TECHNICAL 13 · DETAILING
Purlin spacing: set by the cover, checked by the load.
Purlins carry the cover between arches. Their spacing is set by the cover's own span table first and by snow or wind second — and buyers who skip the first step get a roof that ripples between supports.
| Cover | Purlin spacing | Profile habit |
|---|---|---|
| Film 150–200 μm | 1.0–1.2 m | C 80×40×15×2.0 |
| PC twin-wall 8–10 mm | 0.5–1.0 m | C 100×50×20×2.0 |
| Glass 4 mm | 0.6–0.8 m (glazing bar) | C or RHS rail |
| Shade net | 1.5–2.0 m | C 80 or tensioned wire |
Always check the cover manufacturer's span table; the values above are habits, not substitutes.
What the purlin actually does
Three jobs: hold the cover at the spacing its span table requires, transfer snow and wind into the arch, and provide a straight fixing line. The third one is underestimated — a purlin that sags between arches makes the cover look loose even when the steel is adequate.
The two checks
- Cover span: the maker's table gives maximum support spacing for the sheet or film at your wind/snow case.
- Structural: the purlin bending between arches under snow plus its own dead load, and uplift under wind.
A specification line that avoids argument
“Purlin spacing to suit the covering manufacturer's span table, maximum 1.0 m, with the table submitted for approval.” That puts the number where it belongs — with the cover supplier.
Punched or drilled
Purlins are usually punched to a pattern at the mill so the site crew does not drill. The pattern must match the clamp type and the arch spacing, which means the punching drawing is part of the order. A wrong pattern is a bundle of scrap.
Read next
Pages that answer the question this page raises.
- 04.04C / Z cold-formed profilesCold-formed C and Z steel purlins and rails for greenhouses: 80–120 mm profiles, 1.8–2.5 m→
- 08.10Film selectionGreenhouse film selection: 150–200 μm, UV stabilisation, thermal and anti-condensation gra→
- 08.03Snow loadGreenhouse snow load: ground snow load, shape and exposure coefficients, EN 1991-1-3 and E→
- 05.04Sawtooth PC houseSawtooth polycarbonate greenhouse frame: 9.6 m span, PC sheet support, galvanised steel, r→
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