Interactive lesson

Permanent polytunnel, step by step

Interactive lesson: a 5 × 10 m hooped polytunnel of galvanised tube — setting out, footings and foundation tubes, hoops and braces, ends, film and snow.

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The whole lesson as text

1. The project

Sizes first: a 5 × 10 m polytunnel (about 16 × 33 ft), enough for the warm crops of a family of four.

A 5 × 10 m tunnel — 50 m², three beds and two paths. Enough for the warm crops of a family of four.
Hoops and spacing — Galvanised tube 32 mm, wall 1.5–2 mm. Spacing 1.25 m (9 hoops) in the lowlands, 1.0 m (11 hoops) in upland and snowy areas.
Height and side wall — 2.7 m at the ridge. The side wall is vertical up to 1.2 m, then the curve begins.
Three beds, two paths — Side beds 1.1 m, centre bed 1.4 m for tomatoes in two rows, paths 0.6 m. The film must not touch the plants.

Why: A well-built frame of galvanised tube lasts twenty years or more, and the film is replaced every few seasons. A badly built one can fail in its first winter: under wet snow, in a gale, or because frost has heaved an anchor out of the ground.

Shop-bought kits often have 25 mm hoops at 1.5–2 m spacing: enough for a mild winter, but not for wet snow. Ask for the stronger “storm” or “snow” spec.
The sizes adapt easily to 4 × 8 or 6 × 12 m.

2. Setting out

The tunnel must be a true rectangle, otherwise the hoops won't line up and the film will crease.

Decide the direction — Long axis east–west for winter, north–south for summer.
Four pegs — Drive in four corner pegs at 5 × 10 m.
Check the diagonals — Both diagonals must be 11.18 m. Only then is the tunnel a true rectangle.
String and footing marks — Stretch a string 10 cm above the ground and mark footings every 1.25 m (or 1.0 m). Check the level across with a water level: a clear hose filled with water.

Why: The ground doesn't need to be perfectly level along the length: a slight fall of 1–2 % drains water away. But across it must be level, and you measure the height of every foundation tube from the same string.

Before digging, check where water pipes, drains, power and cables run (in the US and Canada call 811). Don't build under overhead power lines.
Dig a shallow ditch or build a low bank around the tunnel so rainwater from the roof doesn't run inside.