Leaky Home or Office? Our Experts Check Out the New Verification Standards for Weather Tightness

In Australia we are  not immune to the ‘leaky building syndrome’ uncovered recently in New Zealand.  Now the Australian Building Codes Board (ABCB) is introducing changes in the National Construction Code (NCC) that will give builders and owners greater confidence in assessing weathertight buildings. By *Salim Dalla & *Michael Ryan

The ‘leaky building crisis’ is an ongoing construction and legal crisis in New Zealand where an estimated 42,000 buildings constructed between 1994 and 2004 suffered weathertightness problems, and as a result need extensive and costly repairs.

The problem primarily manifested in decay to timber framing which, in extreme cases, made buildings structurally unsound. Many buildings became unacceptable health risks due to mould infestation, leading to a repair bill currently estimated at around $NZ11.3 billion.

Even with Australia’s moderate climate, similar issues now appear to be surfacing locally. In 2014, the Tasmanian government identified their own leaky building syndrome, while Victoria has also established a task force to monitor similar issues.

The problem doesn’t stop at the Murray River either. In the warmer and more humid climates, especially in the Tropics, the constant damp created by humidity creates fertile ground for mould to grow. Many mining communities are struggling with mould infestations in their FIFO (Fly In Fly Out) camps.

Rather than waiting for the problem to create headlines, the ABCB has modified the compliance path for weather tightness Performance Requirements, which will be included in the NCC 2015 for release in May.

 

Performance requirements and assessment methods

The NCC weatherproofing Performance Requirements1 state: A roof and external wall (including openings around windows and doors) must prevent the penetration of water that could cause:
  1. Unhealthy or dangerous conditions, or loss of amenity for occupants; and
  2. Undue dampness or deterioration of building elements.

Weather tightness compliance can be assessed by one or a combination of four different assessment methods, to demonstrate that a solution complies with the Performance Requirements1.

While compliance is mandatory, the assessment methods used are optional. These include:

  1. A verification method including testing conducted by a suitably qualified testing authority.
  2. Evidence to support the use of a material, form of construction or design that meets Performance Requirements1 or a Deemed-to-Satisfy provision. A Codemark Certificate Of Conformity is one way to achieve this option, which has mandatory acceptance.
  3. Comparison with a Deemed-to-Satisfy provision, where applicable.
  4. Judgment by a suitably qualified expert to determine whether a solution complies with the Performance Requirements1.

Defining the risks

The verification methods FV12 and V2.2.13 to satisfy the Performance Requirements1 are the key additions whereby compliance may be demonstrated by physically testing a sample wall. If the maximum ultimate wind load for the building is greater than 2.5kPa then the verification method alone is insufficient. For class 1 and 10 buildings this means buildings with a wind load classification of N44 and above. Class 2 to 9 buildings are subject to detailed wind load analysis5 for the project.

Regardless of the construction systems used, the building must be assessed and a risk score established. If the risk is deemed too high then the building must be assessed using expert judgement and possibly the verification method.

For class 2 – 9 buildings there are no deemed-to-satisfy provisions, therefore demonstrating compliance by comparison is not relevant. For these classes of buildings, suitable evidence, verification or expert judgement are necessary.

Deemed-to-satisy provisions exist for class 1 buildings only with a wind load classification of N1, N2 & N34. These include:

  • Metal wall cladding6
  • Timber weatherboard cladding7
  • Fibre-cement and hardboard wall cladding boards7
  • Fibre-cement, hardboard and plywood sheet wall cladding7

Brick veneer systems with drained and ventilated cavities generally rely on history of use and proven track record to demonstrate weather tightness.

Assessing risk scores

Use of the verification method relies on the building risk score being deemed acceptable. The building risk score assesses the following risk factors:

Assessment Consideration Maximum risk score
Wind region Wind Region A-D – maximum ultimate wind pressure of 2.5kPa. High winds increase risk of water ingress. 2
Number of storeys Single or multi-levels. Wind speed is greater at elevated heights. 4
Roof/wall junctions Protected, partially or fully exposed. Risk of failure increases with exposure. 5
Eaves width Maximum consideration of 600mm. Wider eaves reduce volume of rain hitting facade. 5
Envelope complexity Simple or complex buiding shapes with single or multiple cladding types 6
Decks, porches and balconies Covered, exposed and/or cantilevered at lower or upper storeys 6

Assessment of the risk severity determines the corresponding score, if the building risk score is below 20, and the wind pressure limit is satisfied, then the verification method may be used as the sole method of demonstrating compliance.

If the risk score is greater than 20 and/or the ultimate wind pressure is greater than 2.5kPa, then it’s recommended that verification/testing be conducted in conjunction with expert judgement, using project specific details and wind loads.

Verification Method

A weather tightness test on a sample wall is required for verification of compliance. The sample wall must incorporate all building junctions which have historically been shown to be common points of leakage:

  • Vertical and horizontal control joints.
  • Wall junctions i.e external and internal corners.
  • Windows complying with AS2047 and/or Doors.
  • Electrical boxes.
  • Balcony drainage and parapet flashings.
  • Footer and header termination systems. For example, base of wall to slab junction (footer) and top of wall to eaves/parapet junction (header).

All wall systems using the verification method must also be categorised into one of three external wall system types that are commonly used in construction:

  • Cavity Wall – a wall that incorporates a drained cavity.
  • Direct Fix Cladding – a wall with cladding attached directly to the wall framing without the use of a drained cavity.
  • Unique Wall – a wall which is neither a Direct Fix Cladding nor a (drained) Cavity Wall.

The test procedure is based on the Australian Standard AS/NZS 4284: Testing of Building Facades. The Cavity Wall system test procedure differs slightly from the Direct Fix Cladding and Unique Wall systems.

Testing for compliance

The wall system is considered verified under the following conditions:
  • A Cavity Wall system that reveals no presence of water on the external face of wall membrane (for sarked systems) or insulation (for un-sarked systems).
  • A Direct Fix Cladding or Unique Wall systems that reveals no presence of water on the inside surface of the facade.

In preparation for the proposed changes, CSR conducted testing in conjunction with CSIRO’s facade testing team to ensure compliance for some of its popular residential wall systems.

Double storey prototypes for Cemintel fibre cement cladding, as a drained cavity system, and Hebel Powerpanel XL wall as a unique system, were recently tested at CSIRO’s test facility in North Ryde. Under supervision by CSIRO and AECOM Facade Engineers, the tested wall systems met the Performance Requirements1 for the respective classifications of buildings under FV12 and V2.2.13.

It is likely that some wall systems may require upgrades to comply with the new NCC 2015 verification methods.

Alternately, many construction methods and systems have been in use for years and may have sufficient demonstrable performance to provide evidence of compliance.

However, the verification methods, while not compulsory, are a definitive method by which weather tightness compliance can be demonstrated, and offers newly constructed building facades a rigorous benchmark for testing and endorsing weather tight performance.

While none of the weather tightness provisions absolve builders and designers from their responsibility to provide a weather tight bulding, verification methods certainly help reduce risk and influence building practices, which will go some way to preventing our own leaky building crisis.

References:

  1. FP1.4 (NCC Vol 1) and P2.2.2 (NCC Vol 2)
  2. As defined in NCC Vol. 1
  3. AS defined in NCC Vol. 2
  4. As defined by AS4055
  5. As defined by AS1170.2
  6. Designed and constructed in accordance with AS 1562.1
  7. As per NCC Vol. 2 Accepatable Construction

About the author

Salim Dalla is Hebel Technical Manager and a chartered structural engineer with over 15 years practice in Australia and competencies in the design of Residential, Commercial and Industrial projects. His current core work is the development, testing and assessment of new building systems.

Research engineer at CSR Michael Ryan’s early career was in consulting structural engineering. His key work at CSR includes technical support for Hebel, Gyprock and Cemintel, and fire and acoustic system compliance and testing for material and system performance


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