Finger-jointed robinia decking and cladding – stable and durable
Robinia series · Need: “I’m laying a deck / fitting cladding”
The European timber made for outdoor use, in a version that no longer moves: finger-jointed.
For decking or cladding that lasts, robinia — also known as black locust, and often sold as acacia (Robinia pseudoacacia) — ticks two boxes that no other European timber combines: it is naturally suited to contact with water and soil, with no treatment at all, and it does not stain façades.
In its finger-jointed form — pieces cut free of defects and then glued end to end — it gains what solid timber lacks: stability. Consistent boards up to 6 m long, strength verified in the laboratory, and maintenance that comes down to… nothing compulsory. Here is how to choose it and install it.
88 N/mm² in bending
the mean bending strength measured on our finger-jointed timber at the LEMSC laboratory of UCLouvain (tests of November 2024, report DE042089)
Why finger-jointed rather than solid?
Robinia rarely grows straight, so long, straight sections are precious. Finger-jointing gets round this natural limit: short pieces, cut free of their defects (knots, grain deviation), are joined end to end with glued finger joints. The result is a profile that is stable up to 6 m and barely moves any more — whereas a long solid board risks bowing or twisting over time.
In a school playground in Montpellier, balustrades were built from finger-jointed decking boards: more expensive than solid timber, but with no risk of the barriers distorting year after year. It is the benchmark choice for demanding structures.
Tested in the laboratory: 88 N/mm² in bending
In late 2024, six finger-jointed Acacia Bois boards were tested at the LEMSC laboratory of UCLouvain (Université catholique de Louvain, Belgium) in four-point bending to EN 408:2010 — test report DE042089. Result in the dry state: a mean breaking strength of 88.4 N/mm², up to 90.6 N/mm², for a measured density of 763 kg/m³.
The laboratory also tested the worst case: three boards immersed in water for 14 days. Even saturated — 17 % heavier — they still held 37.8 N/mm² in bending, far beyond anything a deck will ever have to carry.
Manufacture follows the logic of finger-jointed structural timber (EN 15497): grade A timber dried to 12 % moisture content, multiple finger joints, finger-jointing to DIN 68140, bonding with MUF (melamine-urea-formaldehyde) adhesive of class C4 to EN 12765 — the class intended for exterior exposure.
And durability? It does not change: finger-jointing assembles the same robinia heartwood, use class 4 (EN 335) without treatment. Finger-jointing improves dimensional stability; resistance to fungi and insects remains that of the original timber, from our dense, slow-grown Romanian forests.
Decking: boards and joists in the same timber
Finger-jointed decking boards come in 23 or 45 mm thickness, in widths of 80, 100 and 118 mm, and in lengths from 2 to 6 m, in three profiles: smooth, anti-slip (grooved) or rounded. Robinia joists (standard sections from 45 × 60 to 90 × 90 mm, made to measure up to 200 mm, 6 m lengths) carry the deck: using the same use class 4 timber for the substructure and the boards means the deck ages as one — no softwood joist giving way before the decking.
Product | Thicknesses | Widths | Lengths |
Finger-jointed decking board | 23 / 45 mm | 80 / 100 / 118 mm | 2 to 6 m |
Joist | 45 / 90 mm | 60 to 100 mm | 6 m |
Finger-jointed cladding | 23 mm | 60 / 80 / 100 / 110 mm | 6 m |
Robinia’s density (≈ 0.74 at 12 % moisture content) gives excellent wear resistance, even under heavy foot traffic. The anti-slip profile reduces slipperiness, and the timber stays relatively cool in the sun — two qualities valued around swimming pools.
Installation that makes a deck last
Gaps between boards. 5 to 8 mm between boards: water drains away and the timber can move freely.
Ventilation. A ventilated air gap beneath the joists prevents standing moisture — the real enemy, far more than rain.
Fall. A fall of 1 to 2 % so that water runs off rather than sitting on the deck.
Screws and pre-drilling. Stainless steel (A2/A4) is essential, and always pre-drill: robinia is slightly acidic (pH ≈ 4.5) and corrodes ordinary steel, and it is too hard to screw without a pilot hole. The details are in the reference guide “Robinia outdoors – the complete guide”.
Store it properly before installation. On pallets, boards tightly packed, keeping the bundle strapping on until installation: loose storage causes the pieces to bow. Slight lengthwise bow during storage is normal for exterior finger-jointed robinia — it is corrected during installation and has no effect whatsoever on the strength of the finger joints. Check the moisture content as soon as the timber is unloaded: a later reading no longer reflects its condition on delivery.
Cladding: three possible profiles
Finger-jointed robinia cladding (23 mm thick, widths from 60 to 110 mm) can be fixed vertically or horizontally, in consistent lengths up to 6 m. Three profiles, three looks:
Straight profile. Boards are fixed edge to edge or slightly spaced: the most understated look.
Rhombus profile, for open-joint cladding. Each board is bevelled at 15° and fixed with a gap: air circulates between the boards, water runs off the bevels and the façade breathes. It is open cladding par excellence.
Tongue-and-groove profile, for a solid wall. Each board locks into the previous one: no gaps, a closed wall that shelters from wind and prying eyes.
Whatever the profile, robinia is low in tannins, far lower than oak or sweet chestnut: no black streaks running down the façade after rain.
Where finger-jointed timber doesn’t win
To be complete: finger-jointed timber costs more than solid — removing defects and gluing have a price — and considerably more than pressure-treated pine. Over long lengths, slight bowing is possible: it is corrected during installation by fixing the board progressively. Finally, the colour varies from one piece to the next — it is a living material — and evens out as the timber greys naturally.
Maintenance: nothing compulsory
Left unfinished, robinia greys on the surface — a purely aesthetic effect; its strength is unaffected. To keep the honey colour, a penetrating wood oil every 2 to 3 years is enough. No oil is needed for the durability of the timber.
Key points
· Finger-jointed timber is more stable than solid: it doesn’t twist — and its durability is identical (same heartwood).
· Tested at UCLouvain (report DE042089, November 2024): 88.4 N/mm² dry, still 37.8 N/mm² after 14 days in water.
· Decking: boards 23/45 mm × 80–118 mm, 2 to 6 m — 5–8 mm gaps, ventilation, 1–2 % fall, pre-drilled stainless steel screws.
· Cladding: straight profile, 15° rhombus (open-joint) or tongue and groove (solid wall), widths 60–110 mm, length 6 m.
· No treatment, no compulsory oil: the timber greys, the strength remains.
Frequently asked questions
Is finger-jointed timber solid timber?
No. Finger-jointing glues defect-free pieces of timber end to end, joined with multiple finger joints. The result is profiles — decking boards, cladding, joists — that are more stable than solid timber, with the same natural durability, since it is the same robinia heartwood.
Is finger-jointed timber as strong as solid?
Yes. Four-point bending tests carried out at UCLouvain to EN 408:2010 (report DE042089) measured a mean of 88.4 N/mm² dry on Acacia Bois finger-jointed robinia — and still 37.8 N/mm² after 14 days’ immersion in water. The class C4 MUF adhesive (EN 12765) is designed for exterior use, and use class 4 durability belongs to the timber itself: finger-jointing changes nothing there.
What is the difference between the rhombus (open-joint) profile and tongue and groove?
The rhombus profile is bevelled at 15° and fixed with open joints: air circulates between the boards and water drains off the bevels. The tongue-and-groove profile interlocks to form a closed wall. Both can be fixed vertically or horizontally, in finger-jointed lengths up to 6 m.
What gap should you leave between decking boards?
5 to 8 mm. This gap lets water drain away and the timber move with the seasons. Add a ventilated air gap beneath the joists and a fall of 1 to 2 %: it is standing moisture that wears out a deck, not rain.
Does robinia warp? How do you prevent cupping on a deck?
Let’s be frank, because online forums still carry accounts of warped robinia decks: yes, solid robinia is a lively timber that moves as it dries. That is precisely why our boards are finger-jointed — cut free of the defects that cause twisting — offered in sensible widths (80 to 118 mm: the wider a board, the more it cups) and screwed through the face, pre-drilled, rather than clipped. Add the 5–8 mm gap, ventilation under the deck and the fall, and the deck stays flat. The slight lengthwise bow sometimes visible in storage is corrected during installation: it is not cupping.
Which joists should you use under a robinia deck?
The rule is simple: the substructure must last at least as long as the decking. Under use class 4 boards, fit use class 4 joists: our robinia joists (standard sections 45 × 60 to 90 × 90 mm, 6 m lengths) are made for this, and aluminium is the proper alternative. Avoid bargain treated softwood: it will be the first to give way, under decking that still had twenty years ahead of it.
Further reading
Reference guide: “Robinia outdoors – the complete guide”.
See also: “Robinia pergolas – the complete guide from choice to installation” (finger-jointed timber for long spans) · “Where our robinia comes from” (dense forests, slow growth).
Our products: Finger-jointed decking board · Finger-jointed cladding board · Finger-jointed joists · Edged boards.
Contact: Planning a deck or cladding? Send us your areas and quantities: the sawmill will draw up the cutting plan and the quote.
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.
© Xylo Impact SRL — all rights reserved, products marketed under the Acacia Poles brand. Any reproduction, even partial, is prohibited without prior authorisation.
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