Weathertightness and the NZ Building Code

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Building Code clauses do not show how to build — rather, they give minimum performance criteria that the buildings must meet or exceed to ensure that they will meet the purposes of the Building Act.

The key clauses that relate to weathertightness performance are B1 Structure, B2 Durability, E1 Surface water, and E2 External moisture.

A designer must identify all the clauses that are relevant to a particular building and design to meet or exceed the relevant performance requirements.

The following provides very brief extracts of key performance requirements of the above E1 and E2 clauses.

For the full list of requirements, go to www.legislation.govt.nz/regulation/public/1992/0150/latest/DLM162576.html.

Clause E1 Surface water

Surface water resulting from an event having a 10% probability of occurring annually and which is collected or concentrated by buildings or site work shall be disposed of in a way that avoids the likelihood of damage or nuisance to other property.

Surface water resulting from an event having a 2% probability of occurring annually shall not enter buildings.

Drainage systems for the disposal of surface water shall be constructed to:

• convey surface water to an appropriate outfall using gravity flow where possible.

• avoid the likelihood of blockages, leakage, penetration by roots or the entry of ground water where pipes or lined channels are used.

• provide reasonable access for maintenance and clearing blockages.

• avoid the likelihood of damage to any outfall.

• avoid the likelihood of damage from superimposed loads or normal ground movements.

The Acceptable Solution E1/AS2 for stormwater was introduced in November 2020. It references AS/NZS 3500.3:2018 Plumbing and drainage Part 3: Stormwater drainage (with modifications) as an Acceptable Solution for the design and installation of surface water drainage systems.

Clause E2 External moisture

The clause states that building exteriors must prevent the penetration and accumulation of water.

It also allows for the fact that, in extreme situations, water may penetrate the building exterior, and requires that the building be designed to allow water to dissipate without causing damage to components.

Details of Clause E2

Objective

“To safeguard people from illness or injury that could result from external moisture entering the building.”

Leaks may cause deterioration of building components that may make the building:

• structurally unsound, as a result of water-based rot.

• unhealthy, as a result of harmful mould and fungal decay.

Functional requirement

“Buildings must be constructed to provide adequate resistance to penetration by, and the accumulation of, moisture from the outside.”

Performance

E2 incorporates seven performance criteria:

• E2.3.1: Roofs must shed precipitated moisture. In locations subject to snowfalls, roofs must also shed melted snow.

• E2.3.2: Roofs and exterior walls must prevent the penetration of water that could cause undue dampness, damage to building elements, or both.

• E2.3.3: Walls, floors and structural elements in contact with, or in close proximity to, the ground must not absorb or transmit moisture in quantities that could cause undue dampness or damage to building elements, or both.

• E2.3.4: Building elements susceptible to damage must be protected from the adverse effects of moisture entering the space below suspended floors.

• E2.3.5: Concealed spaces and cavities in buildings must be constructed in a way that prevents external moisture being accumulated or transferred, and causing condensation, fungal growth or the degradation of building elements.

• E2.3.6: Excess moisture present at the completion of construction must be capable of being dissipated without permanent damage to building elements.

• E2.3.7: Building elements must be constructed in a way that makes due allowance for the following:

• the consequences of failure.

• the effects of uncertainties resulting from construction, or from the sequence in which different aspects of construction occur.

• variation in the properties of materials, and in the characteristics of the site.

These performance clauses recognise that, at some stage, all buildings may leak.

They allow for some external moisture to enter exterior assemblies, as long as this water does not cause undue dampness or damage to building elements, or that it is not being transferred and causing condensation and the degradation of building elements.

The performance clauses also allow for moisture that may enter and be absorbed by components during construction, as long as this moisture is capable of being dissipated without permanent damage to building elements.

Designers must consider how water will be removed from within wall assemblies when they design construction details.

Acceptable Solution E2/AS1

Building Code clauses have Acceptable Solutions which are documents that set out methods of complying with the Building Code.

Anyone who complies exactly with the methods described in the Acceptable Solution for a Building Code clause is deemed to comply with the Building Code.

It is not mandatory to follow an Acceptable Solution. There are other ways of achieving Building Code compliance.

Designers need to prove to the BCA that these alternative methods of construction comply with the relevant Building Code requirements.

Once they have done this, the alternative methods become Alternative Solutions.

The design of most buildings will be made up of a combination of Acceptable and Alternative Solutions. The main Acceptable Solution for meeting the requirements of E2 is E2/AS1.

This is a non-mandatory guidance document that provides a range of solutions for designing and building weathertight buildings.

When a building is designed and built in line with E2/AS1, it will meet the requirements of clause E2 and will, therefore, be Code compliant with respect to that particular clause.

Designers and builders need to read and understand E2/AS1 as it is often used as a means of Building Code compliance by designers when preparing building consent documentation.

It can also provide useful general guidance information on weathertight construction.

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