FRP purlins suit greenhouses because the air inside is almost always humid and carries fertiliser and crop-protection sprays that rust steel, while FRP does not rust. Greenhouse rafters are usually 3 – 4 m apart, needing SF-120 or SF-160 depending on wind pressure. Hanging crop wires, lights or fans from the purlins needs a separate check.
Why do steel purlins rust fast in greenhouses?
Misting and evaporation from the soil keep the air humid, and at night dew forms on the frame. Dissolved fertiliser and sprays settle on the steel and wear the zinc away faster than in dry outdoor air.
Which FRP purlin size for a greenhouse?
| Rafter spacing | 0.65 kPa (I) | 0.95 kPa (II) | 1.25 kPa (III) | 1.55 kPa (IV) |
|---|---|---|---|---|
| 3 m | SF-100 | SF-120 | SF-120 | SF-160 |
| 3.5 m | SF-120 | SF-120 | SF-160 | SF-160 |
| 4 m | SF-120 | SF-160 | SF-160 | SF-220 |
For light plastic roof sheets with purlins at 660 mm. Film greenhouses on arched frames work differently, with purlins mainly as bracing; send a drawing for advice.
Can crop wires, lights and fans hang from the purlins?
Yes, but they must be calculated separately: the span tables cover the roof sheets, maintenance workers and wind only. Tomatoes and cucumbers on wires can add many kilograms per metre when fruiting. Send the number of wire rows, their spacing and the crop, or hang the wires from a separate system fixed to the rafters. Trellis poles: FRP round tube Ø34.
Do light and heat in a greenhouse affect FRP?
Purlins under the roof get less direct UV than outdoor purlins. Closed greenhouses can get very hot at midday; FRP handles normal under-roof temperatures, but keep it away from heaters and flues. More: how long FRP lasts outdoors.
Frequently asked questions
Which purlins do not rust in greenhouses?
FRP composite purlins, which do not rust in humid air, fertiliser fumes or crop-protection sprays. The size depends on rafter spacing and wind pressure.
Which FRP purlin suits a greenhouse with 4 m rafters?
For plastic sheets with purlins at 660 mm: SF-120 at 0.65 kPa, SF-160 at 0.95 – 1.25 kPa and SF-220 at 1.55 kPa basic wind pressure.
Can tomato wires hang from FRP purlins?
Yes, but the load must be calculated separately because the span tables do not include crop loads, or the wires can hang from a separate system fixed to the rafters.