Why Arch Spacing Matters in UK Wind: 1m vs 0.67m Explained

Greenhouse Arch Spacing for UK Wind: Why 1m vs 0.67m Is the Decision That Matters

Engineering guide · 2,100 words · ~9 min read · Updated 26 June 2026 by Waldenhaus

TL;DR for the impatient: SteelRoot uses 1m as the standard/base arch spacing and offers 0.67m as the close-arch option for more exposed plots. The choice changes the unsupported panel span between arches, which affects deflection, flutter and fixing-point stress under wind load.

If you've researched arched polycarbonate greenhouses in the UK retail market, you've probably noticed that several brands appear to sell broadly similar structures at broadly similar prices. Most of these brands — Dancover TITAN, Gardj, Polyeco, Sigma Lux, and our own SteelRoot — source from a small number of Polish workshops that produce the standardised galvanised-steel arch greenhouse format.

What separates the products at the spec-sheet level is rarely the frame material or the polycarbonate. It's the engineering decisions each brand makes about which configurations to retail in which markets. And the single most important of those decisions is arch spacing.

This guide explains what arch spacing actually does, why it matters disproportionately for UK weather, and how to choose between SteelRoot's standard 1m spacing and the 0.67m close-arch option.

What arch spacing actually means

An arched steel greenhouse is built from a series of identical arch frames running perpendicular to the ridge, joined longitudinally by purlins (horizontal beams along the structure length) and clad with polycarbonate panels. The arch spacing is the centre-to-centre distance between consecutive arch frames.

Two common configurations exist:

• 1.0m spacing — wider gap between arches, fewer arches per metre of structure length
• 0.67m spacing — closer-arch pattern, ~50% more arches per metre of structure length

A 4m-long structure at 1m spacing has 5 arches (at 0m, 1m, 2m, 3m, 4m). The same 4m structure at 0.67m spacing has 7 arches (at 0m, 0.67m, 1.33m, 2m, 2.67m, 3.33m, 4m).

The polycarbonate panels span the gaps between consecutive arches. The panel span is the unsupported distance the polycarbonate has to bridge between fixings. At 1m spacing, the panel span is 1m. At 0.67m spacing, the panel span is 0.67m.

This sounds like a minor engineering detail. It isn't.

Why panel span dominates wind-load performance

Three things happen to a polycarbonate panel under wind load:

1. Deflection — the panel bows inward (or outward) under pressure
2. Flutter — the panel oscillates at sustained wind frequencies
3. Fixing-point stress — the screws or clips holding the panel edge see concentrated load

All three scale with span. Specifically:

• Panel deflection under uniform load scales roughly with span to the fourth power. A panel at 1m span deflects roughly 5× more than the same panel at 0.67m span. Read that again — five times.
• Panel flutter starts at lower wind speeds when the span is longer. A 4mm cellular polycarbonate panel at 1m span begins to oscillate at sustained winds around 25 m/s (~56 mph). The same panel at 0.67m span doesn't reach flutter onset until much higher wind speeds.
• Fixing-point stress at each screw or clip is proportional to the panel area it's holding. A wider span means each fixing sees more load per gust.

The cumulative effect is that arch spacing isn't a small variable — it's the largest single engineering decision affecting whether the greenhouse survives the UK storm season intact.

The UK design wind envelope

The UK has a national wind-load design code, BS EN 1991-1-4. Basic wind speed varies by region:

• London and the South-East: 21.5 m/s basic
• Midlands and Yorkshire: 22.5–25 m/s
• North-West England and Pennines: 25–28 m/s
• Scottish Highlands and Outer Hebrides: 28–31.5 m/s

Site-specific factors add another 10–25% for elevation, coastal exposure, or rural unsheltered positioning. The practical design gust envelope for typical UK domestic plots is 24–34 m/s (54–76 mph), with significantly higher for exposed sites.

Recent named storm reality: - Storm Arwen 2021: 90+ mph gusts across Scotland and North-East England - Storm Eunice 2022: 122 mph maximum gust on the Isle of Wight, 90+ mph widely - Storm Henk 2024: sustained 80+ mph across much of England - Storm Éowyn 2025: 100+ mph across Scotland and Northern Ireland

Shorter arch spacing reduces unsupported panel span and gives each panel more support points. That does not make any domestic greenhouse immune to named-storm exceedance, but it is a real engineering difference for exposed UK plots.

Different physics at different spans.

Why our supplier offers both 0.67m and 1m

The Polish workshop we work with — and which other UK retailers source from in different configurations — manufactures the same arch frame in two spacings because the European market has two different climate envelopes.

Continental Europe (much of France, Germany, Italy, southern Poland) sees lower peak wind speeds and lighter snow loads than the UK. The 1m spacing is sufficient for those climates and lets buyers cover more floor area per arch — making the structure marginally cheaper per square metre of growing space.

Northern Europe (Scandinavia, the Netherlands at coast, Scotland, Ireland) sees significantly higher peak wind speeds and heavier snow loads. The 0.67m spacing is what the supplier recommends for these regions explicitly.

The supplier's product literature describes the 0.67m configuration as "recommended for snowy or windy regions". They produce both and sell both — the choice of which to retail to which market is left to the importing brand.

Most UK retailers selling this format offer both — typically the 1m as the standard and the 0.67m as a paid upgrade. This is how the same supplier ends up in catalogues at radically different storm-resistance profiles, with the buyer often unaware of the engineering significance of the option.

How SteelRoot treats 1m vs 0.67m now

SteelRoot's current commercial configuration is not single-spec. The standard/base arch spacing is 1m. The 0.67m close-arch pattern is available as a paid option for buyers who want the shorter unsupported panel span on a more exposed plot.

Four rules govern how we talk about the choice:

1. The engineering favours 0.67m on exposed plots

As described above, the 0.67m configuration provides more support points and a shorter panel span than the 1m configuration. That matters most on coastal, elevated, rural or otherwise exposed sites.

2. Claims must match the selected configuration

The UK Consumer Rights Act 2015 requires that goods sold be "of satisfactory quality" (Section 9) and "fit for the purpose" they're sold for (Section 10). That means we should not make high-snow or high-wind performance claims for the 1m configuration unless the supplier provides written ratings for that exact configuration.

Ask for written supplier data before relying on any snow-load figure, and do not transfer a close-arch rating to the 1.0 m configuration.

3. Choice is useful only when it is explicit

Offering both spacings is acceptable only if the buyer can see what changes: 1m is the standard/base spacing; 0.67m is the close-arch option. It should not be hidden as a vague "stronger" add-on.

That keeps the configurator honest and keeps customer service clear.

4. Brand integrity means no disguised default

Our positioning — engineered structure for serious British growing — depends on explaining the trade-off, not pretending one configuration is something it is not. SteelRoot starts at 1m spacing; 0.67m is the close-arch option for buyers who want the added support density.

What 0.67m means in practice

For a SteelRoot 3.14m × 8m structure:

The visible difference in the structure is subtle — slightly more vertical lines when you look down the length. The practical difference in a hard winter gust is more material than it looks on a spec sheet.

Combined with our other engineering decisions — mechanical screw-fixing of every panel, Ground Screw Anchors included, closed 40 × 20mm RHS galvanised steel frame and the choice of 4mm or 6mm twin-wall polycarbonate — the 0.67m spacing is one optional mechanism in a four-mechanism build for more exposed plots.

Engineered structure plus correct anchoring — not tonnage.

How to check arch spacing on any brand you're considering

If you're researching arched polycarbonate greenhouses in the UK retail market, three checks:

1. Find the arch spacing on the product page. Brands that publish it are confident in their spec; brands that don't usually have reasons not to.
2. Look for the language "UK-spec", "close-arch", or "snow/wind region" in the description. If the page says "standard 1m spacing", check whether the site guidance explains when a closer-arch option is worth choosing.
3. Check if 0.67m is offered as an option. If it's available as a paid upgrade, the brand has both — the question is why they're defaulting to the 1m.

Brands that retail only 1m in the UK: read the site-exposure guidance carefully. Brands that retail both and let you choose: check whether the configurator explains the trade-off clearly. Brands that retail only 0.67m: check whether the higher structure cost is reflected in the price.

Frequently asked questions

Is 0.67m arch spacing always better than 1m? No. It gives more support points and a shorter panel span, which is useful on exposed plots. A sheltered garden can be well served by the 1m standard/base configuration. That is why SteelRoot offers both rather than hiding the choice.

Does 0.67m spacing cost more than 1m? At the manufacturing level, yes — there are more arches and more connection points. At the retail level, brands that offer both typically price the 0.67m as a paid upgrade. In the current SteelRoot configurator, 1m is the standard/base spacing and 0.67m is the close-arch option.

Will a 1m-spaced greenhouse survive a UK storm? It depends on the exact site, anchoring, glazing, fixings and storm. A sheltered domestic plot is not the same as a coastal or elevated site. The safe public rule is simple: do not apply 0.67m snow/wind ratings to the 1m configuration unless the supplier gives written data for 1m.

Why don't all UK retailers default to 0.67m? Cost pressure and consumer perception. The 1m option appears cheaper on the spec sheet and lets brands compete on headline price. Many buyers don't understand the engineering significance until after the first storm.

Can I check the arch spacing of my existing greenhouse? Yes — measure the centre-to-centre distance between two adjacent arches at the base or the ridge. If it's roughly 0.67m, you have the close-arch configuration. If it's roughly 1m, you have the Continental default.

What about rectangular greenhouses (non-arched)? The equivalent metric is rafter spacing. Quality timber greenhouses use 600–700mm rafter spacing. Entry-tier aluminium often uses 800–1000mm. The same physics applies — closer spacing means shorter panel spans and better wind performance.

Is 0.67m spacing structurally adequate without ground anchors? No. The arch spacing is one mechanism of four — frame strength, arch spacing, panel fixing method, and ground anchoring. All four need to be correct for the published performance. We include the Ground Screw Anchors with every SteelRoot specifically because skipping them is the most common storm-failure cause.

What this means for your buying decision

If you're buying an arched polycarbonate greenhouse for a UK domestic site, the arch spacing decision matters more than most variables on the spec sheet. SteelRoot now gives you the choice: 1m as the standard/base spacing, or 0.67m as the close-arch option for a more exposed plot.

You'll find brands that offer both. Read their engineering rationale. If a retailer defaults to 1m and offers 0.67m as an upgrade, ask what site conditions make the upgrade sensible.

For the SteelRoot range - manufactured in Poland, UK-stocked, UK-backed, 1m standard/base arch spacing, 0.67m close-arch option, 4mm standard/base twin-wall polycarbonate, 6mm glazing option and Ground Screw Anchors included: explore SteelRoot.

Galvanised. Arched. Quiet about it.

See also: Wind-Resistant Greenhouse UK · 6mm vs 4mm Polycarbonate Greenhouse UK · Polytunnel vs Polycarbonate Greenhouse UK · Heavy Duty Polytunnel UK Buyer's Guide