In short: for an exposed or windy UK plot, a galvanised closed-section steel frame holds its geometry and resists racking better than thin aluminium; aluminium wins on weight, rust-immunity and upfront price for sheltered gardens. The rest of this guide compares them on rigidity, corrosion, lifespan and cost so you can match the frame to your site.
Type "steel greenhouse" into a search bar and most of the top results were written for the American market, where they file steel under "commercial farms" and aluminium under "the back garden." That lazy binary does not survive contact with a UK plot. The real question is narrower and more useful: which frame metal actually lasts through wet winters and the strong gusts of a British storm, and where does each one win? This guide answers that without selling you a fairy tale about either side.
Two facts set up the whole debate. Aluminium is the default UK greenhouse frame, and the Royal Horticultural Society says so plainly. A galvanised-steel frame counts as a genuine outlier, rare enough that the RHS and BBC Gardeners' World do not even list it as a primary frame option. This comparison runs between unequals on the shelf, then: the metal almost everyone buys against a metal almost nobody offers, and the interesting part is why that gap exists.
What are the real differences between steel and aluminium frames?
Aluminium is light, never rusts, casts little shade and costs less, which is why it dominates UK greenhouses. Steel is roughly three times heavier and far stiffer, so it resists wind loading better, but it must be galvanised to survive damp. Both are sound choices. The right one depends on your site, your budget and how exposed your plot is to wind.
Quick version before the detail, with figures from the RHS, the Galvanizers Association and published material densities.
| Property | Aluminium | Galvanised steel |
|---|---|---|
| Weight | Light (~2.70 g/cm³) | Heavy (~7.85 g/cm³, about 3× aluminium) |
| Corrosion | Self-healing oxide layer, never rusts | Sacrificial zinc coating, very long-lived in UK air |
| Stiffness | Lower, flexes more under load | Higher, resists wind uplift and lateral push |
| Shade | Thin bars, minimal internal shadow | Chunkier sections, a little more shade |
| Price | Cheaper kits, lower delivery cost | Dearer, rare in the UK domestic market |
| Upkeep | None | None once galvanised |
Where aluminium really wins
Aluminium earns its place at the top of the market, and pretending otherwise would be dishonest. It weighs about one third as much, near 2.70 g/cm³ against roughly 7.85 g/cm³ for steel. That low weight makes a kit easier to lift on your own, simpler to self-assemble at the weekend, and cheaper to send. The thin glazing bars cast little shade, so more winter light reaches your plants, which matters in a small footprint where every hour of January sun counts.
It also never rusts, and the chemistry behind that is real rather than marketing. Exposed aluminium grows a thin oxide skin that seals the metal off from the air. Scratch it and the layer re-forms within moments, so the protection repairs itself. In a damp greenhouse where condensation drips down the frame for months, that self-sealing behaviour is a serious advantage. Gardeners' World adds the practical points: aluminium is cheap, low-maintenance and can be powder-coated in green, grey or black if you dislike bare silver.
Where steel earns its keep
Steel's case rests on stiffness. The same weight that makes it harder to lift also makes it far more rigid, and rigidity is what resists wind. A heavier, stiffer frame flexes less when a gust pushes on the side or tries to lift the roof. For an exposed, coastal or highland plot, that resistance to deformation is the whole argument, and it is a sound one.
The usual objection is that steel rusts, and it is fair to raise it. Galvanising answers it, which we cover in full below. The short version: hot-dip galvanised steel carries a zinc coating that protects the metal for decades in British air, so the rust worry that haunts cheap painted steel does not apply to a properly galvanised frame.
Does galvanised steel rust in the UK climate?
No, not in any timeframe that should worry a gardener. Hot-dip galvanising bonds a zinc layer to the steel, and zinc corrodes sacrificially: it gives itself up before the steel underneath, and it keeps protecting even where the coating is scratched. UK atmospheric conditions are mild for zinc, so the coating lasts for decades.
The numbers come from the Galvanizers Association, the UK authority on the subject. Average zinc-corrosion rates across the UK and Ireland run below one micrometre per year. A typical 85-micrometre hot-dip coating therefore lasts somewhere between 34 and 174 years depending on location, comfortably over 85 years in clean inland air and still 30-plus years even on an exposed coast. These are properties of the galvanising process itself, not a claim about any single product, and they put the "steel rusts" worry to bed for a galvanised frame.
Galvanised steel versus aluminium on corrosion, fairly
- Aluminium wins on simplicity: the oxide layer is automatic, needs no coating step and never fails. There is no zinc to deplete because there is no zinc.
- Galvanised steel wins nothing on corrosion, and it does not need to. It only has to last longer than you will use the greenhouse, and a 30-to-170-year coating life clears that bar with room to spare.
- The honest verdict: corrosion is a wash. Neither a galvanised-steel frame nor an aluminium one will rust away under you. Choose on weight, stiffness and price instead, because those are where the two metals actually differ.
Which frame survives UK wind better?
Steel resists wind better because it is stiffer, but the metal is only half the story. The classic failure is not the frame snapping. It is a door or vent blowing open, wind getting inside, and the structure failing from internal pressure, or an unanchored frame simply lifting off its base. Anchoring and vent discipline matter as much as the metal.
UK winter storms bring the kind of strong gusts that test any structure, and greenhouse retailers see a consistent damage pattern. Greenhouse Stores describes it directly: a door or vent blows open, wind gets inside, and the frame cannot hold. An unanchored 6×8 ft aluminium house can be lifted bodily off the ground. Glass houses start losing panes once wind reaches the upper end of a strong breeze, and most frames take structural damage when it climbs into the gale range. None of that is solved by the frame metal alone.
Stiffness helps, but anchoring decides
A stiffer steel frame deflects less under that lateral and uplift loading, so it has a genuine edge in the worst of it. The bigger lever, though, is fixing the structure down. A greenhouse that is bolted to a firm base or held by ground anchors stays put. One that sits loose on slabs does not, whatever it is made of. Closed sections also matter: a sealed rectangular hollow tube is stiffer than an open channel of the same weight, which is part of why frame design beats raw material in practice.
If you want the engineering detail on how frame geometry and arch spacing handle wind, our piece on arch spacing and UK wind engineering works through the loads in full. The headline is simple: spacing the structural members closer together stiffens the whole shell, and that does more for storm survival than the choice between two sound metals.
The glazing has to hold too
A rigid frame is wasted if the glazing pops out of it. Gardeners' World flags this honestly: twin-wall polycarbonate can blow out of its frame in high winds if the glazing is only clipped in. Panels that bow under pressure work loose, then leave a hole the wind drives through. To fix that, choose a glazing system that locks the panel mechanically rather than relying on friction clips, which is why screw-fixed twin-wall holds steadier on an exposed site than the snap-in glazing common on budget kits.
Weight, freight and the assembly job
Aluminium's low weight is a real, everyday advantage and steel cannot match it. At roughly one third the density, an aluminium kit is lighter to carry through the house, easier to hold up while a second pair of hands drives the bolts, and cheaper to ship. If you are assembling alone at the weekend, that matters.
Steel's weight is the trade-off you accept for stiffness. A heavier frame is more of a two-person job to raise, and it costs more to deliver. That is a fair cost to flag. In return you get a structure that holds its shape under load, which is the reason to choose it in the first place. There is no free lunch here: light metal is easy to handle and flexes, dense metal is harder to handle and stays rigid.
If you have decided the stiffness is worth the weight, our SteelRoot galvanised steel arch greenhouse, from £1,199, is built around exactly that reasoning. It uses closed 40×20mm rectangular hollow section galvanised steel, glazed in screw-fixed twin-wall polycarbonate, with arches set at 0.67m for a closer, stiffer spacing. Ground anchors are standard so no concrete foundation is required, and for extra stability we recommend a firm, level base. Delivery is typically one to two weeks, free to the UK mainland. The advice in this guide holds whatever greenhouse you buy: anchor it down, keep your vents disciplined, and lock the glazing to the frame.
What does each frame cost in the UK?
Aluminium spans the widest price range because it is the volume material. Entry kits start around £300 to £395, a popular 8×6 ft model runs from roughly £509, an 8×10 ft from about £1,099, and top-of-range aluminium houses reach £3,000 and beyond. Galvanised-steel frames are rarer, so the market is thinner, but they sit in the mid-to-upper band rather than the bargain shelf.
The price gap is not arbitrary. Steel costs more to buy as raw material, more to galvanise, and more to ship because of its weight. You are paying for stiffness and for a frame that is uncommon in UK gardens. Whether that surcharge is worth it comes down to your site. On a sheltered suburban plot, an anchored aluminium house will serve you for decades and the extra spend on steel buys headroom you may never call on. On an exposed, windy or coastal site, that headroom is the point.
Reading a price against your plot
- Sheltered garden, modest budget: aluminium is the sensible default. It is what the RHS recommends, it never rusts, and anchored properly it shrugs off normal UK weather.
- Exposed or coastal site: the stiffness of a galvanised-steel frame starts to pay back, especially paired with screw-fixed glazing and a bolt-down or anchored base.
- Tight on light: aluminium's thin bars give you a small edge indoors, worth weighing if your plot loses winter sun early.
For help matching a footprint to what you actually grow, our companion guide on what size greenhouse you need works through bench runs and headroom, and if you are weighing a covered structure against a film tunnel, our polytunnel alternatives guide covers that ground.
Steel, aluminium or timber: how do they compare?
Timber gives you a third option and changes the maths again. Wood looks warmer, holds heat well and is structurally strong, but it costs more than aluminium and needs occasional upkeep. Aluminium stays the low-maintenance volume choice, galvanised steel the stiff outlier. Each suits a different priority, and none is wrong.
The frame metals split cleanly: aluminium for light weight and zero upkeep, steel for rigidity on exposed sites. Timber sits apart as the natural-look choice with the best heat retention and the most character, at the price of dearer kits and a brush of treatment now and then. If your decision is really between wood and metal rather than between two metals, our detailed wooden versus aluminium comparison walks through shade, heat, cost and maintenance side by side.
A quick way to choose
- Pick aluminium if you want the cheapest, lightest, no-upkeep frame and your plot is reasonably sheltered. It is the default for good reasons.
- Pick galvanised steel if your site is exposed and you want the stiffest frame, accepting more weight and a higher price for it.
- Pick timber if you want the warmest look and the best heat retention, and you do not mind dearer kits and a little maintenance.
The honest bottom line on steel versus aluminium
Neither metal will let you down if you anchor the greenhouse and keep the vents under control. For most UK gardens, aluminium is the right call: light, rust-free, cheap and recommended by the RHS for sound reasons. Galvanised steel is the deliberate choice for an exposed or coastal plot where stiffness against strong storm gusts is worth the extra weight and cost, and where galvanising removes the only real objection by giving the frame a coating life measured in decades, 34 to 174 years depending on location.
So buy aluminium with confidence if your plot is sheltered and your budget is tight. Reach for closed-section galvanised steel if the wind off your nearest hill or coast keeps you awake. Either way, spend the £20 to £40 on a vent opener, fix the base down with ground anchors, and lock the glazing to the frame. Those three steps decide whether your greenhouse lasts far more than the badge on the metal does.
