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Vapour barrier (slab)

A vapour barrier is a continuous polyethylene sheet over sub-slab fill before reo and concrete. Blocks moisture into the slab. AS 2870 sets 200 μm minimum thickness.

By Chalkline. Last updated 14 May 2026. Verified 14 May 2026.

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A vapour barrier (also called a damp-proof membrane (DPM) or vapour membrane) is a continuous polyethylene sheet laid over the prepared sub-slab fill before reo placement and concrete pour. It blocks the migration of moisture (and the soluble salts in groundwater) from the soil through the slab into the building above, protecting the slab from chemical attack and the building from rising damp.

Requirements under AS 2870:2011 Appendix B:

  • Minimum thickness: 200 microns (0.2 mm) of polyethylene film, branded as compliant (“AS 2870 vapour barrier” or similar marking).
  • Laps: minimum 200 mm overlap at all sheet joins.
  • Sealing: laps and penetrations sealed with the manufacturer’s specified tape or sealant; some specs allow folded laps without tape on flat ground, but the engineer’s spec is authoritative.
  • Continuous: no gaps in coverage. Tears must be patched before pour.
  • Turn-up at edges: lapped up the inside face of the slab edge form, typically to slab-top level.
  • Penetrations: cut neatly around plumbing and service penetrations, sealed with tape or membrane patches.

Sub-slab fill prep before laying. The vapour barrier sits between fine fill (typically 50 mm sand or fine crushed rock) and the structural concrete. The fine fill provides a smooth surface so the membrane isn’t punctured by coarse stone. Larger crushed rock under the fine layer is acceptable; the membrane must not contact it directly.

Why it matters:

  1. Moisture migration: groundwater can migrate up through soil into a slab via capillary action. Without a vapour barrier, the slab absorbs moisture and transmits it to the floor finish above, causing failure of floor coverings, mould, and timber-floor warping.
  2. Salt attack: soluble sulphates and chlorides in groundwater can chemically attack the slab from below, reducing concrete strength and corroding reinforcement.
  3. Termite path (with chemical barriers): some chemical termite-management systems require the vapour barrier to be intact as part of the protected envelope.

Common defects:

  • Tears not patched: tradies walking on the membrane before pour catch reo on it and tear it. Tape or patch the damage before reo placement.
  • Insufficient lap: laps under 200 mm or unsealed laps. Water tracks through.
  • Penetration cut too generous: large gaps around pipes. Tape or fold the membrane tight to the penetration.
  • Edge turn-up missing: membrane stops at the formwork instead of folding up. Moisture wicks into the slab edge.
  • Wrong-thickness membrane: 150 μm “builder’s plastic” used instead of 200 μm AS-rated membrane. Inadequate puncture and tear resistance; fails the AS 2870 spec.

Pre-pour inspection. The certifier or RBS inspects the vapour barrier at the pre-pour inspection. The checklist:

  • Visual coverage: no gaps, no tears.
  • Lap quantity and sealing.
  • Edge turn-up.
  • Penetration treatments.
  • Branding visible (proves AS-rated membrane was used).

For builders.

  1. Specify AS 2870-compliant 200 μm membrane in the slab subbie’s scope. “Polyethylene sheet” alone is not enough.
  2. Walk the membrane immediately before pour, with a roll of patch tape. Even with great pre-pour install, traffic over the membrane creates new tears.
  3. Photograph the barrier at pre-pour. Years later when slab issues arise, this is the evidence.

Also known as: vapour membrane, damp-proof membrane, DPM, sub-slab membrane.

Category: Materials / slabs / waterproofing.

See also

Last updated: 2026-05-14. Verified: 2026-05-14.

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