One load set, seven packages
These are not seven products that happen to be sold together. They are one structure, divided where procurement needs a boundary, and every package in it is sized against the same four things.
- The wind that tries to lift the roof. A glasshouse is light, large and smooth, standing in the open. Uplift — not weight — is what the foundation resists, what the bracing carries and what the gable columns are sized for.
- The snow that tries to hold it down. The load case that governs wherever snow falls, and one that governs a 26° roof differently from a flat one.
- The crop hanging from the gutters. A tomato or cucumber crop on a hanging gutter system is a substantial, permanent, distributed load on the aluminium — applied years after the building was designed, by someone who assumes the building will take it.
- The equipment a grower adds over twenty years. Screens, heating pipe, lighting rails, internal transport, CO₂ distribution. Each is hung off the gutter or the truss, and each was either in the original load case or it was not.
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Wind Tries to lift a light, smooth roof. The foundation, the bracing and the gable columns are sized for it.
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Snow Holds the roof down, and sits differently on a 26° roof than on a flat one.
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Crop A tomato or cucumber crop on hanging gutters is a heavy, permanent load on the aluminium.
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Equipment Screens, lighting, pipe and transport, hung over twenty years. Either it was in the design case or the structure is working outside it.
The same four cases size every package, from the footing to the gutter.
That last point is the one worth insisting on. The reason these pages carry section sizes rather than descriptions is that most of the load a greenhouse ends up carrying is added after handover. A structure sized for a bare house and then equipped is being used outside its design case, and nothing about it looks wrong until something is hanging from it.
The grid
Everything follows from two dimensions. 8.00 metres is the truss span — the distance between column rows, and so the width of clear floor a grower works in. 5.00 metres is the bay along the gutter. At the gable the column spacing tightens to 4.00 metres, because a gable column is a cantilever against the wind on the whole end wall rather than a member carrying a share of a light roof.
- Internal columns The dots, on the 8.00 × 5.00 m grid. 120 × 60 × 5 mm — the lightest, because they carry the least.
- Side walls Two columns to each 5.00 m bay, 140 × 80 × 3 mm, taking the wind on the long faces.
- Gable faces The accent squares, at 4.00 m centres — one under each gutter line — in 140 × 140 × 4 mm, because they take the wind off the whole end wall.
- Gutters and trusses Gutters (dashed) run the length of the house every 4.00 m. Trusses (thin) cross it at every bay, from one column row to the next.
Two numbers and one exception. Change any of them and the footings, columns, truss, gutter, vents and glass all move together.
The roof is framed at 4.00 metres — one assembled aluminium frame per bay — which turns glazing a hectare into a repeated operation rather than a bespoke one.
Those numbers set the footing layout, the column schedule, the truss, the gutter span, the vent spacing and the glass sizes. Change one and the whole set moves, which is why the grid is settled at design stage and then not revisited.
