Quick Answer

H4 is a hazard rating — it tells you the timber is treated for in-ground contact. CCA and ACQ are the two chemical treatments most commonly used to achieve that rating in Australia. CCA is cheaper and widely used for retaining wall sleepers, but is restricted from close-contact applications. ACQ is arsenic-free and approved for close-contact uses like decking and furniture, but is more corrosive to standard galvanised fasteners.

Below: what each term actually means, how CCA and ACQ compare, why fastener choice matters, and which one is right for a retaining wall.

H4 Is a Rating, Not a Chemical

The most common confusion with treated pine terminology is treating "H4" and "CCA" as if they're the same kind of thing. They're not.

H4 is a hazard class defined in AS 1604, the Australian Standard for preservative-treated timber. It describes the level of exposure the timber is rated to withstand — in this case, in-ground or in-ground-contact use, which is exactly what a retaining wall sleeper needs. CCA and ACQ, on the other hand, are the actual chemical preservatives used to achieve that rating. A sleeper can be H4-rated using either treatment, provided the preservative is applied to the depth and retention level the standard requires.

Think of H4 as the performance requirement and CCA or ACQ as two different ways of meeting it — similar to how a building might need to meet a fire rating, achievable through more than one type of material or construction method.

What Is CCA Treatment?

CCA stands for chromated copper arsenate — a preservative mix of copper, chromium and arsenic compounds. It's the treatment responsible for the familiar green-brown tint of most treated pine sleepers.

Each component plays a role: copper protects against fungal decay, arsenic acts as an insecticide against borers and termites, and chromium fixes the other two chemicals into the timber so they don't easily leach out. CCA has been used in Australia for decades and has a long, well-documented track record in outdoor and in-ground applications.

Because CCA contains arsenic, its use in Australia is restricted by the Australian Pesticides and Veterinary Medicines Authority (APVMA) for applications with regular skin contact. This is why you won't typically find CCA-treated timber sold for garden furniture, handrails, domestic decking or children's play equipment — but it remains permitted and commonly used for retaining wall sleepers, fencing and posts, where the contact profile is different.

CCA is generally the more affordable and more widely available of the two treatments, which is a large part of why it remains the standard choice for treated pine sleepers.

CCA-treated timber also has an unusually long practical track record compared to newer alternatives. Decades of in-ground use across fencing, retaining walls and structural posts have given suppliers and builders a well-established understanding of how it performs over time, which is part of why it remains the default recommendation for applications like sleepers where long-term, predictable performance matters more than close-contact safety considerations.

What Is ACQ Treatment?

ACQ stands for alkaline copper quaternary — a newer, arsenic-free preservative developed as an alternative to CCA. Instead of arsenic, ACQ pairs a higher concentration of copper with a quaternary ammonium compound that provides additional protection against fungi, mould and insects.

Because it doesn't contain arsenic, ACQ-treated timber doesn't carry the same close-contact restrictions as CCA. It's commonly used for decking, garden furniture, handrails and play equipment — precisely the applications CCA is restricted from.

The trade-off is fastener compatibility, covered in detail below, and typically a higher cost than CCA. For a retaining wall sleeper, which isn't a close-contact application in the first place, ACQ's main safety advantage over CCA is less relevant — but the fastener consideration still applies if you buy ACQ-treated sleepers.

ACQ has a shorter track record than CCA simply because it's a newer treatment, but it has been in commercial use for decades now in markets like Australia, Canada and the United States, and its decay and termite resistance at equivalent hazard classes is well established. The main open question for any given project isn't whether ACQ performs adequately as a preservative — it does — but whether the hardware you're pairing it with is suited to its higher copper content.

Why Fastener Choice Matters More With ACQ

This is the part of the CCA vs ACQ comparison that most affects how you actually build with the timber, and it's easy to miss if you're buying sleepers and fixings from different places.

ACQ's higher copper concentration makes it meaningfully more corrosive to standard steel fasteners than CCA. Independent corrosion testing has found ACQ-treated timber accelerates corrosion of mild steel and standard hot-dip galvanised fasteners well beyond the rate seen with CCA-treated timber of the same hazard class. In practical terms, a standard galvanised coach screw or bracket that would last for decades in CCA-treated timber can corrode significantly faster in ACQ-treated timber, sometimes within just a few years in damp or coastal conditions.

Stainless steel fasteners — grade 304 or 316 — are unaffected by either treatment and are the safest choice regardless of which timber you're using. If you're using standard galvanised fixings, they're a reasonable match for CCA-treated sleepers, but for ACQ-treated timber it's worth stepping up to stainless steel, or at minimum confirming with your fastener supplier that the product is specifically rated for use with high-copper treatments. Aluminium fittings should be avoided with ACQ-treated timber altogether, as the reaction between the two can be severe.

This matters for a sleeper retaining wall specifically because posts and brackets are structural — a fastener slowly corroding away isn't just cosmetic, it's a long-term integrity issue. If you don't know which treatment your sleepers use, it's worth asking your supplier before you commit to a fastener type, rather than assuming standard galvanised fittings will perform the same way across any treated timber.

It's also worth thinking about where the corrosion risk is greatest. Fasteners embedded directly in the timber, in constant damp contact, face more exposure than fittings that sit mostly on the surface. Coach screws driving sleepers into steel post channels, or brackets fully recessed into the timber, are exactly the kind of fixing where using the wrong fastener grade for the treatment can cause problems years down the track, well after the wall looks structurally sound from the outside.

CCA vs ACQ at a Glance

Factor	CCA	ACQ
Contains arsenic	Yes	No
Close-contact use (decking, furniture, play equipment)	Restricted by APVMA	Approved
Typical cost	Lower	Higher
Fastener corrosion risk	Lower — standard galvanised generally suitable	Higher — stainless steel or rated fasteners recommended
Common use for retaining wall sleepers	Standard, most widely used	Available, less common for this application

Which Treatment Is Right for a Retaining Wall?

For a retaining wall specifically, CCA H4 is the standard choice, and for good reason. Retaining wall sleepers aren't a close-contact application in the way decking or furniture is, so ACQ's main safety advantage doesn't carry the same weight here. CCA is also more cost-effective and more widely stocked, and it doesn't demand an upgrade to stainless steel fasteners the way ACQ does.

That said, if you're specifically sourcing ACQ-treated sleepers — for consistency with other ACQ-treated timber on the same property, for example — there's nothing wrong with using them for a retaining wall, provided you match your posts, brackets and fasteners accordingly. The timber itself performs comparably to CCA in terms of decay and termite resistance at the same hazard class; the difference is almost entirely about what you connect it with.

There's one other scenario worth mentioning: mixed-material builds where a timber sleeper wall meets steel edging, garden lighting, irrigation fittings or other metal landscaping elements nearby. If any of that hardware is in direct, ongoing contact with the sleepers — rather than just sitting near them — it's worth applying the same fastener-compatibility thinking to those connections too, not just to the main structural brackets.

Whichever treatment you choose, the hazard class matters more than the chemical name for structural performance. Always confirm you're buying H4-rated timber for in-ground use — an H3 sleeper, regardless of whether it's CCA or ACQ treated, isn't rated for the ground contact a retaining wall requires.

How to Identify Which Treatment Your Timber Uses

If you've already got sleepers on site, or you're buying from a supplier who hasn't specified the treatment clearly, there are a few ways to check.

• Check the branding or ink stamp on the timber itself — treated pine is typically stamped with the hazard class and, on many products, the treatment type
• Ask your supplier directly — this is the most reliable method, since stamps can be cut off or hard to read once the timber's been trimmed
• Request a treatment certificate or datasheet, particularly for larger or trade orders, since these documents state the exact preservative and hazard class used

Colour alone isn't reliable, since both treatments can produce a similar greenish tint depending on the specific formulation and how long the timber has weathered. If fastener compatibility genuinely matters for your build — for example, if you've already bought brackets and want to confirm they're suitable — it's worth confirming the treatment before you start rather than guessing from appearance.

Handling and Disposal Safety

Both CCA and ACQ treated timber should be handled with basic precautions when cutting or drilling — gloves, a dust mask, and cleaning up sawdust rather than leaving it to accumulate. This applies to both treatments, not just CCA, since any pressure-treated timber releases fine particles when machined.

Offcuts and sawdust from either treatment shouldn't go into a home compost bin, garden mulch or fire pit. Most councils classify treated timber waste as general waste for landfill disposal rather than green waste, so check your local council's guidance before disposing of leftover material from either type of treated pine.

Common Misunderstandings About Treated Pine Terminology

"H4 means CCA." Not necessarily — H4 is the hazard rating, and either CCA or ACQ can be used to reach it. Always check which chemical treatment your specific sleeper uses if fastener compatibility matters to you.

"ACQ is always better because it's arsenic-free." Safer for close contact, yes — but for a retaining wall sleeper that isn't a close-contact application, this advantage is largely irrelevant, while the fastener corrosion trade-off still applies.

"Any galvanised bracket works with any treated timber." This is the mistake most likely to cause a real problem down the track. Standard galvanised fittings are a reasonable match for CCA but a weaker long-term match for ACQ, where stainless steel is the safer choice.

"The green tint tells you which treatment it is." Both CCA and ACQ can produce a greenish tint in freshly treated pine, so colour alone isn't a reliable way to tell them apart. Check the treatment documentation or ask your supplier directly if it matters for your project.

"CCA is being phased out everywhere, so it must be inferior." CCA has been restricted from certain close-contact applications in various markets, but it remains a fully approved, widely used, and well-tested treatment for exactly the kind of in-ground, low-contact application a retaining wall sleeper represents. Restriction from one use case doesn't mean it's inferior for a different one it was never restricted from.

Why ACQ Exists: A Bit of Background

CCA was the dominant timber preservative in Australia and internationally for most of the twentieth century, valued for its durability and low cost. Over time, concern grew about arsenic exposure from timber in close human contact — particularly around children's play equipment, where hand-to-mouth contact was a realistic pathway for exposure.

In response, regulators in several countries moved to restrict CCA from close-contact residential applications, and the industry developed arsenic-free alternatives, of which ACQ is the most widely adopted. In Australia, the APVMA maintains a list of applications where CCA use is restricted, covering things like garden furniture, handrails, domestic decking and play equipment, while permitting it for applications like retaining walls, fencing, and structural posts where the contact profile is different.

This background is worth knowing because it explains why ACQ exists at all, and why the two treatments aren't simply "old vs new" — CCA hasn't been phased out for every application, and it remains the standard, approved choice for exactly the kind of retaining wall use this guide covers.

Other Treatments You Might See

CCA and ACQ cover most treated pine sleepers on the Australian market, but you may occasionally come across other treatment names, particularly if you're comparing timber from different suppliers or looking at products imported from overseas.

• Copper Azole (CuAz) — another copper-based, arsenic-free preservative, broadly similar in application to ACQ, with comparable fastener corrosion considerations.
• Micronized copper formulations (MCQ, MCA) — newer variants where the copper is suspended as microscopic particles rather than fully dissolved, which can reduce fastener corrosion compared to standard ACQ or CuAz, though these are less commonly seen in Australian sleeper products.
• H2F (blue treatment) — a different treatment entirely, used for termite protection in above-ground framing timber rather than in-ground applications. Not relevant to sleepers, but sometimes confused with H-rated in-ground treatments because of the similar naming.

For a standard retaining wall project, you're very unlikely to need anything beyond CCA or ACQ H4 sleepers, but it's useful to recognise these other names if they come up while you're comparing suppliers.

FAQs

Is CCA or ACQ better for a retaining wall?

CCA H4 is the standard choice for retaining wall sleepers — it's cost-effective, widely available, and works fine with standard galvanised fasteners. ACQ is a suitable alternative but generally needs stainless steel or specifically rated fasteners.

Is CCA treated pine safe to use?

CCA treated pine is approved in Australia for applications like retaining walls, fencing and posts, but is restricted by the APVMA from close-contact uses such as decking, furniture and play equipment due to its arsenic content.

Why does ACQ corrode fasteners more than CCA?

ACQ contains a higher concentration of copper than CCA, and copper accelerates corrosion in standard steel fasteners. Stainless steel fasteners are unaffected by either treatment.

Does H4 mean the timber is CCA treated?

Not necessarily. H4 is a hazard rating for in-ground use set out in AS 1604, and can be achieved with either CCA or ACQ treatment. Check the specific product if the chemical treatment matters for your project.

Can I use standard galvanised brackets with treated pine sleepers?

Standard galvanised fasteners are generally suitable for CCA-treated sleepers. For ACQ-treated sleepers, stainless steel or specifically rated fasteners are the safer choice due to increased corrosion risk.

Is it worth paying extra for ACQ if I'm just building a retaining wall?

Usually not. ACQ's main benefit is close-contact safety, which isn't relevant to a retaining wall sleeper. CCA H4 is generally the more cost-effective and practical choice for this specific application.