*ARCHIVE* - Weathertightness News - No. 6, April 2004
What's inside
- Cavities are here to stay
- Lessons learned from the first weathertightness determinations
- Making sense of the latest weathertightness statistics
Cavities Are Here to Stay
Philip O'Sullivan, Director of Prendos Limited and member of the E2/AS1 working group, believes cavities are here to stay and with good reason.
Cavity-based claddings utilising timber weatherboards, brick veneer or some other cladding material such as fibrecement, are here to stay. Will we return to non-cavity solutions? Europe, Scandinavia and now Canada know that a cavity is the best means of achieving weathertightness. In years to come we will look back with a mix of horror and embarrassment at how we used to build.
The cladding sheds rainwater, so care in design and construction remains necessary. The cavity backs this up by firstly separating potentially damp claddings from the structure and then quickly allowing water to escape by drainage and drying. Claddings directly fixed to the framing, with the building wrap sandwiched between, accumulate and retain leakage moisture for months.
Cavities have become the norm, but there are teething problems and differing views. Should horizontal nog battens be allowed? Should the top of cavities be vented? Such questions will be answered in time.
The proposed BIA documents, Acceptable Solution E2/AS1 and Verification Method E2/VM1, represent two important steps along the road to achieving weathertight and robust buildings. There will always be non-cavity exceptions such as unlined garages and other simple buildings - these are best evaluated on an individual basis using risk analysis.
The other key ingredient to weathertightness is durability. This includes the claddings, fixings and wraps. The durability of underlying timber is covered by B2/AS1. This requires H1.2 timber for framing in external walls, with a very limited exception for single storey brick veneer.
Recladding of apartment with cavity introduced behind fibre cement sheets and weatherboards.
I recently witnessed a large and sophisticated rain penetration test. The test aimed to validate the proposed test method described in E2/VM1 and trial many of the junction details developed for the proposed E2/AS1. Rather than a simple wall, the test panel contained two different claddings, a window, a door, a meter box, a pipe penetration and a small balcony complete with solid balustrade and scupper outlet.
While the test was an outstanding success, I am under no illusions that we - as an industry - have some way to go.
The proposed E2/AS1 is not unlike NZS 3604. No doubt it will become an essential document with its scope and content improving over time. We are proposing many new roof and wall cladding types, and for the first time decks and balconies. Many junction details are proposed too. Our proposals provide details that are buildable, durable and look good. The intention is for specific design to go hand-in-hand with the proposed E2/AS1. Junctions that depart or are not covered will need design, however, the details that are provided will form a guide for those thinking their way through difficult junctions.
Some will be offended by the proposed innovations. Others will be upset that their systems and products are not included. In order for these to be included we are suggesting they will need to be supported by appropriate generic standards.
Those who wish to retain their intellectual property will need to follow the product accreditation/certification route.
In my view the proposed E2/AS1 will be sufficiently broad and well balanced. It will have the appropriate degree of conservatism for a document bearing the official approval of the BIA.
Note: Editorial opinions expressed in Weathertightness News do not necessarily reflect the views of the Authority or BIA staff, unless expressly stated.
Lessons Learned from the First Weathertightness Determinations
Last month the BIA published its first Weathertightness determinations. Here is a summary of the lessons learned from these recent determinations.
Some territorial authorities have refused to issue code compliance certificates (CCCs) for houses built with monolithic cladding that do not include a cavity between the cladding and the framing. In all cases the territorial authorities had approved the building consent without requiring cavity construction.
The Authority takes the view that, under section 43(3) of the Building Act, a territorial authority must refuse to issue a CCC unless it is satisfied on reasonable grounds that the completed building complies with the Building Code, subject to any previously approved waiver or modification. Therefore, if the territorial authority is not satisfied on reasonable grounds that the completed building complies without a cavity it must refuse to issue a CCC even though it had previously issued a building consent for the building without a cavity. The Authority is able to determine only whether or not the completed building complies with the Building Code, it has no power to rule on the approval process that led to the dispute.
The Authority also cannot direct the parties on how the bring the house into compliance, that is for the owner to propose and the territorial authority to accept or reject.
Weathertightness principles
In assessing these cases, the Authority follows a set of principles, based on recent New Zealand data and experience, that define good weathertightness practice. Those principles indicate that the impact of weathertightness problems in monolithic clad houses can be minimised if good and effective design and construction practices are followed.
Installation of exterior cladding to manufacturers' specifications and to accepted good trade practice is a fundamental requirement to assist good weathertightness performance.
The next priority is to reduce the ability of moisture to get through the cladding by a combination of design measures that minimise the effects of the rain impacting on the walls.
The main areas for consideration are as follows:
- an effective deflection mechanism, such as eaves greater than 600 mm wide, has been shown by Canadian data to manage more than 90 percent of rain incidents
- homes in high and very high wind zones are likely to experience wind pressure differentials and thus a higher risk of water ingress
- taller buildings result in an effective increase in the catchment area of the wall and thus the likelihood of leaking
- complex roofs and overall envelope shapes where roofs frequently intersect with walls on upper floors create opportunities for leaks to directly penetrate into the wall
- decks and balconies that are exposed in plan and/or cantilevered out from the external walls are the most frequent location for water leaks.
- The penetration of moisture through the cladding can then be addressed by a combination of effective drainage, ventilation of the drainage cavity, and moisture tolerance in the external wall framing timber. These factors are:
- the structure should allow water that has penetrated the cladding to drain out as quickly as possible. The Authority believes that generally a drainage cavity should be provided behind the outer cladding barrier in monolithic construction.
- the design of the outer walls should allow walls to dry to the outside once moisture penetrates the cladding and the moisture barrier. If walls do not dry, decay fungi can become established in as little as three months. Until scientific data on the optimum depth and configuration of the ventilation mechanism in New Zealand conditions is available, the Authority believes the drainage cavity should not be less than 20 mm, which is in-line with international best practice.
- the external walls should have some degree of decay resistance, or moisture tolerance to allow for situations when moisture circumvents the cladding and moisture barriers and moisture levels in the timber rise to more than 18 percent.
The Authority examined the design of the houses and the way they were constructed to identify the presence of known weathertightness risk factors.
The presence of the risk factors on their own is not necessarily a concern. The lack of a drainage cavity, for instance, does not necessarily mean that the house is not code compliant. The Authority also used moisture readings to determine whether the cladding as constructed is effective in keeping water out. Because these claddings are generally Alternative Solutions, it compared the cladding to any similar Acceptable Solutions, looking for compensating factors that might allow it to comply with the Building Code.
If the building is determined to be non-compliant, wheather or not high moisture levels are found, that non-compliance should be addressed as soon as possible.
Making Sense of the Latest Weathertightness Stats
Adrian Bennett, currently seconded to the BIA from BRANZ, says the latest Weathertight Homes Resolution Service (WHRS) statistics, as analysed by the BIA, continue to highlight monolithic claddings.
The WHRS data relating to the assessment of 329 leaky buildings shows monolithic claddings continue to dominate.
Ninety two percent of buildings have been identified as being at least partly clad with a monolithic cladding. Twenty three percent of the buildings had multiple cladding types.
The numbers of buildings with the main claddings represented are shown in Table 1.
Table 1. Incidence of common claddings in WHRS data.
Note: The cladding type was not identified in 29 buildings.
The figures show that while weatherboards and masonry veneer appear to be commonly associated with leaky buildings, in most cases, leaks involved other claddings. Comparing the cladding statistics from the WHRS data with the estimated market share of these claddings in Table 2, shows that solid plaster (stucco) was still strongly over-represented. In contrast, houses where masonry veneer is the sole cladding were very strongly under-represented.
The most common locations of leaks are shown in Table 3. Cracks in cladding were primarily reported for stucco, fibre cement and EIFS. Problems with window flashings were also common with these claddings but were reported for a range of other claddings including masonry veneer, concrete block and weatherboards.
It is important to note the statistics referred to in this article relate to a small number of total claims received by the WHRS. Data was supplied by individual assessors and, in all but a few cases, was not tested in the resolution process. In some cases data entries were incomplete. Because of these factors, the WHRS information must be considered as indicative only, and more a measure of emerging trends.
Table 2 Comparison of WHRS cladding incidence to the general house population.
1. Percentage of identified WHRS cases where the cladding is the only cladding.
2. BRANZ survey data averaged over 1999-2002.
Table 3 Common location of leaks.
Legality of BIA Interpretations
Only the courts can issue binding interpretations of the Building Act 1991 and Regulations. Indications and guidelines issued by the Department of Building and Housing, either in Weathertightness News or other communications, are provided with the intention of helping people to understand the legislation. They are, however, offered on a “no-liability” basis, and, in any particular case, those concerned should consult their own legal advisers.
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