A greenhouse is designed to a greenhouse standard, not a building standard

The first thing to settle is which document the structure is designed to. Commercial production greenhouses in Europe are normally designed to EN 13031-1, which sets out principles and requirements for mechanical resistance and stability, serviceability and durability. It is not the same document as the Eurocode route used for an industrial shed, and it does not carry the same partial factors or the same deflection allowances.

The practical difference is that a greenhouse is designed to be light. The structure is deliberately slender because every kilogram of frame is a kilogram of shadow, and because the roof has to transmit light rather than carry a heavy deck. That slenderness is bought back with tight limits on deflection and with a load path that assumes the cladding contributes. A glazed panel is not only a window; in many designs it braces the frame, and once it is removed or replaced with a heavier or lighter sheet, the bracing assumption changes.

So the enquiry has to state the standard the design follows, the design life, and whether the cladding is part of the structural model. A supplier who quotes a profile without knowing those three things is quoting a shape, not a structure.

Snow is the governing load, and it fails progressively

On most European greenhouse sites the roof is designed for snow, and the snow case is not a single event. Snow accumulates, and the way it accumulates matters more than the depth. Drift against a ridge, against a valley gutter, and against a wall all produce local loads well above the uniform blanket case, and a valley between two spans collects from both sides.

The failure mode is what makes this a serviceability problem rather than a strength problem. A rafter that is loaded past its deflection limit sags. Once it sags, the roof plane is no longer a plane, water no longer runs off, and the sag becomes a pond. The pond is a load, and the load increases the sag. In a heated greenhouse the same snow sits on a roof that is melting it from underneath, so the snow load is delivered as water and the roof carries water where it was designed to carry snow. This is why a snow assessment that only checks stress and not deflection is not an assessment.

Two controls follow. The first is to fix a deflection limit before the section is chosen, and to check the ponded case, not only the uniform case. The second is to check the connections: a greenhouse rafter is lightly loaded and its joints are lightly made, and the joint that carries the drift case is often a pair of small brackets in a slot.

The glazing bar is a thermal bridge, so the frame drips on the crop

Aluminium conducts heat roughly three orders of magnitude better than glass or polycarbonate. A single-skin glazing bar therefore has a cold inner face in winter, and any moist air that reaches it condenses there. The water then runs down the bar and lands on the crop, which is exactly where it does the most damage: leaf disease, local cold spots and uneven growth.

This is why the glazing bar in a serious greenhouse is not a plain extrusion. It is either thermally broken, with a non-metallic separation between the outer and inner faces, or it is shaped to collect the condensate in a channel and lead it to a gutter instead of letting it drip. Both are profile features, and both have to be specified. A buyer who orders a bar on the basis of its slot and its price has not specified the part that determines whether the house works.

The same logic sets the eave height and the ventilation ratio. Taller eaves give more air volume and a higher condensation point, and a well-designed ridge vent removes the humid air before it reaches the cold bar. Ventilation is a structural requirement as well as an agronomic one, because an open vent is a wind-loaded opening and the mechanism that drives it has to be mounted on something.

The gutter is a structural member that happens to carry water

In a multi-span greenhouse the gutter is the most highly loaded member in the structure. It carries the snow and water from two roof slopes, it spans between the posts, and it also has to be watertight over a length that is measured in tens of metres. Treating it as a drainage accessory is the single most common structural mistake in a greenhouse specification.

Two consequences. First, the gutter section is chosen by bending over the post spacing, and its capacity to carry water is a second, separate calculation. A gutter that is deep enough for the flow and not deep enough for the snow is a winter failure. Second, the joints between gutter lengths are a movement and a sealing problem. Aluminium expands and contracts with temperature, and over a long run the movement is several millimetres; a joint that is rigidly bolted will either leak or crack the seal. The run needs a joint that accommodates movement, and the detail has to say which end is fixed and which end slides.

Aluminium against steel, concrete and cleaning chemicals

Aluminium does not rust, but it corrodes, and a greenhouse is an unusually good place for it to happen. The environment is permanently wet, often warm, and frequently carries fertiliser salts, which are chlorides and nitrates. Three mechanisms matter.

The first is the galvanic couple. Where aluminium is in electrical contact with steel or stainless steel in the presence of an electrolyte, the aluminium is the anodic member and it corrodes. The rate depends strongly on the area ratio: a small aluminium part against a large steel part corrodes fast, because all of the corrosion current is concentrated into a small area. A galvanised steel gutter bolted to an aluminium post is a classic version of this detail, and so is a stainless bolt in an aluminium slot. The control is to break the electrical contact with an isolating washer or a coated interface, or to accept the couple and shorten the inspection interval.

The second is alkalinity. Aluminium’s protective oxide layer is stable over a pH range of roughly 4 to 9, and it is attacked outside it. Wet concrete, cement dust and many alkaline cleaning agents sit above that range. A profile that is set into a concrete kerb without a separation layer, or washed with an alkaline foam cleaner, loses its oxide layer and then its section.

The third is the cut end. An anodised profile has a protective layer on its faces, but a cut end or a drilled hole exposes bare metal. In a wet greenhouse, that is where corrosion starts, and it is where water enters a hollow section. Sealing cut ends is part of the finish specification, not a workshop preference.

Ventilation, screens and the parts that move

Every moving element in a greenhouse — ridge vents, roll-up screens, shade curtains, sliding doors — is mounted on the frame and loads it. These loads are small individually and they are repeated many thousands of times, which makes them a fatigue question rather than a static one.

Aluminium has no fatigue limit. Unlike steel, there is no stress below which an aluminium detail can be cycled indefinitely without accumulating damage; the S-N curve keeps falling. In practice this means that the design of a vent mechanism has to be judged on the stress range at the joint, not on the peak stress, and that a sharp change of section or a thread in a lightly engaged slot is a fatigue initiation site. The controls are ordinary ones: generous radii, bolts that are preloaded so the joint does not fret, and brackets that spread the load into the profile rather than point-loading a slot lip.

The other moving-part problem is wear at the slot. A slot that carries a sliding shoe will wear, and the wear opens the mouth and reduces the grip of everything else mounted in it. Where a slot is a running track, it should be a dedicated track profile or it should carry a hardened insert, not the same slot that holds the frame together.

What CBAM changed for aluminium bought into the EU

Since 1 January 2026 the EU carbon border adjustment mechanism has been in its definitive period, and aluminium is one of the covered sectors. A European greenhouse builder importing aluminium profiles or finished frame sections is now inside a regime where the embedded emissions of the goods matter, with the first annual declaration and certificate surrender due on 30 September 2027 and certificate sales starting on 1 February 2027.

Two details decide whether this affects a given order. The first is the de minimis threshold: an importer whose annual net mass of the relevant iron and steel, aluminium, fertiliser and cement imports stays under 50 tonnes is outside the obligation. The threshold is annual and per importer, and it aggregates across those sectors, so it is not a free allowance on every shipment. The second is the emissions route. Aluminium sits in the group that currently accounts for direct emissions, and the choice between verified actual values and published default values changes the certificate requirement. A default value is not automatically the cheaper or the more expensive option; it depends on the plant.

For the supplier, the practical consequence is that the emissions data has to be available with the product, tied to the correct production installation and the correct CN code, and reconciled with the invoice net weight. That is a documentation capability as much as a manufacturing one, and it is worth asking about before the first order rather than after it.

How to write the greenhouse enquiry so it can be answered

An aluminium supplier cannot size a greenhouse from a span and a photograph. The information that makes a quotation usable is the same list that makes the design checkable, and it is short. State the design standard and the design life. State the snow and wind loads, or the site, and say whether drift has been considered. State the cladding and whether it is structural. State the post spacing and the gutter run, because those two numbers set the gutter section. State the finish and the environment, including whether fertiliser salts and alkaline cleaning are present. State who owns the structural design.

Everything else — the slot, the alloy, the temper, the cut lengths — follows from those answers. A profile quotation that arrives before them is a price for a shape.