chalkline. Snap the line. Build to it.
regulation Compliance and regulation 5 min read

AS/NZS 2904 (DPC and flashings): the product spec under every brick wall

AS/NZS 2904 sets the materials and durability classes for damp-proof courses and flashings in masonry walls. Five material classes; NCC ABCB Part 5.7 calls it up.

By Dan Waring. Last updated 16 May 2026. Verified 16 May 2026.

Ask Chalkline about this →

In plain English

AS/NZS 2904:1995, Damp-proof courses and flashings (with Amendment 1, 1998 and Amendment 2, 2013), is the joint Australia/New Zealand product standard for the materials used as damp-proof courses (DPCs) and concealed flashings in masonry construction. It sets the material classes (polyethylene, embossed alkathene, butyl rubber, lead-cored, aluminium-cored), the minimum thicknesses, durability tests, water-tightness performance, and chemical compatibility with mortars and brick mortar admixtures. It is called up by the ABCB Housing Provisions Standard 2022 Part 5.7 (Weatherproofing of masonry) and through that into NCC 2022 Volume Two Part H2 (verified 2026-05-16).

A DPC sits horizontally in a masonry wall to break the capillary path of rising damp. A concealed flashing sits horizontally or sloped within the wall to catch incidental water and direct it out via weep holes. Both use AS/NZS 2904-compliant material. The same product types are used at:

  • Wall-to-floor junction (bottom course of brickwork).
  • Window heads and sills, through the cavity.
  • Wall-to-slab junctions in stepped sites.
  • Parapet copings.
  • Through-wall penetrations at services.

Builders read AS/NZS 2904 in conjunction with the glossary entry on the standard for quick reference; this article is the deeper compliance treatment.

What it requires

For the DPC and flashing supplier:

  1. Material classification. Each product falls into one of five recognised classes:
    • Class 1: General-purpose embossed polyethylene (most common residential DPC).
    • Class 2: Improved-durability polyethylene with higher tear strength.
    • Class 3: Bitumen-coated aluminium-cored sheet.
    • Class 4: Bitumen-coated lead-cored sheet (legacy, lead-restricted on modern jobs).
    • Class 5: Butyl rubber-based or modified-bitumen self-adhesive.
  2. Minimum gauge or thickness. Set by class. Embossed polyethylene typically 0.5 mm or 0.75 mm; aluminium-cored 0.3 mm core with bitumen coat.
  3. Water-tightness performance. No water transmission under a specified head pressure for a specified period.
  4. Tensile strength and tear resistance. To survive trade handling and brick-laying loads.
  5. Chemical compatibility. With Portland cement, mortar additives (calcium chloride is the historical incompatibility hazard), and adjoining wall materials.
  6. UV stability where exposed. Most DPCs are not designed for long-term UV exposure; the design must keep them concealed in the wall.

What it doesn’t cover

  • Exposed metal flashings (head flashings on lightweight cladding, parapet cappings). Those are designed and detailed under AS/NZS 2179 (metal cladding) and AS 1562 (sheet-metal cladding). AS/NZS 2904 is the concealed-flashing standard.
  • External waterproofing membranes for wet areas or roofs. Those are AS 3740 (wet areas), AS 4654 (above-ground external), and AS 4858 (below-ground tanking).
  • Sarking and wall wraps. Those are AS/NZS 4200.1 (pliable building membranes).
  • Termite barriers. Those are AS 3660.1, even though physical termite-barrier sheet products often look like DPC.

Practical implications

  • Substitution between classes is not a free choice. A Class 1 polyethylene DPC and a Class 5 self-adhesive membrane are both “DPC” but perform very differently under stress, mortar load and chemical exposure. The architectural detail must specify the class, and substitution requires equivalent-class confirmation.
  • DPC lap dimension matters. AS/NZS 2904 specifies minimum laps at joints (150 mm common on horizontal runs, more at corners). A 50 mm lap is a defect: water tracks past, mortar squeezes in, the joint fails on the first storm.
  • DPC must be continuous through the wall thickness. A common defect on cavity masonry is laying the outer-leaf DPC and forgetting the inner-leaf DPC, or vice versa. The DPC has to break the capillary path on both leaves and across the cavity.
  • Calcium chloride mortar additive is incompatible with most DPC classes. Avoid on jobs that use Class 1, 2 or 3 DPCs. The historical “antifreeze admix” use case is the cause of long-term DPC failures in older houses.
  • Lead-cored DPCs (Class 4) are now rare. Modern residential jobs use embossed polyethylene (Class 1 or 2) or self-adhesive bitumen-based (Class 5). Lead-cored remains in some heritage repair contexts.

References

See also

Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.

Related articles