All building work must comply with the New Zealand Building Code. Of all the Code clauses that deal with weathertightness, clause E2 External moisture is the most significant. A design can meet the requirements of E2 if it follows Acceptable Solution E2/AS1.
The Building Code has three clauses of general provisions and 38 clauses on specific aspects of building performance. Building Code clauses do not show how to build – they give performance criteria that the building must meet. Buildings must comply with these performance criteria 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
- E2 External moisture.
A designer must identify all the clauses that are relevant to a particular building and design to meet the relevant performance requirements. This page provides very brief extracts of key performance requirements of the above clauses. For the full list of requirements, read the Building Code.
Clause B1 Structure
Buildings, building elements and sitework:
- shall have a low probability of rupturing, becoming unstable, losing equilibrium or collapsing during construction or alteration and throughout their lives
- shall have a low probability of causing loss of amenity through undue deformation, vibratory response, degradation or other physical characteristics throughout their lives, or during construction or alteration when the building is in use.
Clause B2 Durability
Building elements must, with only normal maintenance, continue to satisfy the performance requirements of the Code for the lesser of the specified intended life of the building, if stated, or:
(a) the life of the building (not less than 50 years) if those building elements (including floors, walls and fixings) provide structural stability to the building or are difficult to access or replace or their failure would go undetected during normal use and maintenance
(b) not less than 15 years if those building elements (including the building envelope, exposed plumbing in the subfloor space and in-built chimneys and flues) are moderately difficult to access or replace or their failure would go undetected during normal use of the building but would be easily detected during normal maintenance
(c) not less than 5 years if the building elements (including services, linings, renewable protective coatings and fixtures) are easy to access and replace and their failure would be easily detected during normal use of the building.
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 sitework 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.
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.
“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.
“Buildings must be constructed to provide adequate resistance to penetration by, and the accumulation of, moisture from the outside”.
This means that the building exterior must be designed and built to ensure that water is kept on the outside.
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 consider how water will be removed from within wall assemblies when they design construction details.
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.
Clause E2 (which includes E2/AS1) can be downloaded from the The Ministry of Business, Innovation and Employment’s (MBIE) Building Performance website.
Reasons for using E2/AS1
Designers often follow E2/AS1 as a means of compliance with E2 because:
- it provides specific guidance
- it is understood and regularly used by BCAs
- it is a MBIE document, so if a building built to E2/AS1 fails, then the MBIE has some responsibility
- in a lot of cases, it is the easiest option as it is accepted by BCAs (it can be difficult in some cases to convince BCAs that alternative methods of construction are Code compliant.)
Most BCAs have E2/AS1 as their benchmark document for weathertight design and E2 compliance. They often use the solutions in it as a comparison when they are assessing alternative methods at building consent stage.
Risks of using E2/AS1
However, there is a risk if E2/AS1 is applied to construction that is outside the scope of the Acceptable Solution, for example, where wind speeds are greater than extra high.
Following E2/AS1 is not the only way of designing and constructing a weathertight building. There are a number of alternative and equally effective methods that designers can follow.
What does E2/AS1 cover?
E2/AS1 is limited in scope to timber-framed buildings up to three storeys high, with a maximum height from the ground to the highest point of the roof of 10 m. Buildings outside of this scope need to be specifically designed.
It incorporates information and details on a wide range of building materials and systems. There are some notable exceptions, for example, timber-framed windows are not generally covered by E2/AS1, and nor is a steel framing.
Updated: 12 July 2019