External membranes and Building Code compliance

This article discusses how to establish compliance with Clause E2 External Moisture of the Building Code (the Code), particularly in relation to external membranes for roofs and decks.

The article was developed in response to variations in building consent authority (BCA) approaches to this issue.

Clause E2 External Moisture has a Verification Method (test or calculation methods that prescribes one way to comply with the Code) and two Acceptable Solutions (step-by-step instructions that show one way to comply with the Code). The relevant Acceptable Solution for membrane roofs and decks is E2/AS1.

Designs that comply with the verification method or acceptable solution must be accepted by a BCA as complying with the Building Code. BCAs do not have the power to set new or different standards from those in the Building Code and Compliance Documents.

E2/AS1 - External Moisture

E2/AS1 is the Acceptable Solution that covers external membranes for certain buildings. It has a clearly defined and relatively narrow scope that does not apply to all building designs. This means it can't always be used to demonstrate compliance with clause E2 of the Code for every building project.

E2/AS1 also has limitations as to the types of membrane roofs and decks that are covered.

These tables are summaries only, and designers or applicants should always refer to the full text of E2/AS1 and NZS 3604: 1999.

Summary of types of building covered by E2/AS1


E2/AS1 includes:

  • materials, products, and processes contained in E2/AS1 for buildings within the scope of clause 1.1.2 of New Zealand Standard NZS 3604: 1999 Timber Framed Buildings:
    • up to three storeys of timber framing, with a maximum height from ground to eaves of 10 metres
    • with a floor plan area limited only by seismic and structural control joints.

Additionally, some of the specific inclusions covered in clause 1.1.2 of NZS 3604: 1999 are as follows:

  • buildings should be founded on good ground (see section 1.3, definitions,of NZS 3604: 1999)
  • buildings that fall within the building wind zones as described in NZS 3604: 1999 and are low, medium, high or very high floor and roof live loadings applicable to domestic, residential, institutional, and educational buildings fall within the scope of NZS 3604: 1999 (provided that the
  • floor loading shall not exceed 1.5 kPa for the uppermost floor of three storey buildings).

Note: The plan floor area can be unlimited for one or two storey buildings where all storeys are of timber frame.


E2/AS1 does not cover buildings over three storeys, (with a maximum height from ground to eaves of 10 metres).

E2/AS1 also specifically excludes:

  • outbuildings (such as garages and other unlined structures)
  • buildings with drained cavities and spread of flame requirements as specified in clause C3.3 of the Code
  • buildings with drained cavities and acoustic requirements as specified within clause G6 of the Code. Buildings not founded on good ground, as defined in section 1.3 of NZS 3604: 1999 are not covered.

NZS 3604: 1999 also specifically excludes:

  • buildings above very high wind zones (specific engineering design is required here).

Summary of membrane roofs and decks covered by E2/AS1


Membranes composed of butyl or EPDM (ethylene propylene diene monomer) installed over plywood substrates for:

  • roofs with a minimum fall of 1.5 degrees (1:40)
  • decks with a minimum fall of 1 degree (1:60)
  • decks with a maximum area of 40 square metres
  • internal gutters with a minimum fall of 1 in 100 (see also the exclusion opposite)
  • decks with removable raised surfaces to give level access.


All membranes other than butyl and EPDM fall outside E2/AS1.

Other building elements not covered include:

  • decks with steps within the same level of the deck area (except into gutters)
  • decks with integral roof gardens
  • decks with a downpipe directly discharging to the deck
  • internal gutters with a minimum fall of 1 in 100, with seams in the gutters closer than 1 metre to an outlet
  • the application of directly-applied wearing or decorative surfaces to membranes (eg, tiled surfaces)
  • deck substrates other than plywood (17 mm minimum thickness).

Note: for roof and deck areas over 40 m2, roof vents will be required. Roof vents are not covered by the Acceptable Solution.

Specific design and alternative solutions

Building consent applications for designs that are not covered by this scope cannot rely on E2/AS1 to demonstrate compliance with the Code. Such designs are referred to as 'alternative solution proposals'.

The building consent applicant or designer needs to demonstrate how their proposal complies with the Code, before the BCA can approve it as an alternative solution.

Sound decision-making on external membranes

Every building consent application should be assessed on its own merits by the BCA for compliance with the Building Code.

In general terms, a given building consent application might seek to demonstrate compliance with clause E2 by:

  • fully adhering to the requirements of E2/AS1
  • partially or substantially complying with E2/AS1, but also proposing an alternative solution for one or more components of the design. Additional supporting information to help establish compliance will be needed for any such alternative solution component
  • not referring to E2/AS1 at all, but using a specific design and alternative solutions to demonstrate compliance. Supporting information must be provided to help establish compliance.

Designers should ensure the proposed means of compliance is clear and well detailed in their application, to help the BCA make an efficient and informed compliance decision.

Relevant Building Act 2004 requirements

Some of the key requirements in the Act include:

  • Section 17 - which requires building work to comply with the Building Code
  • Section 18 - which specifies that building work does not have to achieve performance criteria additional to or more restrictive than the Building Code
  • Section 19 - which specifies a number of ways that compliance with the Building Code can be established. Some examples mentioned include:

    • complying with a Compliance Document published by the Department
    • complying with a determination issued by the Department.

The decision-making process then used by BCA staff to check any given building consent application is likely to depend on which compliance pathway the applicant or designer is proposing. The flow chart opposite outlines some key decisions that should be considered when assessing external membranes.

Additional information on external membranes

Other useful information on external membranes includes:

  • BRANZ's Weathertight Solutions Vol. 6: Membrane roofs. This resource can be purchased from the BRANZ website at www.branz.co.nz
  • The Membrane Group New Zealand Incorporated's Code of Practice (for Torch-on Membrane Systems for Roofs and Decks), available from their website at www.membrane.org.nz

Solution pathways.

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