Screws and fixings for robinia

Ferdinand Terlinden

1 day ago

12 min read

Robinia series · Installing and assembling

A2 or A4 stainless steel, hardened steel with a C4 coating, pilot holes at 70 % of the diameter: installation values for a timber at 740 kg/m³.

Robinia (Robinia pseudoacacia) — also known as black locust, and often sold as acacia — is an acidic timber, with a pH of around 4.5, and its extractives — tannins and organic acids — react with iron. Two families of fixings hold up in these conditions: stainless steel, A2 (EN ISO 3506, equivalent to 304) for the great majority of jobs and A4 (equivalent to 316) wherever there are chlorides in the air; and hardened steel protected by a class C4 anti-corrosion coating, which we use ourselves for joinery. What does not hold up is plain zinc-plated steel. And the two families are never mixed in the same damp area, or galvanic corrosion follows.

Robinia has a density of around 740 kg/m³: pre-drilling is a must. Eurocode 5 (EN 1995-1-1) requires a pilot hole for every screw driven into hardwood and sets its diameter at about 70 % of the screw diameter, i.e. 3.5 mm for a 5 mm screw. With that pilot hole, the same screw goes in without effort and holds for thirty years.

3.5 mm for a 5 mm screw

the pilot-hole diameter in robinia: about 70 % of the screw diameter (EN 1995-1-1)

Detail of a bolted robinia joint with stainless-steel washers on a pergola, seen from below — A bolted stainless-steel pergola joint, with large washers at the bearing faces: in a timber at 740 kg/m³, it is the pre-drilled pilot hole that prevents splitting and snapped screws.

Why are stainless-steel screws essential in robinia?

Robinia is an acidic timber, with a pH of around 4.5: its extractives attack iron as soon as the joint is damp. The zinc on a galvanised screw is used up within a few months; the exposed steel rusts and, with the extractives, forms iron tannate — the black halo that runs down beneath every screw head. Eurocode 5 specifies stainless steel as the minimum protection for joints in service class 3, the class for exposed timber — the situation of robinia in use class 4 (EN 335). Like oak, sweet chestnut and western red cedar, robinia belongs to the timbers with a pH below 5, for which stainless steel is the obvious specification. That is the reference for specifications, and it is what we recommend by default for public contracts and structural work. There is a second route, described next, which we prefer for joinery.

A2 or A4 stainless: which to choose, and at what extra cost?

EN ISO 3506 defines the two grades: A2 corresponds to 304 stainless, A4 to 316 stainless, which adds 2 to 3 % molybdenum and raises the pitting resistance equivalent number from 18–20 to 23–28. A2 is enough inland; A4 is essential by the sea, around swimming pools and in livestock buildings with ammonia. The extra cost is real: around 1.3 to 1.5 times the price of A2, a market order of magnitude rather than a standardised value.

What we actually screw in the workshop

We have tried a great many screws in robinia, and the workshop's findings do not quite match the catalogues. Ordinary zinc-plated steel is soft: it drives well, but it rusts within a few months in an acidic timber, and once corroded it snaps when the wood moves. Standard A2 stainless — off-the-shelf stainless screws and their equivalents — does not rust, but it snaps too, for the opposite reason: austenitic stainless steel is not hardened, its torsional strength is low, and in a timber at 740 kg/m³ you twist or shear the screw before it is home. Two opposite failures, one symptom.

The compromise we have settled on for our garden gates and all our outdoor joinery is a case-hardened steel screw with an anti-corrosion coating: the Dynaplus Unischroef AR-coating (AR = anti-rust), countersunk Torx TX-25 head, 5.0 × 60 mm with a partial thread. Its advantage comes down to two points, both documented by the manufacturer. First, the metallurgy: the skin of the screw is hardened to take the driving torque, while the core stays ductile to cope with the movement of the wood — Dynaplus states a bending angle of at least 65° without fracture and a breaking torque of around twice that of an A2 stainless screw of the same diameter, i.e. 7.3 N·m at Ø 5 mm. That is exactly what austenitic stainless steel lacks in a dense hardwood. Second, the protection: a duplex coating — electroplated zinc followed by three organic layers, 15 µm minimum — rated C4, which the manufacturer explicitly approves for species with a pH below 5, where it firmly advises against zinc-plated screws.

The partial thread matters as much as the steel. On a 60 mm screw threaded over 35 mm, the 25 mm of smooth shank passes freely through the upper piece: the screw pulls the two pieces together instead of pushing them apart. That is the detail that keeps a gate tight. The reasoning does, however, assume an upper piece no thicker than 25 mm; beyond that, the thread bites into both pieces and the clamping effect disappears — in that case use a longer screw, not a fully threaded one.

Three caveats, because they need saying. This is not stainless steel but protected steel, and a coating can be scored during driving: hence systematic pre-drilling at 3.5 mm for a 5 mm screw, which the manufacturer also recommends in hardwoods. The same manufacturer gives no guarantee in coastal areas, where A4 stainless remains the only answer. And a specifier will call for stainless steel for timbers with a pH below 5: for a public contract or structural work, stay with stainless. Our workshop experience applies to everyday outdoor joinery; it does not replace a specification.

Finally, there is a third route, the most expensive: hardened austenitic stainless steel, offered by a few manufacturers specialising in dense hardwoods and robinia. It combines torque resistance with corrosion resistance, and it is what the German suppliers who specialise in this species recommend. If the budget allows and the structure is exposed, it is the best of both worlds.

Should robinia be pre-drilled, and at what diameter?

Pre-drilling is not negotiable. Eurocode 5 makes a pilot hole mandatory for every screw driven into hardwood, and for nails above a characteristic density of 500 kg/m³; robinia is around 740 kg/m³. The pilot hole is about 70 % of the outside diameter of the screw, i.e. the core diameter — the body of the screw measured at the root of the thread — over the full length of the screw, with the head seated in a counterbore.

For a load-bearing joint, Eurocode 5 requires an end distance — from the cut end of the piece — of at least 7 times the diameter with a minimum of 80 mm, an edge distance of 3 times the diameter, and spacing of 4 to 5 times the diameter. On a non-load-bearing piece, trade practice is 5 times the diameter from the edge and 20 to 25 mm from the end.

Which fixing for which joint?

Four families cover almost every job: stainless-steel wood screws, coach screws — large hexagon-head screws driven with a spanner — through bolts with large washers for load-bearing points, and threaded rods set in resin for base plates on concrete. A screw should penetrate at least 6 times its diameter into the receiving piece; a bolt takes a washer at least 3 times the rod diameter, in a hole no more than 1 mm larger than the rod.

Joint	Stainless-steel fixing	Size guide
23 mm decking board on joist	Countersunk screw, twin thread	5 × 60 mm, 2 per bearer, 3.5 mm pilot hole
23 mm cladding on batten	Countersunk screw or ring-shank nail	4.5 × 60 or 2.8 × 60 mm, 3 mm pilot hole
12–14 mm shingle on batten	Flat-head ring-shank nail	2.5 × 50 mm, 2 nails per shingle
Rail on post	Hexagon-head coach screw	8 × 100 to 10 × 140 mm, 5.5 to 7 mm pilot hole
Beam on pergola post	Through bolt, large washers	M10 or M12, hole = diameter + 1 mm
Base plate on concrete slab	Threaded rod, chemical anchor	M10 or M12, embedment as per resin
Joist hanger or post base	Connector screws or coach screws	5 × 40 to 8 × 80 mm, 3.5 to 5.5 mm pilot hole

The through bolt: the most reliable fixing, and the only one you can take apart

On anything load-bearing, the standard through bolt — an M10 or M12 bolt, nut and two large washers — remains the safest fixing for robinia, for a reason that is often forgotten: it takes no torque during installation. Unlike a screw, you cannot snap it by tightening it. Its strength is that of the rod, not of a thread cut into the wood; it can be fully inspected; and above all, it can be retightened. On a timber that shrinks noticeably in its first year, that is decisive: a quarter turn after the first summer takes up the play, whereas a screw that has worked loose cannot be recovered.

The second advantage is practical: a bolt can be undone. Our tables, benches and furniture are assembled this way, and it makes life much easier for whoever manages them — a picnic table can be flat-packed for the winter or moved from one site to another, a damaged top can be replaced without touching the legs, a structure can be dismantled instead of cut up. A screwed joint, on the other hand, can only be undone once: the second time, the thread no longer bites.

The installation rules fit in a few lines. Drill to the rod diameter, with no more than one millimetre of clearance: beyond that, the play makes the joint work. Washers at least three times the rod diameter under the head and under the nut — robinia is hard, but it crushes across the grain, and an undersized washer sinks in and loses the clamping force. A lock nut or lock-nutted pair, because a bolt that works comes undone. Eurocode 5 spacings: five times the diameter between bolts, seven times at a loaded end, three times at an unloaded edge. And a single family of metal throughout the structure, washers and nuts included: preferably A4 stainless — hot-dip galvanised steel holds mechanically, but the zinc reacts with the acidity of the wood and leaves black streaks on visible faces.

Pergolas and carports: which structural hardware?

Structural timber hardware — joist hangers (the U-shaped stirrups that take the end of a beam), base plates, post bases — is mostly galvanised: on exposed robinia, look for the stainless-steel versions, because two different metals in the presence of water form a cell — this is galvanic corrosion. Post bases follow a single rule: never a cup that holds water. A raised post base lifts the timber 5 to 10 cm clear, and a post set directly in concrete should finish with a sloping top that sheds water.

Acacia (robinia) pergola being assembled, post-and-beam structure fixed with stainless-steel bolts — Assembling an acacia pergola: it is the through bolts at the load-bearing points that are checked again after the first summer — screwed joints cannot be retightened.

Robinia decking: how should the boards be screwed?

Finger-jointed robinia decking boards are face-fixed with countersunk, twin-thread stainless-steel screws: the upper thread pulls the board down onto the joist. For our 23 mm boards, the usual choice is a 5 × 60 mm screw, two per bearer, with a 3.5 mm pilot hole drilled right through, 20–25 mm from the ends and edges. Hidden clips require boards grooved on their edges: on a timber that moves this much, face-fixing is still the recommended method.

Shingles and cladding: ring-shank nails or pre-drilled screws?

Robinia shingles — 45 cm long, 12 to 14 mm thick — are fixed with stainless-steel ring-shank nails, whose rings markedly improve pull-out resistance. Diameter 2.5 mm, length 45 to 50 mm: the nail passes through two thicknesses of shingle and still penetrates at least 20 mm into the batten, the horizontal lath that carries the covering. Two nails per shingle, 2 or 3 cm from the edges and above the exposure line. Solid-board cladding works differently: it is screwed rather than nailed, and each board is pre-drilled before screwing. In a timber at 740 kg/m³, a screw driven without a pilot hole splits the board at the end or snaps at the head, and the ventilated vertical counter-batten cannot make up for it.

Vertical solid robinia board cladding on a gable end, boards pre-drilled before screwing — Solid robinia board cladding: each board is pre-drilled before being screwed to its battens — without a pilot hole, the screw splits the wood or snaps at the head.

Which mistakes should be avoided, and what should be checked after a season?

An impact driver set too high is the leading cause of snapped screws: A2 and A4 stainless screws are ductile, not hardened, and in a timber at 740 kg/m³ the torque builds quickly. Next come screws placed too close to the end, missing pilot holes, mixed metals, and tightening hard on wood that is still damp — the piece shrinks as it dries, the bolt works loose or the washer sinks in and starts a split.

Hence the follow-up: bolted joints are checked after the first summer, then once a year, a quarter turn at most; screwed joints cannot be retightened. It also has to be said plainly: installing robinia costs more than installing pressure-treated pine. Pre-drilling adds a drilling and a counterboring step at every fixing point and roughly doubles installation time, and stainless steel costs far more than zinc-plated. It is a real line in the quote. Greying and maintenance are covered in a dedicated article, “Greying and maintenance of robinia”.

Key points

· Acidic timber (pH around 4.5): stainless steel, or hardened steel with a C4 anti-corrosion coating — never plain zinc-plated steel, which blackens the wood within a few months.

· A2 (EN ISO 3506, equivalent to 304) inland; A4 (equivalent to 316) by the sea and in livestock buildings.

· Pilot hole at about 70 % of the diameter: 3.5 mm for a 5 mm screw, 5.5 mm for an 8 mm coach screw.

· 23 mm boards: 5 × 60 mm countersunk stainless-steel screws, two per bearer, 20–25 mm from the ends and edges.

· A2 stainless does not snap because it is brittle but because it is not hardened: its torsional strength is low in a dense hardwood. Pre-drill and drive slowly, or use hardened stainless.

· The through bolt remains the most reliable fixing: no torque during installation, can be retightened, and the only truly demountable joint — hence its use on our tables and benches.

· Bolted joints are retightened after a drying season; screwed joints are not.

Frequently asked questions

A2 or A4 stainless-steel screws: which should I choose for robinia?

A2 stainless (EN ISO 3506, equivalent to 304) suits the great majority of robinia jobs inland: decking, pergolas, fencing, cladding. A4 stainless (equivalent to 316, with 2 to 3 % molybdenum) is essential wherever there are chlorides or ammonia: by the sea, around swimming pools, in livestock buildings. At the same property class, A2-70 and A4-70 have the same mechanical strength; only resistance to pitting corrosion differs, for around 1.3 to 1.5 times the price. One point product sheets rarely mention: austenitic stainless steel is not hardened, its resistance to driving torque is low, and that is why it snaps in a dense timber even with a correct pilot hole. Pre-drill, drive slowly, or move to hardened stainless — see the section on what we actually screw in the workshop.

What pilot-hole diameter for a 5 mm screw in robinia?

3.5 mm. Eurocode 5 (EN 1995-1-1) requires pre-drilling for every screw driven into hardwood and sets the pilot hole at about 70 % of the screw diameter, which amounts to drilling at the core diameter. In a timber at 740 kg/m³, drill the full length of the screw and counterbore the head. Other guides: 4 mm screw, 3 mm bit; 6 mm screw, 4.5 mm bit; 8 mm coach screw, 5.5 mm bit; 10 mm coach screw, 7 mm bit.

Can hidden clips be used on robinia decking?

Technically yes, provided the boards are grooved on their edges and the clip manufacturer approves use with a dense hardwood. In practice, face-fixing is still recommended on robinia: the wood moves a great deal from one season to the next, a clip can only resist limited pull-out, and a board that lifts is hard to put right. Our finger-jointed boards are supplied smooth, anti-slip or crowned, with no side groove: two 5 × 60 mm stainless-steel screws per bearer remain the safest solution.

Which screw should I use for a robinia gate or outdoor joinery?

We use a case-hardened steel screw with a class C4 anti-corrosion coating, the Dynaplus Unischroef AR-coating (AR = anti-rust) type, countersunk Torx TX-25 head, 5.0 × 60 mm with a partial thread, always pre-drilled at 3.5 mm. The compromise is this: a hardened skin that takes the driving torque — the breaking torque is around twice that of an A2 stainless screw of the same diameter — and a ductile core that copes with the movement of the wood, where an austenitic stainless screw shears and a zinc-plated screw rusts and then snaps. The manufacturer approves this coating for species with a pH below 5, which is the case for robinia. Two limits: it is not stainless steel, so there is no guarantee by the sea, where A4 is essential; and for structural work or a public contract, the specification remains stainless steel. Never mix stainless and coated screws in the same damp area.

Why have my galvanised screws left black streaks on the wood?

Because robinia is an acidic timber (pH around 4.5) rich in extractives. The zinc layer on a galvanised screw, a few microns thick, is used up within a few months; the exposed steel rusts, and the dissolved iron reacts with the extractives to form iron tannate, a black compound that rain carries down around each head. Oxalic acid lightens the stain, but it comes back as long as the source of iron is there. The only lasting solution: replace the fixings with stainless steel.

Should pergola bolts be retightened after the first season?

Yes. Robinia delivered at around 12 % moisture content moves with the seasons, and a solid piece installed wetter shrinks noticeably in its first year. Check the through bolts and coach screws after a full summer, then once a year: a quarter turn is enough — the aim is not to crush the wood. Screwed joints, on the other hand, cannot be retightened — tightening a screw in a thread that has already been cut makes it lose its grip. Hence bolts at the load-bearing points.