Accoya: the acetylated radiata pine for joinery and cladding

Accoya is acetylated radiata pine: FSC-certified Pinus radiata softwood chemically modified at the cell-wall level to deliver Durability Class 1 performance and near-zero movement, without using a preservative biocide. It is made under licence by Accsys Technologies at a plant in Arnhem, Netherlands, and distributed in Australia through Mathews Timber, Britton Timbers, Duce Architectural Timbers, Gowan Lea and McKay Timber (verified 2026-05-28, Accoya Australia suppliers). The result is a plantation-grown softwood that carries a 50-year above-ground warranty and a 25-year in-ground or freshwater warranty (verified 2026-05-28, Accoya warranty page).

What it is

Accoya starts as plantation-grown radiata pine, the same species behind most Australian framing pine, sourced FSC-certified from sustainably managed forests (verified 2026-05-28). It is then put through an industrial acetylation process at the Accsys plant: the timber is loaded into pressure vessels and treated with acetic anhydride (a concentrated form of vinegar), under heat and pressure, until the modification has taken across the full cross-section of the board.

The result is not a chemically protected pine like an H3-treated LOSP product, and not a coated or sealed surface treatment. The wood itself has been structurally changed at the cellular level. The reaction happens through-and-through, so a cut, planed or shaped surface has the same properties as the outer face. Nothing is left untreated when you machine it on site.

The acetylation chemistry in plain terms

Untreated softwood holds water because the cell walls are full of hydroxyl groups, OH sites that grab water molecules out of the air and hold them. That water cycling drives most of the things builders hate about external softwood: swelling, shrinking, end-grain checking, paint blistering, and the moist conditions fungi need to break the wood down.

Acetylation swaps those hydroxyl groups for acetyl groups, which do not attract water. The cell walls stop absorbing moisture, so the wood:

Swells and shrinks far less. Accsys reports the modification reduces water absorption in the cell walls by about 75 to 80 per cent compared with untreated radiata, giving the timber close to the dimensional stability of teak (verified 2026-05-28, accoya.com).
Stops feeding decay fungi and termites. Fungi cannot recognise the modified wood as food, and termites cannot digest it the way they digest untreated softwood. Accoya is rated EN 350 Durability Class 1 (the highest), good for Use Classes 1 through 4 (interior through in-ground contact) (verified 2026-05-28, Accoya warranty page).

The only by-product of the reaction is acetic acid, the same compound that makes vinegar smell sharp. Freshly machined Accoya often smells faintly of vinegar; the smell fades on exposure. No toxic biocide is introduced and none leaches out: the wood is recyclable as ordinary timber and can be used safely around water and food-preparation surfaces.

Where it fits in the build

The combination of dimensional stability and Class 1 durability makes Accoya a natural fit for the parts of a building where untreated softwood would normally fail too quickly to be worth using:

External windows and doors. This is the volume market in Australia. A timber-framed window has to hold square against the seasonal swing in humidity or its seals fail and the sashes get sticky; Accoya’s near-zero movement keeps the frame square and the gaps tight, so a window operates and seals the same in February and August. Major Australian timber-window makers including Duce and Paarhammer run production frames in Accoya for that reason (verified 2026-05-28).
External cladding. Stable, paint-friendly and Class 1 durable: Accoya cladding takes finishes well and runs to long maintenance intervals. Britton Timbers and Mathews stock cladding profiles in Accoya for architectural work (verified 2026-05-28).
Decking and boardwalks. Permitted in ground or freshwater contact for 25 years, useful for pool surrounds, jetty timber and waterside boardwalks where hardwood durability is wanted without the weight, cost or sourcing footprint of teak or merbau.
External joinery and trim. Anywhere a timber section sits exposed: fascia, soffit linings, pergola structure, garden screens, where the painted-softwood failure mode is the problem you are trying to dodge.

It is not a framing timber: too expensive for studs and plates, and there are better-priced graded options (pine framing grades) for structural concealed work.

Accoya, cedar and treated pine

	Accoya (acetylated radiata)	Western red cedar	Treated pine (H3 LOSP)
Source	Plantation radiata pine	Plantation or old-growth cedar	Plantation radiata pine
Modification	Acetylated through the full section	None	Envelope treatment, surface biocide
Durability class	Class 1 (EN 350)	Class 2 typical	Class 4 untreated; treated to H3 hazard
Dimensional stability	Very high (75 to 80 per cent less water absorption)	Moderate	Low (untreated pine baseline)
Above-ground warranty	50 years	Not standard	Not standard; treatment warranty varies
In-ground / freshwater	25 years	Not rated	Not rated (LOSP is above-ground only)
Coating life	Long; stable substrate barely cycles the film	Moderate; soft grain and knot bleed	Short to moderate; pine movement cycles film
Cost band	High (premium softwood, near hardwood)	High (cedar pricing)	Low
Cut-end treatment	None needed for durability	None	Required; brush-treat every cut on site

The practical differences are what matter on site. A treated-pine window needs every cut end brush-treated to keep its envelope intact; a western red cedar cladding needs careful detailing for its softer grain and knot bleed; an untreated pine simply fails too fast outside. Accoya removes all three issues by changing the wood itself rather than coating, treating or accepting it as-is.

Performance and warranty conditions

Accoya’s standard warranties (verified 2026-05-28, accoya.com warranties):

50 years against decay in above-ground use (EN 335 Use Class 3.1 and 3.2).
25 years against decay in in-ground or freshwater contact (Use Class 4).
Conditions: the wood must be installed and used per Accoya’s installation guidance, including stainless-steel fixings in direct contact (residual acetic acid will corrode plain steel and galvanising over time), proper ventilation behind cladding, and a coating system rated for acetylated timber.

The warranties are backed by independent durability testing (BRE in the UK, TRADA, and long-term in-ground stake tests) and by the field history of the product, in commercial use since 2007.

Buying it in Australia

The Australian Accoya supply is run through Accsys’s appointed local distributors (verified 2026-05-28, Accoya AU where to buy):

Mathews Timber (Sydney, Melbourne, Brisbane): Australia’s first Accoya distributor, since 2011.
Britton Timbers (Melbourne, Sydney, Brisbane): cladding and joinery sections.
Duce Architectural Timbers: integrated with Duce’s timber-window-and-door manufacturing.
Gowan Lea Timbers (Sunshine Coast): regional Queensland supply.
McKay Timber (Tasmania, Victoria): regional supply.

It is stocked in joinery and cladding sections rather than commodity framing sizes; expect to specify section, finish and quantity rather than walking in for a few lengths. Lead time on a non-stock section runs to weeks.

For a builder

Use stainless-steel fixings in direct contact. The residual acetic acid corrodes plain steel and standard galvanising over time. A2 (304) stainless is the general external grade; A4 (316) is the right call for coastal exposure or pool environments.
Glue with rated products. Polyurethane (PUR) and EPI adhesives bond well; verify with the glue maker that the product is rated for acetylated wood. PVA holds indoors; PUR or EPI is needed for external joinery.
Seal cut ends for finish, not for durability. No end-grain preservative is required for the warranty, but coat cut ends with the chosen finish to keep the painted look even.
Pick a coating system rated for Accoya. Because the substrate barely moves, paint films and clear coatings last far longer than on untreated pine; spec a coating manufacturer that lists Accoya on its substrate sheet.
Cost is the deciding factor. Accoya runs at hardwood-bracket pricing, much more than untreated pine and broadly comparable to cedar or merbau on a section-for-section basis. The case for it is long-life external applications where rework cost outweighs the up-front saving.