Risk evaluation

Location and design both influence the riskiness of a building when it comes to weathertightness. There are a number of tools available that can help assess risk and help ensure Building Code compliance.

E2/AS1 risk matrix

One of the key tools for designers and builders in E2/AS1 is the risk matrix, which allows the designer to calculate the weathertightness risk of a design. Information about the proposed building design can be put into six different risk factor categories, and the levels of risk in each category can be identified.

Including a completed risk matrix in the building consent documentation will clearly identify the level of weathertightness risk to the BCA and will allow it to assess the building’s ability to meet that level of risk.

Risk factors

The six risk factor categories included in the E2/AS1 risk matrix relate to aspects of design that have been proven to affect the weathertightness of a building:

  • Wind zone – Wind drives rain against a building and increases the potential for leaks, in particular at high, very high or extra high levels.
  • Number of storeys – Taller buildings have more wall area exposed to wind and rain and water running over vulnerable areas such as window and door openings or junctions; taller buildings are also less likely to be sheltered by neighbouring buildings or vegetation.
  • Roof and wall intersection design – Junctions between roofs and walls are potential sources of leaks and are often difficult to detail and build, meaning the risk of failure is higher than for less exposed junctions.
  • Eaves width – Eaves provide shelter to the walls of a building and reduce the wetted area during rain; narrower eaves, or no eaves, mean less shelter and therefore greater risk.
  • Envelope complexity – Complex buildings have more junctions and often more penetrations in the cladding for windows and other elements, creating increased risk of leaks. These details may be more difficult to design and build than simpler forms, so the risk of failure is greater.
  • Deck design – Waterproof decks and solid balconies provide catchment areas for rain and so are potential sources of leaks. These risks are greater for cantilevered decks that penetrate the cladding and for decks with no upper storey, which are more exposed to the elements. Waterproof decks are also difficult to detail and build, increasing the risk of failure.
Complex designs increase the weathertightness risk..

Complex designs increase the weathertightness risk.

Risk severity

Within the six risk factor categories, relevant building features are defined as low, medium, high and very high risk severity (see Table 1 from E2/AS1 at the bottom of this page).

Risk scores

Designers then complete a risk matrix for each face of the building by entering a risk severity score for each risk factor and adding them up to reach a total risk score.

The total risk score applicable to the design of each face puts it into a particular overall risk category. The total risk scores for each risk category are:

  • 0–6 = low risk
  • 7–12 = medium risk
  • 13–20 = high risk
  • over 20 = very high risk

There may be aspects of the design that makes one face a higher risk than the others – in fact, each face of the building could end up with a different total risk scores (see Table 2 from E2/AS1 at the bottom of this page).

Cladding options

Once the risk score has been calculated, the designer uses the wall claddings table to select a cladding type and construction details suitable for that level of risk (see Table 3 from E2/AS1 at the bottom of this page).

Cladding options cover both direct-fixed claddings and drained and vented cavity claddings. As the weathertightness risk score increases, E2/AS1 calls for cavity-based cladding systems to be used. This is because cavity systems offer more potential to drain or dry water that has penetrated the cladding.

For example:

  • for a risk score up to and including 12, bevel-back timber weatherboards can be direct-fixed
  • for a risk score of 13–20, bevel-back timber weatherboards must be fixed over a drained and vented cavity – in this case, the cavity provides secondary protection to manage and remove any water that may penetrate the cladding.

In some cases, if the designer believes the risk is too high, they may redesign that face of the building and remove some of the high-risk features to end up with a lower risk score.

E2/AS1 can’t be used as the means of compliance for a building that scores 21 or more, as this is outside the scope of the Acceptable Solution. A different means of demonstrating compliance must be used, and it must be specifically designed to meet the higher level of risk.

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Risk features

A number of factors related to house design or the environment in which the house is built have also been identified as increasing the risk of weathertightness failure. Many of these features have been referred to in risk factors above, for example, taller buildings, buildings that lack eaves, and buildings with complex envelopes or membrane decks are all at greater risk of weathertightness failure.

Other risk features include parapets and monolithic cladding.

Monolithic cladding can be constructed in flush-joint flat fibre-cement sheet, EIFS or stucco and must be installed over a cavity under E2/AS1. They rely on surface coatings as a weatherskin, and houses using these claddings often incorporate higher-risk features such as flat roofs, parapets and no eaves – all features that are not generally appropriate in wet climates.

Many direct-fixed monolithic claddings have suffered weathertightness failure. If water penetrates these cladding systems, there is little opportunity for drainage or drying behind the cladding. Instead, water is held within the assembly, increasing the potential for deterioration. Unfortunately, some cladding leaks do not often become evident until deterioration of the wall assembly is well advanced.

Also see common leaks for more details of risk features.

Updated: 9 September 2014