detailing, either by the designer or the builder. Examples include lack of flashings, flashings that are poorly detailed or installed, insufficient drainage from surfaces, insufficient level differences between inside floor levels and outside ground or deck levels, poorly detailed or constructed joints between claddings, poorly detailed or installed membranes, lack of overlap between claddings and other elements such as flashings and window facings, and inadequately detailed or constructed penetrations in claddings.
Some of these problems may have arisen because of poor understanding of building materials or inadequate documentation (see above).
Also see common leaks.
With most residential construction today, the designer has no on-site role. This means that the builder often has overall project management and construction responsibility, and that the designer is only involved at arm’s length. This puts greater responsibility on the builder, who must also have a greater skill set.
In some cases, there may be no-one on site with overall responsibility for the project, and this has meant that on-site roles and responsibilities have become blurred, with an adverse effect on build quality and building performance.
Builders should insist on the involvement of the designer (or a similarly qualified person) as a member of the project control group if the type of construction justifies this. Builders should also understand who is responsible for what on the job and the contractual and legal obligations.
Other contributing factors include:
- unqualified and unskilled builders
- no requirements for trade-trained/qualified builders to construct dwellings
- poor inspection regime from TAs.
Some weathertightness issues have arisen because of a lack of building maintenance, especially coatings on monolithic claddings.
Updated: 9 September 2014