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ACQ (Alkaline Copper Quaternary)

ACQ is the arsenic-free alternative to CCA for H3 and H4 hazard classes. Standard for modern residential decking, posts, ground-contact. Builder primer.

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

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ACQ stands for Alkaline Copper Quaternary, a water-borne timber preservative that is the modern arsenic-free alternative to CCA. The actives are copper (broad-spectrum fungicide and insecticide) and a quaternary ammonium compound (didecyl-dimethyl-ammonium chloride or DDAC) acting as a secondary fungicide and termiticide. Brand names include Tanalith E, Permapine ACQ, and Wolmanit CX.

Hazard class range. ACQ achieves H3 and H4 ratings under AS 1604.1, covering the residential applications most builders meet:

  • H3 ACQ: above-ground exterior, weather-exposed (decking surfaces, balustrades, pergola, exposed cladding battens).
  • H4 ACQ: in-ground contact (fence posts, garden edging, landscape sleepers, posts for decks at ground contact).

For H5 and H6 applications (marine, in-water), creosote or specialised heavy-duty CCA variants remain in use; ACQ is not typically rated above H4.

Why ACQ replaced CCA on most residential decking. Three drivers:

  1. No arsenic. Post-APVMA 2006 restrictions on high-contact CCA uses, ACQ became the standard for elements with prolonged human contact: decking surfaces, handrails, exposed seating, pool decks, playground frames.
  2. Similar appearance. ACQ produces a green-tinted treatment look comparable to CCA. Most builders cannot distinguish them visually at delivery.
  3. Disposal flexibility. ACQ off-cuts can be disposed of through regular waste streams in most jurisdictions (where CCA off-cuts must go to landfill and never be burned). Confirm local council rules; some areas treat all preservative-treated timber identically.

Trade-offs vs CCA.

  • Higher cost at the mill: ACQ-treated decking is typically 5 to 15% more than equivalent CCA-treated.
  • Higher copper content in ACQ means stainless or hot-dip-galvanised fixings only. Standard galvanised nails and screws corrode quickly in ACQ. Use Type 304 or 316 stainless or HDG to AS 1604.1 fixing-corrosion requirements.
  • More aggressive on aluminium in direct contact than CCA; isolate ACQ-treated framing from aluminium flashings, joist hangers, or window frames with a barrier (DPC, plastic shim).
  • Slightly lower depth of penetration on dense or wet stock than CCA at equivalent retentions; mostly an issue at the mill, not on site.

Identifying ACQ. The grade stamp on treated timber identifies the chemical: look for ACQ, Tanalith E, Permapine ACQ, or similar mill-specific markings. The visible green tint is similar to CCA; the stamp is the definitive identifier.

For builders.

  1. Spec ACQ for decking and handrails. Don’t fight the architect on this; ACQ is the contemporary default and avoids the contact-restriction problem.
  2. Use stainless or HDG fixings. Plain electroplated zinc fasteners will fail in years.
  3. Isolate from aluminium. Direct ACQ-aluminium contact corrodes both quickly.
  4. Off-cut storage. Treat ACQ off-cuts as treated timber waste; check the local council rules before bulk disposal.

Also known as: Alkaline Copper Quaternary, ACQ-treated pine, Tanalith E.

Category: Materials / timber / treatment.

See also

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

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