Robinia pergolas – the complete guide from choice to installation
Robinia series · Need: “building a pergola”
A pergola is an open structure, permanently exposed to rain, sun and, above all, wind. The timber that carries it has to last without heavy maintenance: that is exactly where robinia (Robinia pseudoacacia) – also known as black locust, and often sold as acacia – comes into its own. This guide covers choosing the timber, the planning rules, dimensions, joints and anchoring. For the details of why robinia resists decay without treatment, see the complete guide “Robinia outdoors – the complete guide”.
In this article
· 1. Why robinia for a pergola
· 2. Planning rules (UK and Ireland)
· 3. Getting the dimensions right (height, sections, spacing)
· 4. Standard plans and quantities – 4 plans included: 3×3 m module · 3×4 m · 6×9 m · 6 m long span
· 5. Design and joints
· 6. Wind, the governing load
· 7. Anchoring and the post base – the critical point
· 8. Maintenance: the bare minimum
· Frequently asked questions
1. Why robinia for a pergola
Durable without treatment. Use class 4: it withstands permanent exposure without stain or pressure treatment – whereas a pergola in pressure-treated pine often greys and weakens within 5 to 8 years.
Mechanically strong. Its high density and modulus of elasticity allow generous spans with reasonable sections.
No run-off stains. Unlike oak or sweet chestnut, it releases virtually no tannins: no brown streaks on the ground or walls.
Joints without metal. Pegged mortise-and-tenon joints with robinia dowels avoid visible hardware – and therefore any corrosion caused by the acidity of the wood.
2. Planning rules (UK and Ireland)
In the UK, many garden structures fall under permitted development, but height, position and listed-building or conservation-area status can change that – check with your local planning authority before you build.Permitted development: many pergolas can be put up without a planning application – but the conditions depend on the structure and where it stands.Planning permission: if your project falls outside those conditions, you will need planning permission – and you need it before work starts.Listed buildings and conservation areas: stricter rules can apply here, even to small structures.Attached or freestanding, open or covered: this can also affect the assessment. State it clearly when you make your enquiry.In Ireland, the same question goes to your local council. Elsewhere, rules vary from country to country: ask your local authority. |
Checking up front avoids unpleasant surprises: always ask your local planning authority before you buy.
3. Getting the dimensions right
A few reference formats to get your bearings (and to check early on whether permission is needed):
Dimensions | Area | Typical use |
3 × 3 m | 9 m² | Relaxation corner, garden seating |
3 × 6 m | 18 m² | Large table, outdoor living room |
3 × 9 m | 27 m² | Dining area + seating (check whether permission is needed) |
6 × 6 m | 36 m² | Large covered terrace (check whether permission is needed) |
As for the material, robinia can be supplied as posts in very long lengths (up to ~13 m) and large diameters – span is not an obstacle.
4. Standard plans and quantities: from 2×2 to 6×9 m
A round-log robinia pergola is designed in modules. The basic module, from 2×2 to 3×3 m, is built with 4 sanded round posts Ø 14-16 cm (2.50 m long – about 2.50 m of clear headroom under the beam, on base plates), 2 beams Ø 12-14 cm screwed onto the post tops, 6 to 8 rafters Ø 8-10 cm screwed onto the beams (at ≈ 40-43 cm centres – choose Ø 10 over a 3 m span), 4 knee braces at 45° and 4 base plates, with stainless steel (A2/A4) bolts and screws: the acidity of robinia attacks ordinary steel.
Extending without limit: the 3 m grid. Beyond 3 m a side, you do not look for longer logs: you repeat the grid with intermediate rows of posts, and the beams are joined over the posts. A 3×4 m pergola stays on 4 posts (4.50 m beams with overhangs, Ø 14 recommended over this span); a 6×9 m pergola is laid out as 3 rows of 4 posts – 12 posts, 9 beams of 3.00 m, 44 rafters in 2 layers and 24 knee braces. Each span is thus limited to about 3 m.
Bear in mind. These plans apply to an uncovered pergola: fabric, reed screening, polycarbonate or climbing plants change the loads (including snow) and call for the structure to be recalculated – and beyond 6×6, it is a job for a structural engineer. If you set the posts directly in concrete, lengthen them by about 80 cm to keep at least 2.20 m of headroom. Quantities are indicative and must be checked by a builder or an architect; the sawmill then adjusts the cutting list to your project before dispatch.
And a 6 m span with no intermediate post? It can be done – provided you move up a size class, because at this span it is deflection that governs, not strength. Allow for a round-log beam of Ø 25 cm (Ø 22 minimum), with its natural curve turned upwards as a camber, or a twin beam of 2 × 75 × 250 mm in finger-jointed robinia with spacer blocks; rafters of Ø 10-12 cm (at ≈ 43 cm centres) and Ø 30 cm posts with reinforced bracing – an “XXL” version that also gives 3.50 m of clear headroom. Our logs up to Ø 40 cm make these sections possible. This format goes beyond the modular standard plan: sign-off by a structural engineer is essential.
5. Design and joints
Lean-to (resting against the house wall, the simplest and most sheltered) or freestanding (four or more posts, placed wherever you like). The roof stays open: poles, spaced slats, shingles or reed screening, depending on the shade you want.
The traditional robinia joint is the pegged mortise and tenon: a robinia dowel passes through the joint and takes the tension, with no metal. As the wood is very hard, you should always drill a pilot hole before pegging or screwing. Knee braces (diagonals between posts and beams) provide the bracing – essential on an open structure.
6. Wind, the governing load
On an open structure, it is not the weight that governs the design but the wind: it tries to lift and overturn the whole thing (the Eurocodes, EN 1991, cover these loads). Hence the importance of knee braces and, above all, of solid anchoring in the ground.
7. Anchoring and the post base – the critical point
The only real rot risk on a robinia pergola is the base of a post left standing in water. The right approach:
Frost-proof concrete pads, at least 30 × 30 × 40 cm; never stand posts directly on tiles or a smooth slab.
Lift the post base off the ground with a hot-dip galvanised base plate or post shoe: the wood no longer touches water and dries out between showers.
Square up during installation using the diagonals method (two equal diagonals = right angles).
8. Maintenance: the bare minimum
No treatment is needed for durability. Left as it is, the pergola takes on a silvery-grey patina. If you want to keep the honey colour, a penetrating oil every 2 to 3 years is enough, after an annual low-pressure clean.
Frequently asked questions
Do I need planning permission for a pergola?
In the UK, many garden structures fall under permitted development, but height, position and listed-building or conservation-area status can change that – check with your local planning authority before you build. In Ireland, the same question goes to your local council.
What should the posts be anchored to?
To frost-proof concrete pads, with a galvanised post shoe or base plate that lifts the post off the ground. This avoids the only possible rot zone. Setting posts directly in concrete is still possible with a use class 4 timber such as robinia: coat the buried part with a bituminous protection – between timber and concrete, the proper term is a damp-proof course against capillary rise – lay a bed of free-draining gravel at the bottom of the hole, and finish the concrete with a sloping top that sheds water.
Does robinia leave run-off stains like oak?
No. It releases virtually no tannins: no brown streaks on the terrace or the facade.
What span can be bridged – up to 6 m without an intermediate post?
Up to a 3 m span, Ø 12-14 cm round-log beams are enough. To span 6 m without an intermediate post, our “long span” plan moves up to Ø 25 cm beams or finger-jointed twin beams (2 × 75 × 250 mm) installed with a camber. The design always follows the load (snow, wind) according to the Eurocode.
What post diameter for a 3×3 or 3×4 m pergola?
A sanded Ø 14-16 cm robinia round log suits the posts of standard modules (2×2 to 3×3 m and their extensions), combined with Ø 12-14 cm beams and Ø 8-10 cm rafters – choose Ø 10 over a 3 m span.
How much timber for a 6×9 m pergola?
In sanded round logs: 12 posts Ø 14-16 cm (3 rows of 4, 3 m grid), 9 beams Ø 12-14 cm, 3.00 m long, screwed onto the post tops, 44 rafters Ø 8-10 cm in 2 layers (at ≈ 40-43 cm centres) and 24 knee braces, on 12 base plates. Indicative quantities, to be checked by a builder or an architect.
Further reading
Reference guide: “Robinia outdoors – the complete guide”.
Our products: Sanded posts · Milled posts · Edged boards.
Sources: Cover Green; Eurocode EN 1991.
Contact: Need a price for a pergola? Send us the footprint dimensions and the height you want, and we will send back the cutting list.
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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