Robinia or oak – the comparison

Ferdinand Terlinden

1 day ago

8 min read

Robinia series · Comparing species

Europe's noble forest timber set against its most durable hardwood.

Oak is Europe's great building timber: centuries-old roof frames, floors, barrels. In ground contact, however, it is robinia (Robinia pseudoacacia) — also known as black locust, and often sold as acacia — that lasts longest: around 30 years for a stake, against five to ten years for a comparable oak stake.

EN 350 places robinia heartwood in durability class 1-2; since its 2016 revision, it puts the heartwood of European oak (Quercus robur, Quercus petraea) between classes 2 and 4. Here is what that gap changes, and where oak keeps the advantage.

~30 years against 5 to 10

indicative service life of a stake in ground contact: robinia against oak

Pasture fence of split robinia stakes and wire mesh, holding up in ground contact — A split robinia stake in ground contact: it is at ground level, and nowhere else, that the gap with oak opens up.

Durability: what do the standards really say about oak and robinia?

EN 350 rates the natural durability of timber against fungi from 1 (very durable) to 5 (not durable). Robinia heartwood — the core, the mature wood — is rated 1 to 2: it is the only European hardwood that can be used in use class 4 (EN 335), in permanent ground contact, without treatment.

European oak is more variable than its reputation. The old EN 350-2:1994 simply rated it "durable", class 2; the 2016 revision describes the heartwood of Quercus robur and Quercus petraea across a range from class 2 to class 4 (slightly durable), depending on provenance and on the individual tree. A robinia stake can therefore be guaranteed for use class 4; an oak stake never can.

The myth of everlasting oak: why did old posts last so long?

Everyone has in mind a telegraph pole or an oak sleeper still standing after fifty years. Three explanations account for it, and none of them applies to a stake bought today.

First, the quality of the wood — and this is the main explanation. Those pieces came from dense high forest, grown close for a century and a half or two: slow growth, narrow rings, dense wood, and heartwood largely formed by the time of felling. Today's oak, harvested younger from stands thinned to grow fast, does not have those characteristics. It is the same tree, but not the same wood. Treatment, by contrast, explains less than people think in the case of oak: in France, beech and pine sleepers were impregnated with creosote, while oak was commonly laid untreated, its natural durability being enough.

Next, the cross-section: a 25 x 25 cm beam can lose several centimetres at its outer edge without failing, whereas an 8 cm stake has no reserve at all. Finally, exposure: a farm roof frame works under cover, in use class 1 or 2, where almost any timber lasts for centuries. In ground contact and in small sections, oak is given five to ten years.

Sapwood: for the same cross-section, how much truly durable wood?

Sapwood (the pale ring of young wood at the outside of the trunk) is not durable in any species: EN 350 rates it class 5 across the board. So the question is not "which species" but "what share of the cross-section is heartwood".

Oak has a wide, clearly defined band of sapwood, often several centimetres, which must be removed entirely or the piece will rot from the outside in. The width of that sapwood also depends on growth rate: an oak grown fast in a thinned stand carries noticeably more of it, in centimetres, than a narrow-ringed oak from dense high forest. Robinia has only a thin sapwood of a few growth rings: on a 12 cm round, the difference is decisive. Our article on round, sawn or split stakes covers this in detail.

Stack of split robinia stakes seen end-on, dark heartwood and a narrow pale ring of sapwood — Split stakes seen end-on: robinia sapwood — the pale ring around the edge — is narrow, so for the same cross-section more durable wood remains than in oak.

Mechanics and tannins: hardness, run-off stains and black marks around screws

On density, the two species are close: around 740 kg/m³ for robinia and 650 to 750 for oak, at 12 % moisture content. The gap widens on hardness, that is, resistance to indentation. The French Monnin test (NF B 51-013) gives around 9.5 for robinia against around 4.2 for oak (CIRAD values). On the Janka scale, more familiar to English-speaking readers, robinia reaches about 7,560 N against about 4,980 N for oak: it is about 50 % harder.

A word on the Monnin figure, because it is often misread: it is not a mark out of ten. The Monnin hardness test (standard NF B 51-013, also known as the Chalais-Meudon test) presses a steel cylinder 30 mm in diameter into the wood under a standard load, then measures the impression left behind. The value N is the inverse of the depth of that impression in millimetres: the deeper the cylinder sinks, the smaller the number. The scale is therefore open-ended and does not stop at ten.

The usual benchmarks in the French timber trade: below 1.5 for very soft woods such as poplar, 1.5 to 3 for softwoods, 3 to 6 for medium-hard hardwoods — this is where oak sits, at around 4 — 6 to 9 for hard woods, and above 9 for very hard woods, the category of tropical decking timbers. At around 9.5, our robinia falls into this last class. In practice, that means a surface that does not dent under a heel, a hoof or a wheelbarrow wheel, a decking board that does not wear hollow along the walkways, and pre-drilling before every screw — the other side of the coin, covered in detail in "Screws and fixings for robinia".

Above all, oak is a very tannin-rich wood: its hydrolysable tannins, the ellagitannins, make up about 10 % of its dry mass, against 2 to 5 % for timber on average. Water carries them to the surface and deposits them as brown-black run-off streaks on render and stone. These tannins also corrode ordinary steel: the iron released combines with them to form blackish iron tannates, which stain the wood around fixings. Robinia is acidic too (pH around 4.5) and requires the same stainless steel (A2/A4) — see our article on screws and fixings — but it produces practically no run-off.

Where oak wins, clearly

Traditional timber framing is oak's territory, and robinia makes no claim to it: oak yields straight, large-section beams six, eight, ten metres long. Oak also dominates interior joinery, flooring and cabinetmaking, where its even appearance is the benchmark, and cooperage belongs to it — stave wood, those staves split along the quarter, exists only in oak. Add to that a strong timber industry — in France, oak is the country's leading hardwood species — and a unique heritage value.

Robinia, for its part, has limits that should be stated. The tree is harvested at 25-30 years for diameters of 15 to 30 cm and grows crooked: long straight lengths are rare and are obtained by finger-jointing. Coming out of our sawmill in Teiu, Hunedoara County (Romania), robinia keeps a more irregular look than oak, and this hard wood machines less well for fine joinery.

Price and availability: what are we really comparing?

Oak is a major European resource, but one in heavy demand: cooperage, joinery, flooring and export take up the fine straight logs. The wood left over for a stake or a landscape edging is therefore neither cheap nor the best oak.

The right unit of comparison remains the cost per year of service: at around 30 years against five to ten, you need to install three to six oak stakes for one robinia stake, labour included — the calculation is set out in our article "The price of robinia explained". Out of ground contact the gap narrows, and oak, sweet chestnut or a modified wood remain defensible choices.

Robinia log yard at the Acacia Bois sawmill in Teiu, sorting logs before sawing — The sawmill log yard: robinia is harvested at around 25-30 years for diameters of 15 to 30 cm, whereas a fine oak log is counted in centuries.

Criterion	Robinia	European oak
Heartwood durability (EN 350)	Class 1-2	Class 2 to 4 (since 2016)
Use class 4 without treatment	Yes, the only European hardwood	Not guaranteed
Stake in ground contact	~30 years	5 to 10 years
Sapwood	Thin	Wide, not durable (class 5)
Density at 12 % moisture content	≈ 740 kg/m³	≈ 650-750 kg/m³
Hardness: French Monnin test (Janka)	≈ 9.5 (Janka ≈ 7,560 N)	≈ 4.2 (Janka ≈ 4,980 N)
Tannins	Hardly any run-off	Run-off and iron tannates
Long straight lengths	Rare, finger-jointing	Excellent

Key points

· EN 350: robinia heartwood rated 1-2, European oak heartwood rated 2 to 4 since 2016.

· In ground contact, a robinia stake lasts around 30 years, an oak stake five to ten years.

· Old oak owes its longevity to slow growth in dense high forest, large cross-sections and use under cover — not to treatment.

· Oak sapwood is wide and not durable (class 5): for the same cross-section, there is less durable wood than in robinia.

· Oak remains superior for timber framing, interior joinery, flooring and cooperage.

Frequently asked questions

Isn't oak rot-proof?

No timber is rot-proof, and oak is no exception. EN 350 puts European oak heartwood between classes 2 and 4 depending on provenance and the individual tree, and its sapwood in class 5. Out of ground contact, an oak roof frame lasts for centuries; in permanent ground contact, an oak stake lasts five to ten years. Robinia, rated 1-2, is the only European hardwood that can be used in use class 4 without treatment.

Why did old oak sleepers and posts last so long?

For three reasons, and treatment is not the main one. First, those pieces came from dense high forest: a century and a half of slow growth gives narrow rings, dense wood and well-formed heartwood, a far cry from a plantation oak harvested young. Many also had huge cross-sections, able to lose several centimetres at the outer edge without failing. Finally, old roof frames work sheltered from the rain, an exposure that has nothing in common with ground contact. In France, oak sleepers were in fact commonly laid untreated: it was beech and pine that had to be creosoted.

Does oak turn black around screws and ironwork?

Yes, it often does. Oak is rich in hydrolysable tannins, the ellagitannins, at about 10 % of its dry mass. In the presence of moisture, these tannins corrode ordinary steel; the iron released combines with them to form blackish iron tannates, which stain the wood around the fixing. Stainless steel screws are the only reliable answer. Robinia, also acidic, requires the same stainless steel but produces practically none of these stains.

Oak or robinia for an outdoor deck?

Both species hold up out of ground contact. Robinia has the edge on hardness — about 50 % harder than oak on the Janka scale — on heartwood durability rated 1-2, and on the absence of tannin run-off. Oak offers in return a more even appearance and longer lengths. In both cases, use class 4 joists and stainless steel screws are essential.