What steel is a Venlo greenhouse structure made of?

Hollow box sections in St 37, roughly equivalent to S235. Internal columns are 120 × 60 × 5 mm, side-wall columns 140 × 80 × 3 mm and gable columns 140 × 140 × 4 mm at 4 metre centres. The 8 metre truss is 490 mm deep with 60 × 40 × 4 mm chords, 30 × 30 × 3 mm diagonals and a 50 × 5 mm top plate, bolted with two M12 at each end. Everything is bolted rather than site-welded and designed to NEN 3859.

Why is greenhouse steel hot-dip galvanised rather than painted?

Because a greenhouse runs near saturation humidity for most of its life and condensate forms on every cold steel surface, which is a corrosivity class few buildings experience. Hot-dip galvanising to ISO 1461 puts a metallurgically bonded zinc layer on every face including the inside of hollow sections, and it protects cut edges sacrificially. Better Farm specifies a 60 µm minimum coating, with mechanically galvanised bolts and nuts, and that thickness is what sets the structure's service life.

How far apart are the columns in a Venlo greenhouse?

The standard grid is 8.00 metres between column rows — the truss span — by 5.00 metre bays along the gutter. At the gable face the columns close up to 4.00 metre centres because they resist the wind load off the whole end wall and hold the gable glazing line straight.

What span do greenhouse trusses use, and why?

Eight metres between column rows is the standard for a Venlo glasshouse, with a truss 490 mm deep in 60 × 40 × 4 mm chords. It is a compromise between three things: more span gives fewer columns and more usable floor, but bending moment rises with the square of the span so a deeper truss is needed, and a deeper truss shades the crop; more span also means a heavier column and a heavier footing. Eight metres is where those settle for a glass Venlo roof — shallow enough to stay largely out of the light path, light enough for a 120 × 60 × 5 mm internal column, and wide enough for a workable number of obstructions per hectare.

Why is a greenhouse structure bolted rather than welded on site?

Because every member is hot-dip galvanised after fabrication and before it leaves the factory. Welding on site would burn through that coating at exactly the connections — which, being joints, are the places most likely to trap moisture in a high-humidity building. Bolting keeps the coating intact everywhere. The bolts themselves are mechanically galvanised rather than hot dipped, because a hot-dipped thread picks up enough zinc to bind unless the nut is tapped oversize.

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Galvanised steel columns and trusses

Columns, trusses and bracing in hot-dip galvanised St 37

The steel is what makes a Venlo greenhouse a building rather than a cover. It carries the roof, the glass, the ventilation gear, the screens, the heating pipe, the crop hanging from the gutters and the wind and snow that arrive on top of all of it — and it has to do that while occupying as little of the light path as the calculation permits.

Everything is hollow box section in St 37 (equivalent to S235), bolted rather than site-welded, and hot-dip galvanised after fabrication. A greenhouse runs at high humidity for its whole life with condensation forming on the inside face of every cold surface. Paint does not survive that; zinc does.

Galvanised steel frame of a Venlo greenhouse being erected at the gable end against a clear blue sky

Columns

Internal columns are 120 × 60 × 5 mm box, set on the 8 × 5 metre grid and bolted to the precast footings. They are the slenderest members in the building because they carry the least: a share of a light roof, and no wind at all except what the bracing hands them.

Side-wall columns are 140 × 80 × 3 mm, two per 5 metre bay. Gable columns step up again to 140 × 140 × 4 mm at 4 metre centres. The section grows as the wind share does — a gable column is a cantilever against the full pressure on the end wall, and it also holds the gable glazing line straight, which is what stops the panes at the corner from being loaded in the one direction glass has no answer to.

Side elevation of a Venlo glass greenhouse, gutter line and glazed side wall running to the horizon — Better Farm · on site A side wall running to the horizon: the columns, the rod bracing between them, and the gutter line along the top.

The 8 metre truss

The truss spans between column rows and defines the whole economy of the type. It is 490 mm deep, with 60 × 40 × 4 mm chords top and bottom, 30 × 30 × 3 mm diagonals, and a 50 × 5 mm plate on top. Each end is bolted with two M12.

Those are small sections for an eight metre span, and they can be small because a Venlo truss does not carry a roof — it carries a strip of gutters, each of which is itself a beam spanning the bay it drains. Splitting the roof load into two stages is the reason the members above the crop are 40 mm deep instead of 400.

The truss also becomes the rail for the ventilation mechanism. The push-pull system that opens the roof vents is mounted on it, so the top chord is set out and levelled as a running surface, not only as a load path.

Truss spacing, and why eight metres

The 8.00 m span between column rows is the dimension the whole type is organised around, and it is a compromise between three things that pull in different directions.

Diagram What a longer span buys, and what it costs

More span, fewer columns Columns interrupt transport, take planting positions and shade. A longer span leaves more of the floor clear.
More span, a deeper truss Bending rises with the square of the span, so the depth needed grows faster than the span — and depth above the crop is shadow on it.
More span, heavier columns and footings Each column carries a larger share of roof, and each footing resists a larger share of the uplift.

Eight metres, on a 490 mm truss, is where the three settle for a glass Venlo roof.

More span means fewer columns and more usable floor. Columns are obstructions: they interrupt internal transport, they occupy planting positions, and they shade. A grower working a hectare wants as few of them as the structure allows.

More span means a deeper truss, and a deeper truss shades more. Bending moment rises with the square of the span, so doubling the span more than doubles the depth needed to carry it in the same sections. Past some point the roof structure costs more yield in shadow than the removed columns give back in floor.

More span means a heavier column and a heavier footing. Each column carries a larger share of roof, and the uplift each footing resists rises with it.

Eight metres with a 490 mm truss in 60 × 40 × 4 mm chords is where those three settle for a Venlo roof in glass. It is shallow enough to stay largely out of the light path, light enough that a 120 × 60 × 5 mm internal column carries it, and wide enough that a hectare has a workable number of obstructions in it. The sections look small to an engineer used to buildings. They are right here because the load is genuinely small — thin glass on extruded aluminium at a 26° slope — and the path it takes to the truss is genuinely short.

Bracing

Side-wall bracing is two rows of Ø10 mm solid steel rod per bay, bolted at both ends. A bottom tie beam of 120 × 50 × 3 mm box runs between the concrete columns; a top tie of 50 × 50 × 2 mm sits 300 mm below the gutter.

Roof bracing is of the same family and is placed where the analysis puts it — its purpose is to stiffen the roof plane against bending and, in doing so, to reduce the steel per square metre rather than add to it. That is worth stating plainly: bracing is how a greenhouse gets lighter, not heavier.

What ISO 1461 at 60 µm actually buys

All steelwork is hot-dip galvanised to ISO 1461 with a minimum coating of 60 µm, and the bolts and nuts are mechanically galvanised. Inside a greenhouse the relative humidity sits near saturation for long periods and condensate runs down every steel face — a corrosion environment most buildings never see. The specification has three parts, and each does a different job.

Hot dip, not spray or paint. The fabricated member is pickled and then lowered into molten zinc, so the zinc reaches every surface the liquid touches — including the whole inside face of every hollow section. That is the point. The inside of a sealed hollow column is the one place in the building that nobody will ever inspect or maintain, and condensation forms there too. Sprayed and painted coatings do not reach it.

Diagram Three reasons the steel is dipped rather than painted

Inside as well as out Molten zinc reaches every surface it touches, including the inside of a hollow column — the one place condensation forms that nobody will ever inspect.
Bonded and sacrificial Zinc alloys with the steel and protects a scratch electrochemically. Under paint, corrosion starts at the scratch and spreads beneath the film.
Bolted, not welded Members are dipped complete and bolted on site. A weld would burn through the zinc at the joint — the place most likely to hold moisture.

The coating goes on after fabrication, which is also why the structure is bolted: a site weld would burn through it at the joint.

Bonded and sacrificial. Hot dipping forms iron-zinc alloy layers at the steel surface rather than a film sitting on top of it, so it does not peel or blister. Where it is scratched, the surrounding zinc protects the exposed steel electrochemically instead of letting rust creep under the edge. Paint does the opposite: a scratch in paint is where corrosion starts.

Thickness is time. Zinc is consumed at a rate set by the atmosphere around it, so coating life is close to proportional to thickness. Specifying a minimum rather than a nominal value matters, because a coating measured as an average can be thin exactly where the section is thin. A thinner coat, or a painted system with site-cut edges, gives back years of service life for a saving measured in a single season's crop.

Bolts, and no site welds. A hot-dipped thread picks up enough zinc to bind unless the nut is tapped oversize, so bolts are mechanically galvanised instead — a coating built up cold, at a controlled thickness that leaves the thread usable. And the structure is bolted rather than welded on site for the same reason the coating exists: every member is galvanised complete, after fabrication, and a site weld would burn through the zinc at precisely the connections, which are the places most likely to hold moisture.

Reference

Specification

Galvanised steel columns and trusses — specification
Parameter	Typical range
Material and protection
Section type	Hollow box, St 37 (≈ S235)
Zinc coating	Hot dip to ISO 1461, 60 µm minimum, after fabrication
Fasteners	Bolts and nuts mechanically galvanised
Site connections	Bolted throughout; no site welding
Columns
Interior	120 × 60 × 5 mm
Side wall	140 × 80 × 3 mm, two per 5.00 m bay
Gable	140 × 140 × 4 mm, at 4.00 m centres
Truss
Span and depth	8.00 m, 490 mm deep
Chords	60 × 40 × 4 mm top and bottom; 50 × 5 mm top plate
Web members	30 × 30 × 3 mm
End connection	Two M12 bolts each end
Stability
Wall bracing	Two rows of Ø10 mm solid rod per bay
Lower tie	120 × 50 × 3 mm box, between the concrete columns
Upper tie	50 × 50 × 2 mm box, 300 mm below the gutter
Design code	NEN 3859

Values for the standard Better Farm bay set, referenced to the governing codes. Project values are confirmed against the site's wind, snow and seismic figures, the crop and the equipment schedule before fabrication is released.

Design notes

What decides the detail

Bolted, not site-welded

A site weld burns off the galvanising it is made through and has to be repaired by hand at every joint. Bolting keeps the coating intact and turns erection into an assembly sequence.
Two-stage load path

Gutters span the bay; trusses span between column rows. Splitting the roof load this way is why the members above the crop are shallow.
The truss is also a rail

Vent drive gear runs on the top chord, so its line and level are a mechanical tolerance, not just a structural one.
Section follows exposure

120 × 60 internally, 140 × 80 at the side, 140 × 140 at the gable — the steel grows only where the wind share does.

Reference

Common questions

What steel is a Venlo greenhouse structure made of?

Hollow box sections in St 37, roughly equivalent to S235. Internal columns are 120 × 60 × 5 mm, side-wall columns 140 × 80 × 3 mm and gable columns 140 × 140 × 4 mm at 4 metre centres. The 8 metre truss is 490 mm deep with 60 × 40 × 4 mm chords, 30 × 30 × 3 mm diagonals and a 50 × 5 mm top plate, bolted with two M12 at each end. Everything is bolted rather than site-welded and designed to NEN 3859.
Why is greenhouse steel hot-dip galvanised rather than painted?

Because a greenhouse runs near saturation humidity for most of its life and condensate forms on every cold steel surface, which is a corrosivity class few buildings experience. Hot-dip galvanising to ISO 1461 puts a metallurgically bonded zinc layer on every face including the inside of hollow sections, and it protects cut edges sacrificially. Better Farm specifies a 60 µm minimum coating, with mechanically galvanised bolts and nuts, and that thickness is what sets the structure's service life.
How far apart are the columns in a Venlo greenhouse?

The standard grid is 8.00 metres between column rows — the truss span — by 5.00 metre bays along the gutter. At the gable face the columns close up to 4.00 metre centres because they resist the wind load off the whole end wall and hold the gable glazing line straight.
What span do greenhouse trusses use, and why?

Eight metres between column rows is the standard for a Venlo glasshouse, with a truss 490 mm deep in 60 × 40 × 4 mm chords. It is a compromise between three things: more span gives fewer columns and more usable floor, but bending moment rises with the square of the span so a deeper truss is needed, and a deeper truss shades the crop; more span also means a heavier column and a heavier footing. Eight metres is where those settle for a glass Venlo roof — shallow enough to stay largely out of the light path, light enough for a 120 × 60 × 5 mm internal column, and wide enough for a workable number of obstructions per hectare.
Why is a greenhouse structure bolted rather than welded on site?

Because every member is hot-dip galvanised after fabrication and before it leaves the factory. Welding on site would burn through that coating at exactly the connections — which, being joints, are the places most likely to trap moisture in a high-humidity building. Bolting keeps the coating intact everywhere. The bolts themselves are mechanically galvanised rather than hot dipped, because a hot-dipped thread picks up enough zinc to bind unless the nut is tapped oversize.

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