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When buyers compare greenhouses, they tend to focus on the glazing — panel thickness, light transmission, UV warranties. These are important. But the frame is the part of the greenhouse that determines whether it is still standing and performing correctly in year eight, or whether it has gradually racked, twisted, and settled out of true by year three.
Frame material is also where the most consequential quality differences between greenhouse tiers are found — and where the gap between a serious growing structure and an entry-level kit is widest.
KLASIKA and BALTIC LT greenhouses are built on CE-certified galvanised steel frames. This is not the default material in the broader greenhouse market, where aluminium has become dominant, and it is a deliberate engineering choice. This article makes the case for that choice honestly — including the one genuine trade-off that any fair account has to acknowledge — and explains why galvanised steel is the frame material that suits the working gardener best.
Before comparing materials, it is worth being precise about what a greenhouse frame is doing and what it needs to do well.
A greenhouse frame carries the weight of the glazing panels across its full surface area. On a lean, this load is modest. Under snow accumulation, it can be substantial. The frame must distribute this load to the ground without deforming — panels that shift out of their channels leak, lose their insulating air seal, and eventually crack.
The frame also resists lateral forces. Wind does not just push on a greenhouse — it applies racking forces that try to distort the rectangular structure into a parallelogram, twisting it out of square. Over time, in an exposed position, a frame that is not sufficiently rigid will gradually move out of true. Doors that opened easily in year one stick or swing open in year three. Glazing bars that held panels securely in year two develop small gaps in year four.
And the frame is the structure that everything else in the greenhouse connects to — shelving, plant-binding systems, irrigation pipework, staging brackets. A frame that cannot accept load at fastener points, or that flexes under the weight of a fully-laden shelf, limits how useful the greenhouse can be as a working space.
Steel addresses all three of these requirements more effectively than any other commonly used frame material.
Steel has a higher modulus of elasticity than aluminium — the engineering measure of a material’s resistance to deformation under load. In practical terms, a steel section of given dimensions is stiffer than an aluminium section of the same dimensions. It deflects less under the same load, resists distortion more effectively under racking forces, and returns to its original geometry after load removal rather than retaining a small permanent set.
For a greenhouse frame that will spend its working life exposed to wind, varying snow load, and the cumulative effect of repeated thermal expansion and contraction cycles, this stiffness advantage is not theoretical. It accumulates over years of use. A steel-framed greenhouse that is well-anchored and properly assembled will be as square and true in year ten as it was in year one. This is the foundation of the KLASIKA and BALTIC LT range’s design life — not just the panel warranty, but the structural integrity that makes the panels perform correctly for its duration.
The omega-profile steel sections used in KLASIKA and BALTIC LT frames are assembled with M5 screws — a thread size and engagement strength suited to the 1mm steel section. Steel holds screws more firmly than aluminium at equivalent section thicknesses. The thread engagement in steel is tighter, the pull-out resistance is greater, and the joint does not work loose under repeated vibration or thermal cycling in the way that aluminium joints can over time.
This matters at every connection point in the greenhouse — between arch sections, between arches and base rails, between glazing bars and main frame members. A frame with firm, stable joints maintains its geometry. A frame where joints gradually loosen under service loads does not.
The profiles used in KLASIKA greenhouse frames are manufactured from 78mm wide steel strip, cut with precision guillotines and formed into omega-section profile. The steel is 1mm thick — a specification that may sound modest but is specifically calibrated for the omega profile geometry. The omega section — a pressed C-shape — is more structurally efficient than its cross-sectional area might suggest, because the flanged edges of the section significantly increase its moment of inertia compared to a flat plate of the same thickness. In plain terms: the shape makes the section much stronger than the raw material thickness alone would imply.
This is not the thin, lightweight steel used in budget-category greenhouses. Those products use rolled steel at thicknesses of 0.5–0.7mm, sometimes less, where the section is proportionally weaker and the zinc coating proportionally thinner. The difference is measurable in the stiffness of the assembled frame, the load it can carry, and the structural confidence it provides in service.
Galvanising is the process of applying a protective zinc layer to steel. Zinc is sacrificially anodic to steel — it corrodes preferentially, meaning that the zinc layer degrades before the underlying steel begins to corrode. As long as the zinc layer is present, the steel beneath it is protected.
The thickness and quality of the zinc coating is the variable that determines how long this protection lasts in outdoor conditions. The KLASIKA frame profiles carry a zinc coating of 275gr/m², applied to hot-dip galvanised steel specified to EN10346: DX51D+Z — a European standard for continuously hot-dip coated flat steel products.
To contextualise that specification: the minimum zinc coating for structural hot-dip galvanising in outdoor applications under European standards is typically Z100 (100gr/m²). A Z275 coating is a heavyweight protective specification — more than twice the minimum, appropriate for structural components that will spend their entire service life exposed to outdoor weather.
At 275gr/m², even in a UK climate where winter humidity, rain, and frost provide sustained corrosion challenge, the zinc coating provides decades of effective protection under normal operating conditions. This is not a cosmetic coating — it is a structural asset that contributes directly to the greenhouse’s ability to reach and exceed its intended service life.
The zinc coating specification was independently verified as part of the CE marking process by KIWA Inspecta, confirming that the material complies with its declared specification. This means the 275gr/m² figure is not a marketing claim — it is a certified material fact.
Every KLASIKA greenhouse frame carries a CE marking certificate issued by KIWA Inspecta — one of Europe’s leading independent testing, inspection, and certification organisations, operating across more than 30 countries.
For structural steel products, CE marking under EN 1090-1:2009+A1:2011 is a serious and demanding standard. It requires the manufacturer to demonstrate that their production process, quality management systems, and material specifications all meet defined performance criteria — not as a one-time assessment, but as an ongoing verified commitment.
The EXC2 performance class to which KLASIKA frames are certified is the class applied to load-bearing structural steelwork in buildings and structures — the same performance class used for residential building steel. It is not a category designed for garden products; it is a category that greenhouse frames happen to qualify for when they are built properly.
The practical meaning for a buyer is straightforward. An independent expert has examined the manufacturing process, tested the materials, and confirmed that what is being sold matches what is being claimed. This is external verification — not self-certification, not a retailer’s description, not a marketing assertion. The CE marking is evidence that the frame specification is real.
There is one advantage of a steel greenhouse frame over aluminium that appears in no comparison table and is rarely discussed in product descriptions, but which any experienced greenhouse grower will immediately recognise as significant.
You can tie plants directly to a steel frame.
KLASIKA and BALTIC LT greenhouses are strong enough to support climbing plants tied or clipped to the frame arches. Indeterminate tomatoes trained on vertical strings attached to the ridge. Cucumbers clipped to horizontal wires running between arches. Beans twined around the uprights. The steel frame carries this load without flexing at the fixings, without loosening at the joints, and without any concern about progressive damage to the frame structure.
For a grower who wants to maximise vertical growing space — and in a greenhouse, vertical space is some of the most productive space available — this is not a minor detail. It changes how you plan and plant your greenhouse. You can grow more, because you are using the full internal volume rather than just the floor area.
Aluminium frames can accept light loads at fixing points, but the softer, more ductile material is more susceptible to gradual deformation around fasteners under sustained load. Repeatedly removing and replacing fixings in aluminium — as you would across multiple growing seasons — can widen fixing holes and reduce the reliability of the fixing over time. Steel does not have this problem.
KLASIKA’s own specification notes that the frame strength allows you to tie plants to the frame and hang pots and shelves. This is not a feature being added — it is a natural consequence of building a greenhouse frame from properly specified structural steel.
Any honest account of steel frames has to acknowledge one material disadvantage, and the honest account here is this: steel is a better thermal conductor than aluminium, and both are dramatically better conductors than timber or plastic.
Cold bridging occurs where a material with high thermal conductivity creates a direct path for heat to travel from the warm interior of a structure to the cold exterior. In a greenhouse, this means that where steel frame sections contact both the interior air and the exterior, heat escapes more readily at those points than through the insulating polycarbonate panels.
The KLASIKA omega-profile design addresses this directly, though not by eliminating the physics — it manages the engineering consequences. The omega section makes contact with the interior of the greenhouse along its flanged edges rather than across a broad flat face, minimising the contact area that is simultaneously exposed to interior and exterior temperatures. The arch spacing — 67cm in the KLASIKA ARCHED and KLASIKA HOUSE, 100cm in the BALTIC LT — means that frame sections represent a small proportion of the total greenhouse envelope surface area. Most of the envelope is insulating polycarbonate panel.
The thermal consequence of this geometry is that cold bridging through frame sections is a minor and localised effect within the overall greenhouse thermal performance. The R-value figures for the polycarbonate panels — approximately 1.43 for 4mm and 1.54 for 6mm — describe the envelope performance where it covers the majority of the surface. Frame sections are a small fraction of that surface.
In practical growing terms, the difference is not measurable in overnight temperature retention compared to an aluminium-framed greenhouse of equivalent specification. The cold bridging effect of steel over aluminium in a greenhouse context is real in principle and negligible in practice.
Aluminium became the dominant greenhouse frame material for straightforward commercial reasons: it is lighter, does not require galvanising to resist corrosion, and is cheaper to extrude into complex profiles. These are real advantages.
What aluminium does not have is the rigidity of steel. An aluminium extrusion of the same cross-sectional dimensions as a steel section is meaningfully less stiff — aluminium’s modulus of elasticity is approximately one-third that of steel. To achieve comparable rigidity, aluminium profiles need to be thicker or more geometrically complex, which increases material cost and section weight. Most aluminium greenhouse frames achieve adequate rigidity for normal conditions but are noticeably less solid in feel and perform less well under the sustained loads — wind racking, heavy snow, canopy weight — that a serious working greenhouse encounters over a decade.
Aluminium also holds screws less firmly than steel, and repeated fixings and refixings over multiple growing seasons gradually enlarge holes in aluminium sections in a way that does not occur with steel. For a greenhouse used actively and fitted out with shelving, irrigation systems, and plant-support fixtures, this becomes a maintenance consideration over time.
Neither of these is a fatal objection to aluminium. Aluminium greenhouses are sold in large numbers and perform adequately for many buyers. The case for steel is not that aluminium fails — it is that steel, at the specification used in KLASIKA and BALTIC LT greenhouses, performs more robustly in the situations that a serious growing greenhouse encounters across a decade of use.
Timber-framed greenhouses — western red cedar being the most commonly used species — occupy the premium end of the greenhouse market, and their appeal is genuine and well-founded. Timber is an excellent thermal insulator, which means timber frame sections do not contribute to cold bridging in the way metal frames do. Cedar has natural oils that resist decay without treatment, and a well-maintained cedar greenhouse has a warmth and character that no metal frame can replicate.
The case for timber in aesthetic and insulation terms is strong. The case for timber in practical and economic terms is more nuanced.
Timber-framed greenhouses at comparable quality to the KLASIKA range are typically priced at two to three times the equivalent steel model. The material cost of cedar is higher, the manufacturing process is more labour-intensive, and the complexity of timber joinery adds cost at every connection point.
Timber also requires maintenance that steel does not. Cedar weathers to silver-grey without treatment, which some gardeners prefer; if the original colour is to be maintained, annual or biennial oiling is necessary. Glazing bars may eventually shrink and require re-sealing. Frames must be checked periodically for any ingress of moisture at joints.
For the buyer for whom aesthetics are the paramount consideration and budget permits the premium, timber is a beautiful and legitimate choice. For the buyer who wants a long-lasting, hard-working greenhouse that requires minimal maintenance, performs structurally throughout its service life, and supports the full range of growing activities without restriction, galvanised steel at the KLASIKA specification is the more practical and considerably more economical answer.
The most important comparison to understand is not steel versus aluminium or steel versus timber — it is the difference between properly specified galvanised steel and the lightweight steel used in low-cost imported greenhouse kits.
Budget greenhouse frames are almost always steel — steel is cheaper than aluminium at thin gauges — but the specification varies enormously. Common characteristics of budget import frames include steel thickness of 0.5–0.7mm (versus 1mm in KLASIKA profiles), zinc coatings at 80–120gr/m² or lower (versus 275gr/m² in KLASIKA profiles), and no independent quality certification of any kind.
A steel frame with a thin zinc coating at lightweight section is not comparable to a CE-certified galvanised frame at 275gr/m². The thin coating begins to show surface rust within a few years of outdoor exposure — typically at cut edges, fixing holes, and areas of surface abrasion where the coating is thinnest. Once rust begins at the surface, it progresses through the zinc layer rapidly and into the steel beneath. A lightweight frame that is rusting structurally is a frame that is weakening, and in a greenhouse context this means panels that no longer sit correctly, doors that bind, and eventually sections that fail entirely.
The Z275 coating on KLASIKA profiles is a protective layer thick enough to degrade for years before the underlying steel is at any risk. It is the difference between a consumable protective layer that needs replacing and a durable one that sees out the greenhouse’s intended service life without intervention.
One of the most coherent aspects of the KLASIKA and BALTIC LT design philosophy is that the frame specification is calibrated to match the panel warranty. Brett Martin polycarbonate carries a 10-year UV warranty. The galvanised steel frame is built to a specification — CE-certified, 275gr/m² zinc coating, 1mm section — that is designed to outlast those panels comfortably, providing a structural envelope into which replacement panels can be fitted at year ten without the frame itself requiring attention.
This is the right way to think about a greenhouse investment: a frame that performs for the life of the glazing, and then continues performing through the next glazing cycle. The steel is not a cost item that will need replacing in parallel with the panels — it is the permanent structural element around which the renewable glazing system operates.
That alignment — between frame longevity and panel warranty, between structural specification and intended service life — is what distinguishes a properly designed greenhouse from a product that happens to keep rain off plants for a few years.
| Galvanised steel (KLASIKA) | Aluminium | Timber (cedar) | |
|---|---|---|---|
| Structural rigidity | Excellent | Good | Good |
| Screw retention | Excellent | Moderate | Good |
| Corrosion resistance | Excellent (Z275 zinc) | Excellent (inherent) | Good (with maintenance) |
| Plant-tying suitability | Excellent | Moderate | Good |
| Cold bridging | Minor (managed by omega profile) | Minor | Negligible |
| Maintenance required | None | None | Periodic oiling/re-sealing |
| Relative price | Mid | Mid–low | Premium (2–3× higher) |
| CE certification | Yes (KIWA Inspecta) | Varies | Not applicable |
Galvanised steel at the KLASIKA specification is not a compromise or a budget material. It is the choice that delivers the most complete combination of structural performance, long-term durability, practical growing functionality, and value across the service life of a serious greenhouse. It is the working gardener’s frame — built to hold, to last, and to work as hard as the plants it protects.
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