In short: a metal arch greenhouse suits an open or windy plot where you want a rigid, walk-through growing space without the cover-replacement cycle of a film polytunnel. Budget aluminium-and-glass kits cost less but flex and shed panels in wind. Below: what arch construction actually buys you, the glazing and anchoring to check, and when a traditional apex suits better.
Search for a metal greenhouse in the UK and the results split fast. On one side sit budget aluminium-and-glass kits from around £399. On the other sit curved steel-arch greenhouses, the shape that looks half polytunnel, half greenhouse. The arch ones raise an honest question before you spend anything: are they actually any good, or just a cheaper-looking compromise? This guide answers that with construction facts, not sales gloss, so you can tell a well-built steel arch from a flimsy one.
We sell one. SteelRoot is a galvanised steel-arch greenhouse, so treat the brand mentions below as one worked example, checked against the same yardstick as everything else. The yardstick matters more than the badge.
What is a metal arch greenhouse, exactly?
A metal arch greenhouse uses curved steel ribs instead of a peaked roof and straight gable ends. The ribs are spaced along the length and clad in polycarbonate sheet rather than glass. You end up with a rigid, shatter-resistant tunnel-shaped shell that costs less than a glasshouse and stands up to wind better than a peaked aluminium frame.
Two things set the better arches apart from the cheapest ones. Start with the steel itself. Budget ranges use light galvanised tube, often 40x20 mm or thin-walled round section near 56x1 mm. Then look at how the glazing attaches. Cheap kits clip thin 4 mm polycarbonate into channels; the rigid ones screw every panel down. Both decisions show up the first time a gale hits.
The frame: galvanised steel as the budget-strength choice
Garden frames come in three materials. The RHS notes that aluminium is the usual choice for a glasshouse because it needs no upkeep and its thin glazing bars cast little shade. Timber looks handsome but needs periodic treatment and casts more shadow. Galvanised steel sits between the two on price: stronger than the aluminium extrusion used in budget kits, cheaper than a hardwood frame, rust-protected by its zinc coating.
The catch is that "galvanised steel" covers a wide range. A closed rectangular hollow section, screwed and bolted at every joint, behaves very differently from a thin round tube pushed into a plastic connector. When you compare arches, look past the word "steel" to the wall thickness, the profile and the fixings.
Why does close-arch spacing make a metal greenhouse more rigid?
Close-arch spacing means more support ribs along the same length, so each polycarbonate panel spans a shorter gap and the whole shell resists flexing and wind uplift better. It is the single biggest structural difference between a budget arch and a sturdy one, and it costs more because it uses more steel per metre.
Picture two greenhouses of identical length. One places its arches every metre; the other places them every 0.67 m. The closer-spaced frame fits roughly 50 per cent more ribs into the same run. Each panel of glazing has less unsupported area to billow, and the frame has more points carrying the load when wind pushes on the side.
The aerodynamics help too. A curved roof has lower drag than a flat or peaked one. Instead of catching wind and concentrating stress at corners and the ridge, the arch deflects air over and around the shell and spreads load more evenly. Tunnel and Gothic-arch shapes create less turbulence than a peaked roof. We have written the full engineering case, including how rib count changes the maths, in our guide to arch spacing and wind engineering for UK greenhouses.
Polycarbonate vs polythene film: where the arch shape earns its keep
A metal arch greenhouse and a polytunnel share a silhouette, then part ways at the cover. The arch carries rigid twin-wall polycarbonate, screwed to the steel. A polytunnel stretches polythene film over hoops. That difference decides how the structure ages, how it feels inside, and how often you reach for your wallet.
Most professional growers cover a tunnel with 720-gauge, 180-micron polythene. UV warranties run four to five years, and real-world life is roughly five to seven years before the film clouds, occasionally stretching to ten. UV thins the cover fastest along the ridge, the same line that takes the most sun. When it goes, you re-skin the whole tunnel.
Twin-wall polycarbonate on a steel arch lasts longer and behaves differently day to day. Genuine UV-stabilised 4 mm twin-wall is rated at roughly 10 to 15 years in UK weather, with 6 mm closer to 15 to 20, and quality sheet usually carries a 10-year non-yellowing guarantee. A rigid shell does not billow in wind or drip condensation off a flexing membrane the way film can. If you want the full cost-per-year comparison against a tunnel, we keep it in polytunnel vs polycarbonate greenhouse rather than repeating it here, and our wider look at polytunnel alternatives sets the arch in context.
What twin-wall polycarbonate trades away
Polycarbonate is not free of compromise. Horticultural glass transmits about 90 per cent of light and never degrades in sun. Twin-wall sheet transmits less: roughly 80 per cent at 4 mm and 75 to 78 per cent at 6 mm, because each extra wall traps a little more light along with the heat it holds. For most UK growing that trade is worth it, since the same twin-wall structure that loses a few per cent of light gives far better heat retention. A 6 mm sheet runs near a 2.8 W/m²K U-value against roughly 5.5 for single skin, about half the heat loss.
One installation detail decides whether you get the rated lifespan. Twin-wall sheet has a UV-protected face that must point outward. Fitted upside down, panels can start to yellow within three summers. On a screw-fixed kit with marked sheets this is hard to get wrong, which is a quiet advantage of buying a proper arch over cutting film to fit.
Where a metal arch beats budget aluminium
A budget aluminium-and-glass greenhouse and a steel arch fail, or survive, for the same reason: how the glazing is held. This is where the arch usually wins for an exposed UK garden, because it tends to screw its panels down rather than relying on friction.
Most cheap aluminium kits hold their panes with spring glazing clips that grip by friction alone, often four clips a pane in exposed spots. Once wind finds a way in, internal pressure pushes against a pane, a clip lets go, and the damage cascades from there. Perished or missing rubber seals make it worse. BBC Gardeners' World gives the same caution about plastic: polycarbonate is cheaper, does not break and insulates better, "but allows less light through and can pop out of its frame in high winds". Both cases call for the same fix. Screw-fixing or bar-capping, fastened down rather than clipped, holds far better than friction.
Clipped polycarbonate can lift in a strong gust once wind catches the underside, and once it lifts the failure can run along a row of panels. Screwing each panel to the frame, with a washer to seal the hole, removes the weak link. So does anchoring the whole structure into the ground rather than letting it sit loose on a base.
| Feature | Budget aluminium + glass | Sturdy steel arch + twin-wall |
|---|---|---|
| Glazing fixing | Spring clips (friction) | Screw-fixed with washer |
| Wind behaviour | Panes pop out in gales | Rigid shell, panels stay put |
| Glazing | 3 mm horticultural glass | Twin-wall polycarbonate |
| Light transmission | ~90% | ~75 to 83% |
| Break risk | Shatters | Shatter-resistant |
| Ground fixing | Often a loose base | Ground anchors standard |
When does a traditional apex greenhouse suit you better?
An apex greenhouse, the classic peaked shape, wins on light and on edge headroom. Glass transmits around 90 per cent of available light against roughly 80 per cent for twin-wall, which matters for seedlings and high-light crops. Straight walls also give you usable height and easy staging right to the edge, where an arch curves the space away.
Choose an apex if your priority is raising plenty of seedlings in early spring, if you want to grow tall crops along the walls, or if you simply prefer the look of a traditional glasshouse in a sheltered garden. A timber apex adds warmth and steadier temperatures; our wooden vs aluminium greenhouse comparison works through that choice in detail, and polycarbonate vs glass glazing covers the light-versus-insulation maths.
The arch trades a little light and some edge headroom for two things glass cannot match at the price: a shell that shrugs off wind, and panels that bounce rather than shatter when a football or a falling branch finds them. Be honest with yourself about which one your garden needs. An exposed plot leans towards the arch; a sheltered, sunny corner can take the apex.
How to read a steel arch spec sheet
Once you have decided an arch suits your garden, the spec sheet tells you whether a given kit is built to last. Five lines do most of the work.
- Steel profile and wall: a closed rectangular hollow section resists twisting better than thin round tube, and budget arches often use 40x20 mm tube or thin 56x1 mm round section.
- Arch spacing: closer ribs mean a stiffer shell, so an option to drop from 1 m to 0.67 m spacing is worth taking on an exposed site.
- Glazing fixing: screw-fixed with a sealing washer beats clipped sheet for wind, every time.
- Ground anchoring: count the anchors and check they come in the kit, not as an upsell.
- Ventilation: the RHS wants roof-vent area equal to 15 to 20 per cent of the floor, ideally on both sides of the ridge, while budget kits often ship only 5 to 8 per cent.
That last line catches a lot of buyers. A 6x8 ft greenhouse ideally has two hinged roof vents plus a side vent, and a wax-cylinder automatic opener is worth the £20 to £40 it costs because it works on temperature with no electricity. The RHS adds a fair caveat: those openers respond slowly, so you still crack the door each warm morning. If a kit ships short on vents, budget for add-on openers from the start rather than discovering the problem in a July heatwave.
How SteelRoot reads against that checklist
To make the checklist concrete, here is our own arch held to it. SteelRoot uses 40x20 mm rectangular hollow-section galvanised steel arches with a ZAM coating, pre-drilled for bolt assembly. Arch spacing is selectable at 1 m or 0.67 m, with 0.67 m recommended for the UK because it adds about 50 per cent more support ribs. The glazing is twin-wall polycarbonate with a UV-stabilised top layer, available in two thicknesses.
On fixings, SteelRoot screws each panel down with an EPDM washer at every edge in place of spring clips, and ships ground anchors scaling from 8 to 24 per kit by length. Every model includes two opening windows, with roof auto-vents available as an option, and carries a 10-year anti-corrosion warranty on the galvanised steel frame. It arrives flat for bolt-together assembly with free delivery to the UK mainland, typically in one to two weeks. None of that makes it the right pick for every garden; it makes it a worked example of what the checklist is asking for.
SteelRoot galvanised steel arch greenhouse
Closed 40x20 mm RHS galvanised steel, screw-fixed twin-wall polycarbonate and 0.67 m close-arch spacing. The advice above works whatever greenhouse you choose; if a rigid steel arch fits your garden, this is ours, with current pricing on the product page.
Sizing, base and budget for a metal arch
Get the footprint right before anything else. The smallest standard sizes run about 24 sq ft, but the usual recommended beginner minimum is 8x6 ft, near 48 sq ft, which lets you stage both sides and keep an 18 to 24 in path down the middle. The metal-greenhouse size grid runs through 6x8, 6x10 and up to 8x12. If you are unsure, our guide to what size greenhouse you need walks through bench layout and crop space.
Aim for an eave height of at least 1.5 m, ideally 1.8 m or more, with the ridge sitting at least 60 cm above the eaves for door access and rain run-off. On the base, a firm level surface of paving or gravel is the standard, and a base typically takes 15 to 20 per cent of the build budget. For an arch, ground anchors are the relevant fix: anchors are standard, so no concrete foundation is required, though we recommend a firm, level base for extra stability.
On price, expect entry polycarbonate metal greenhouses from around £399 for a small model, with budget steel arches around the £1,175 to £1,305 mark for a 3x6 m footprint. Spend the difference on the things the spec sheet flagged: closer arch spacing, screw fixings and enough vents. Those are the lines that decide whether your greenhouse is still standing, and still clear, in ten years. Walk the spec sheet top to bottom before you buy, and let those five lines, not the headline price, make the call.
