Poultry and pig farm roofs should use FRP purlins, because ammonia and humidity in the shed rust galvanised steel purlins many times faster than in ordinary buildings. Farm roofs usually fail the same way: fine for a few years, then the screw points rust, brown streaks run down the sheets, rust flakes fall into the feeders, and once the purlins thin out a strong wind tears the roof off.
Why do steel purlins rust fast in livestock buildings?
Manure and urine release ammonia, which rises and condenses with moisture under the roof. Closed houses with exhaust fans and cooling pads are even more humid, and at night water drips from the roof. The zinc coating is eaten in this ammonia-laden film, and every drill hole or scratch starts a rust spot. Under ISO 9223 such buildings are often C4, sometimes C5, where a Z275 coating may last only about 2.5 – 9 years.
FRP purlins contain no metal, so ammonia cannot rust them, the roof needs no painting, and no rust flakes fall on the animals.
Which FRP purlin size for a livestock farm?
| Rafter spacing | 0.65 kPa (I) | 0.95 kPa (II) | 1.25 kPa (III) | 1.55 kPa (IV) | 1.85 kPa (V) |
|---|---|---|---|---|---|
| 2.5 m | SF-100 | SF-100 | SF-100 | SF-120 | SF-120 |
| 3 m | SF-100 | SF-120 | SF-120 | SF-160 | SF-160 |
| 3.5 m | SF-120 | SF-120 | SF-160 | SF-160 | SF-160 |
| 4 m | SF-120 | SF-160 | SF-160 | SF-220 | SF-220 |
For plastic roof sheets or tiles with purlins at 660 mm. Use one size larger along the eaves and gables. Sizes: SF-100, SF-120, SF-160.
Which small details decide whether a farm roof lasts?
- Bolts, nuts and bracket screws in 304 stainless; plated steel rusts before the purlin and the roof fails at the joints.
- Stainless roofing screws with rubber washers.
- End caps so rats and birds cannot nest in the boxes.
- Plastic roof sheets or tiles so the whole roof is rust-free.
- Steel rafters that stay should be well painted, as they sit in the same air.
Do disinfectant spraying and washing harm FRP purlins?
No. Most farm disinfectants and detergents at working dilution do not affect polyester resin, and pressure washing does not damage FRP purlins. If the farm uses special chemicals, tell Santiago FRP which ones so they can be checked.
Can the roof be replaced while animals are inside?
Yes. A 6 m SF-100 weighs about 11.7 kg, so one person can carry it without a crane. Purlins arrive cut and drilled, so on the roof workers only place them and tighten bolts, with little cutting noise to stress the animals, and the roof can be replaced house by house.
Frequently asked questions
Why do steel purlins in pig and poultry farms rust fast?
Ammonia from manure and urine condenses with humidity under the roof and eats the zinc coating. Drill holes and screw points lose their zinc during installation, so they rust first.
Which FRP purlin suits a pig farm with 3 m rafters?
SF-100 at 0.65 kPa basic wind pressure, SF-120 at 0.95 – 1.25 kPa, SF-160 at 1.55 – 1.85 kPa, for plastic sheets with purlins at 660 mm. Use one size larger along the eaves.
Can FRP purlins withstand farm disinfectants?
Yes. Most farm disinfectants and detergents at working dilution do not affect the polyester resin of FRP purlins.