External membranes for roofs and decks

We developed this guidance 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 Building Code) and two Acceptable Solutions (step-by-step instructions that show one way to comply with the Building 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, Acceptable Solutions and Verification Methods.

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 Building 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 Timber framed buildings.

Types of building covered by E2/AS1

Included

E2/AS1 includes:

• materials, products, and processes 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 of 10 metres from ground to highest point of the roof
  • 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 are as follows:

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

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

Excluded

E2/AS1 does not cover buildings over three storeys (with a maximum height of 10 metres from ground to highest point of the roof).

E2/AS1 also specifically excludes:

• outbuildings (such as stand-alone garages and other unlined structures)
• buildings with drained cavities which also have spread of flame requirements as specified in clause C3.3 of the Building Code
• buildings with drained cavities which also have acoustic requirements as specified within clause G6 of the Building Code.

NZS 3604 also specifically excludes:

• buildings not founded on good ground, as defined in section 1.3 of NZS 3604
• buildings above extra high wind zone as described in NZS 3604

Membrane roofs and decks covered by E2/AS1

Included

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

• roofs with a minimum fall of 2.0 degrees (1:30)
• decks with a minimum fall of 1.5 degrees (1:40)
• decks with a maximum area of 40 square metres
• internal gutters with a minimum fall of 1 in 100 and with no cross seams
• decks with removable raised surfaces to give level access.

Excluded

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

Other building elements not covered include:

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

For roof and deck areas over 40m², roof vents will be required in accordance with the membrane manufacturer’s instructions. 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 Building 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 an Acceptable Solution or Verification Method published by MBIE
  • complying with a determination issued by MBIE.

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 below outlines some key decisions that should be considered when assessing external membranes.

Additional information on external membranes

Other useful information on external membranes includes; the Code of practice for torch-on membrane systems for roofs and decks, which you can read about on the Waterproofing Membrane Association Incorporated website.

All guidance related to E2 External moisture