Weathertightness solutions

In July 2005, specific solutions for the way buildings can be designed and built to help avoid weathertightness problems took effect. They are about better protection for homeowners and contain detail and advice on how buildings should be designed and built to achieve weathertightness.

They apply to most standard houses and low-rise apartments. If you are building or thinking about building, we recommend you talk to your designer about these changes now.

What are the weathertightness solutions?

The weathertightness solutions are set out in the Acceptable Solution for Building Code Clause E2 External Moisture, otherwise known as Acceptable Solution E2/AS1.

The new Acceptable Solution E2/AS1:

  • provides details and information about how buildings can be built to achieve weathertightness
  • is based on risk management principles for potential water leaks
  • sets out a method for building cavities when required between wall claddings and wall framing
  • details methods for installing various roof and wall cladding systems to minimise the risk of leaks
  • describes methods for building things such as balustrades or balconies, solid decks, and junctions between roofs and walls which would otherwise have a high risk of leaking
  • provides detail on designing and installing flashings.

The Acceptable Solution E2/AS1 is one means of establishing compliance with the Building Code. An owner, through their designer, may choose an alternative means of compliance with the Building Code. Find out more about how to comply with the Building Code.

It is strongly recommended that owners engage a competent architect or designer to prepare documents for building consent, to obtain competitive prices where required and for instructing the builder. In some instances with simpler work, the builder may be engaged to provide design services as well, however, in doing so they will take on the full design responsibility that work complies with the Building Code. The owner can obtain independent design advice by engaging an independent designer.

When did these weathertightness solutions take effect?

The amended Acceptable Solution E2/AS1 took effect on 1 July 2005 and represents current best practice in building for weathertightness. Please talk to your designer to make sure weathertightness has been carefully considered before your building design is completed.

How do these solutions affect building projects?

Planning to build?

If you are now planning to build and apply for a building consent using the Acceptable Solution for weathertightness designs, talk to your designer now about the solutions. Meeting them means your building automatically complies with the relevant clauses of the Building Code. However, you can choose alternative designs but you will need to demonstrate to the satisfaction of the building consent authority that these comply with the Building Code.

Already building?

The changes may not affect you if you have a building consent issued under the previous weathertightness solution. (See BIA Update No 42 for more information.) Talk with your local building consent authority to clarify whether this is the case.

Even if the changes don’t affect you, we suggest you talk to your designer now, as the amended Acceptable Solutions are based on the best current building science and practice. It may be possible to include some aspects of the new solutions in your project, although to do so you may need to apply for an amendment to your building consent.

Why are these solutions important?

If the design and construction of your building accurately follows these solutions then:

  • it will automatically comply with the Building Code requirements for managing external moisture
  • you will minimise the risk of leaky building problems.

How do the solutions do this?

The weathertightness solutions detail how to design and construct buildings for managing the risk of weathertightness problems for individual buildings.

They provide a method for assessing that risk based on six factors:

  • Wind zone - buildings in areas that are subject to strong winds are at risk of rain being driven behind the cladding.
  • Number of storeys - the higher the building, the greater the catchment area for rain and exposure to wind.
  • Roof/wall intersection design - buildings with complex roofs intersecting with walls create opportunities for leaks to penetrate into the walls.
  • Eaves width - increasing the width of eaves provides additional shelter to the walls from rain.
  • Envelope complexity - the more complex the building envelope (essentially the outside shape and the walls of a building), the more difficult it becomes to make it weathertight because it will have more junctions and penetrations which are prone to leaks.
  • Deck design - certain deck and balustrade designs are less risky than others. For example, timber open-slatted decks at ground-floor level are less risky than enclosed or sealed decks at second-floor level.

Where do I find the details?

The detailed solutions are set out in the Acceptable Solution in the Compliance Document for Clause E2 External Moisture of the Building Code (called ‘E2/AS1’). E2/AS1 is a technical and detailed document that your architect, designer or builder should be familiar with. For more information, see Compliance Documents in the section 'Weathertightness - Information for building professionals.'