16 10 月, 2026
Greenhouse Steel Frame: How to Specify Galvanized Sections
A greenhouse steel frame is a galvanized structure built to a span and a bay, and most of the regret in a greenhouse order sits in the coating rather than in the steel. The steel is ordinary structural steel. What decides whether the house lasts is that every section inside it is protected against a building that is wet almost every day, and that the frame is specified as a system.
Short answer
A greenhouse steel frame is built from galvanized square and rectangular tube for the arches, columns and glazing bars, galvanized angle for the bracing and the base rail, and formed sections for the secondary members. The sections are hot-dip galvanized, or formed from pre-galvanized coil, because a greenhouse holds moisture for most of the day and bare steel will not last in it. What decides the order is the span and the bay, the section family, the coating route, the connections and the foundation.
What a greenhouse frame is made of
| Part of the frame | Usual section | What it does |
|---|---|---|
| Arch or truss | Galvanized square or rectangular tube | Carries the roof |
| Column or post | Galvanized square tube or angle | Carries the arch |
| Purlin and glazing bar | Small tube or formed section | Supports the covering |
| Bracing and base rail | Galvanized angle or flat | Steadies the frame |
| Gutter and connection | Formed sheet or plate | Water and joints |
The covering, the glazing and the ventilation all sit on the same frame, so the section follows the covering. A film house and a glass house load the frame and the glazing bars differently, so the section family is chosen once the covering is known.
Why the frame is galvanized
A greenhouse is a wet building: condensation, irrigation and airborne fertiliser all attack steel, and a frame that rusts along a glazing bar or at the base fails before the covering wears out. That is why the frame is specified as galvanized rather than painted. Two coating routes are common, and they are not the same thing:
- Hot-dip galvanizing – the finished member is dipped in molten zinc, so the coating covers the inside and the cut ends; it is chosen where the house is humid or the frame has to outlast the covering
- Pre-galvanized coil – the strip is coated before the section is formed, so the coating is thinner and the cut ends are bare unless treated; it is chosen where the frame is standard hardware rather than a long-life structure
The coating is specified as a coating type and a coating mass, not a colour or a brand.
What to fix before ordering
Most greenhouse disputes are created before production starts. These points decide whether the frame goes together:
- The span, the bay and the height, because the whole frame is sized from them
- The covering, since film, polycarbonate and glass load the frame differently
- The section family and the standard, so the tube, the angle and the connections come from one system
- The coating route and the coating mass, written as a specification rather than a product name
- The foundation and the anchor detail, because the base of a greenhouse is where corrosion starts
- The ventilation and the door openings, since they interrupt the frame and have to be framed in
Greenhouse steel frames are supplied to the drawing and the standard named on the order; the covering, the coating and the foundation are confirmed at quotation.
What this means for your order
Because the frame, the covering and the ventilation are one system, a greenhouse enquiry settles fastest when the span, the covering, the coating and the foundation are named together. Where the covering is still open, say so: the frame can be designed for the heaviest option, but only if that is known before the sections are cut.
Related reading
- Galvanized square and rectangular tube
- Galvanized angle steel
- Solar mounting steel: grades and coatings
- Steel weight calculator
Frequently asked questions
What steel is used for a greenhouse frame?
Galvanized square and rectangular tube carries the arches, columns and glazing bars, galvanized angle carries the bracing and the base rail, and formed sections carry the secondary members. The steel is ordinary structural steel; the galvanizing is what suits it to a wet building.
Should a greenhouse frame be hot-dip galvanized or pre-galvanized?
Hot-dip galvanizing coats the formed member inside and out and covers the cut ends. Pre-galvanized coil is coated before forming, so the coating is thinner and the cut ends need treating. The choice follows how long the frame has to last.
What span and bay should a greenhouse be ordered to?
The span and the bay are set by the covering, the site and the growing plan, not by the steel. They are named on the drawings and the sections are sized from them, so they are the first thing to fix in an enquiry.
How is a greenhouse frame protected at the base?
The base is the wettest part of the house and the part that meets the ground and the fertiliser, so the coating and the foundation detail are specified together. A frame protected everywhere except at the base still fails there.
Send the span, the covering, the coating and the destination port, and the enquiry is worked against those points. See the steel product catalogue or request a quotation.