Robinia or pressure-treated pine (use class 4) – the comparison

Robinia series · Authority: “choosing between robinia and treated timber”

Two “use class 4” timbers with very different stories: one is injected, the other grows that way.

Use class 4 pressure-treated pine (often sold as “tanalised”) really does exist. But durability obtained by injecting biocides is not natural durability: in ground contact, reckon on around 10 years for a treated pine stake, against about 30 years for robinia — also known as black locust, and often sold as acacia — with no treatment at all.

And be wary of labels: “pressure-treated Douglas fir, use class 4” is doubtful, because Douglas fir barely takes up treatment. Here is the comparison, point by point.

~30 years against ~10

indicative service life in ground contact: untreated robinia against pressure-treated pine

Robinia stakes in stock under a shed: use class 4 is in the timber, not in a treatment.

How pressure treatment works — and why everything depends on the species

Pressure treatment impregnates timber with biocidal salts (mainly copper-based) under vacuum and then under pressure. The crucial point: the preservatives only penetrate the sapwood. Scots pine, with its thick sapwood, takes treatment well: properly treated, it can genuinely reach use class 4. Douglas fir, fir and spruce, on the other hand, resist impregnation — a few millimetres of penetration — and in practice top out at use class 3, whatever the label says. Faced with “pressure-treated Douglas fir, use class 4”, ask for the treatment certificate.

Another consequence: as soon as a cut, notch or split goes through the impregnated zone, the untreated core is exposed. The durability of pressure-treated timber is an envelope; that of robinia runs right through the wood.

Robinia: use class 4 from birth

The heartwood of robinia (Robinia pseudoacacia) is naturally durability class 1–2 (EN 350) and is used in use class 4 (EN 335) without any treatment: its flavonoids (robinetin) and the blocking of its vessels by tyloses protect it from within. Nothing to inject, nothing to leach into the soil — an argument that carries weight in organic farming, in viticulture and wherever timber touches soil or water.

Head to head, point by point

Criterion	Robinia	Pressure-treated pine
Service life in the ground	~30 years	~10 years
Source of durability	Natural, right through the wood	Biocides injected into the sapwood
Notches and cuts	No effect on durability	Expose the untreated core
Contact with soil / water / organic	No chemicals	Salts may leach out
End of life	Natural timber — ends up as firewood	Treated waste — must not be burnt
Purchase price	Higher	Cheaper
Cost per year of service	Favourable (30 years)	Unfavourable (replacements)
Appearance	Honey-coloured, greys without treatment	Green/brown, also greys

Over the life of a fence, the difference in purchase price reverses: a stake replaced every 10 years costs two to three times its value in materials and labour over 30 years. It is the economics of the whole installation you should compare, not the unit price.

End of life, the forgotten point. A robinia stake at the end of its service is still simply wood: it ends up as firewood and gives its last kilowatt-hours in your stove. A pressure-treated stake, never. Burning timber treated with copper salts gives off toxic fumes and leaves contaminated ash — it is prohibited, including in a stove or a garden fire pit; it must go back through the treated-waste stream. Thirty years after purchase, one heats the house, the other is waste that has to be collected.

Pressure-treated pine against robinia: where durability comes from, in one image — the cross-section, the service life in the ground and the three reasons why robinia heartwood holds out.

Where pressure-treated pine still scores

To be fair: treated pine is cheaper to buy, available in every DIY store, lighter to handle and supplied in very consistent sections. It is also more stable: dried, sized pine twists little, whereas solid robinia — a lively timber — can bow as it dries (finger-jointed timber solves this problem, but at a price). For long, straight lengths, rails or heavy boards, treated pine therefore keeps a practical advantage — with one proviso: several European countries are regulating the use of biocide-treated timber in certain sensitive applications more and more strictly. For short-term, above-ground or temporary use, it may be enough. It is in ground contact, over time, and whenever chemistry becomes an issue, that robinia has no European rival.

Key points

· Pressure-treated Scots pine: use class 4 is possible (thick sapwood, takes treatment well) — but ~10 years in the ground in practice.

· Douglas fir, fir, spruce: resist impregnation → use class 3 in reality; “pressure-treated Douglas fir, use class 4” needs justifying.

· Robinia: natural use class 4 right through the wood (EN 335, durability class 1–2 EN 350), ~30 years in the ground, zero chemicals.

· Any cut exposes the untreated core of pressure-treated timber; robinia can be recut without losing its durability.

· End of life: robinia ends up as firewood; pressure-treated timber must never be burnt (toxic fumes) — dedicated waste stream.

· Compare the cost per year of service, not the purchase price.

Frequently asked questions

Can pressure-treated Douglas fir really be use class 4?

It is very doubtful. Douglas fir resists impregnation: the salts only penetrate a few millimetres, and in practice the treatment tops out at use class 3. Only Scots pine, with its thick sapwood, takes up enough to aim for use class 4. Faced with a “Douglas fir, use class 4” label, ask for the treatment certificate.

How long does a pressure-treated pine stake last?

In ground contact, around 10 years in real conditions — that is the replacement cycle seen in farm fencing. Untreated robinia lasts about 30 years in the same conditions; sweet chestnut, about 15 years.

Is robinia more expensive than pressure-treated pine?

To buy, yes. Per year of service, no: one stake that lasts 30 years works out cheaper than three 10-year stakes, not counting the labour for replacement and for refitting the fences. Over the life of a plot, the difference clearly reverses.

What happens to pressure-treated timber at the end of its life?

It is treated-wood waste: it must not be burnt in the open or chipped for mulch, and it belongs in dedicated collection streams. A naturally durable timber such as robinia, by contrast, remains simply wood at the end of its life.

Further reading

Reference guide: “Robinia outdoors – the complete guide”.

See also: “Robinia outdoors – the complete guide” (section on treated timber) · “Round, sawn or split stakes – which to choose (sapwood and durability)”.

Our products: Agricultural posts · Round stakes · Large posts · Cleft palings.

Contact: Want a fence priced in robinia rather than treated pine? Send us the lengths: the quote also compares the cost over 30 years.

Disclaimer. Non-contractual article, provided for information only (photos and drawings included). Robinia is a natural material: the values given are orders of magnitude and may change without notice; in case of discrepancy, only the product’s technical data sheet and the general terms of sale apply. Always check the regulations that apply to your project.

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