Class 4 pressure-treated or acetylated wood: which to choose in the Caribbean?

What “class 4” means, and what it leaves out

Standard EN 335 sorts wood uses into classes. Class 4 (UC4) covers wood in permanent contact with the ground or freshwater: posts, low decking, decks on pedestals. Under our rains, with soil that stays damp and termites active all year, it is the sensible threshold for a deck in Guadeloupe or Martinique.

The class says where the wood can be used. It does not say how it ages, whether it warps, or what it contains. Two UC4 woods can behave very differently over ten years.

First route: chemical impregnation of pressure-treated pine

Pine is a softwood, not durable in its natural state. It goes into an autoclave: under pressure, a preservative is forced into the timber. That product gives it resistance to fungi and insects, and lets it reach class 4. The process is old and well mastered, and it explains pressure-treated pine’s main selling point: it is clearly cheaper to buy than the other options.

Its downside lies in the material itself. Pine remains a wood that moves: cupping, splits, boards that twist after a few rain-and-sun cycles. The treatment does not always reach the core, so a cut on site exposes less protected wood that must be treated again. And the protection comes from an added substance, which calls for care in handling and will weigh on recycling.

Second route: the acetylation of Accoya®

Accoya® starts from a fast-growing wood from sustainably managed forests, which the manufacturer Accsys treats with acetic anhydride, a vinegar derivative. The reaction changes the wood’s chemistry through its whole thickness: it absorbs far less water, so it swells and shrinks far less, and the organisms that feed on wood no longer recognise it as food. It reaches class 4 “thanks to its natural durability of class 1” under standard EN 350, the top one, with no added preservative.

In practice, a board cut with a saw on site stays protected on its edge, since the modification runs through the whole timber. And the stability shows after installation: joints that stay even, boards that stay straight.

The two side by side

• Purchase price: pressure-treated pine is cheaper. Accoya Color Grey is €110 incl. tax per m², all sizes
• Stability: pine moves more; Accoya keeps its joints and flatness
• Protection: an added product for pine, a modification of the wood itself for Accoya
• Cuts on site: to be re-treated on pine, not an issue on Accoya
• Warranty: the manufacturer Accsys’s for Accoya, up to 50 years above ground (25 years in ground or freshwater contact), subject to installation and use conditions, transferable if you sell

So which one?

Pressure-treated pine holds up for a small project on a tight budget, a hidden structure, or a use where you accept re-treating and replacing a few boards. Accoya makes sense when you want a deck that keeps its look, around a pool, in front of a rental villa, or anywhere you would rather pay once than maintain continuously. Our full materials comparison also weighs hardwoods and composite.

If you are unsure, order your boards by the m² or ask for a free site visit: installation is quoted, with warranties set out in writing.