This risk matrix has been developed to assess the possible risk of water ingress to various types of detailing and construction. Nothing in this assessment certifies or warranties the construction elements from failure. Construction detailing and compliance with the NZ building code remains the responsibility of the LBP's involved in the individual projects.
Details have been selected from the existing G.J. Gardner Risk matrix which has been in use since 2011, as well as details identified that are high risk. These have then been split into two groups. Firstly those that have an impact on weathertightness and NZBC E2/AS1 and secondly those that have an effect on overall durability of the home.
The weathertightness risk matrix uses 7 scoring criteria that take into account the consequence of failure as well as the performance of the details. These are weighted to give preference to those details that have an acceptable solution for E2 Compliance.
The first 3 of the 7 criteria relate to the risk of failure to the detail.
Details that are easier to detail and construct receive a lower score. Examples of difficult detailing would be multiple junctions occuring at one point.
Construction that would cause damage across large areas or hard to see and repair damage would score more than easily fixed or limited areas.
The existence of an E2/AS1 Acceptable solution or a manufacturer approved detail greatly reduces risk as it will already have undergone testing to comply with the building code. This score is weighted to twice the value of the other criteria. Existence of a detail does not negate the other factors as those risks will still remain.
The final 4 criteria relate to the performance of the construction detail and is based on the 4D principle that the E2 Risk matrix is based on.
Further Info - BRANZ 4Ds of weathertightness
The more a wall is exposed to water, the higher the risk of water penetration. Deflection devices (such as cladding and window head flashings) intercept water at the building exterior and deflect it away from critical junctions.
Wall assemblies also need to be designed and built to incorporate drainage to allow water that may have penetrated the exterior cladding to drain down the back of the wall cladding and out of the wall assembly.
Wall assemblies dry as a result of air movement through and behind the cladding systems.
Components in the assembly have varying levels of durability. Treated timber framing will be durable if it remains dry, but it may rot if it is kept wet for long periods of time. Kraft-based wall underlays are durable but will deteriorate if they are kept wet, while synthetic-based wall underlays will have greater durability
This score is the average risk score from the 7 criteria. All scores are based out of 10 except for the E2 or manufacturer Detail that scores 20 if there is no detail and 0 if it exists.
This risk score and zones do not make a definitive line of details that will or will not leak but is a line the represents an acceptable or unacceptable risk to the G.J. Brand.
These calculated scores are broken into three zones.
This zone covers details that if well detailed and constructed should comply with the building code and provide acceptable levels of risk.
This Zone covers details that while compliance with the building code should be achieved, they present a higher risk and where possible should be designed away from. No more than two details from the Amber range per dwelling are permitted
These details are outside of the risk tolerance and should not be constructed.