An FRP purlin is a roof purlin made of glass fibre reinforced polymer instead of steel or timber. It does not rust or rot, weighs about a quarter of steel for the same volume and does not conduct electricity. Santiago supplies FRP box purlins in six sizes, SF-60 to SF-220, for rafters 1.2 – 6.8 m apart.
FRP purlins on a roof frame
Where are FRP purlins worth using?
Where steel purlins rust quickly: poultry and pig farms with ammonia, buildings near the sea, shrimp ponds and fish stores, greenhouses, fertiliser stores, plating shops and wastewater plants. In a dry town house, galvanised steel purlins are cheaper and work well.
FRP purlins in a farm building
FRP, steel or timber?
| Property | FRP | Galvanised steel | Timber |
|---|---|---|---|
| Rust or rot | No | Rusts, fast in ammonia and salt | Rots, termites |
| Weight | About 1/4 of steel by volume | Heaviest | Lightest |
| Stiffness | 8 – 10 times less stiff than steel | Stiffest | Medium |
| Painting | Not needed | Repainting in wet or corrosive air | Termite treatment |
| Conducts electricity | No | Yes | No when dry |
| Price per metre | Higher than steel | Lowest | Depends on wood |
Because FRP is less stiff, an FRP purlin is taller than a steel purlin for the same span. Sizes are chosen from the rafter spacing and wind pressure, not by copying the steel section.
FRP box purlins
Purlins fixed to the rafters
Which sizes are available?
| Code | Box (mm) | kg/m |
|---|---|---|
| SF-60 | 60 × 30 × 3 | 0.98 |
| SF-80 | 80 × 40 × 3 | 1.33 |
| SF-100 | 100 × 50 × 3.5 | 1.95 |
| SF-120 | 120 × 60 × 3.5 | 2.36 |
| SF-160 | 160 × 55 × 4, double cell | 3.60 |
| SF-220 | 220 × 55 × 4, double cell | 4.53 |
Span tables for each size are on the SF series page.
FRP purlin profile
FRP purlins on site
How are FRP purlins installed?
- Fix 304 stainless L-brackets to the rafters.
- Stand the box upright, tall side vertical.
- Put a sleeve inside the box at each bolt so tightening does not crush it.
- Join purlins only over a rafter.
- Fit end caps, then screw the roof sheets on with stainless screws.
Installing FRP purlins
Bracket and bolt connection
Purlin rows on the frame
Purlins ready for the roof sheets
Finished purlin layout
What should I avoid?
- Laying the box flat: it is several times less stiff that way.
- Ordinary steel bolts and screws, which rust before the purlin.
- Hanging fans, lights or crop wires from purlins without a separate calculation.
- Using FRP next to heat sources such as dryers or chimneys.
FRP purlins under a plastic roof
FRP purlin roof
FRP purlin detail
Frequently asked questions
Do FRP purlins rust?
No. FRP is glass fibre and resin with no iron.
How heavy is an FRP purlin compared with steel?
About a quarter of steel for the same volume. A 6 m SF-100 weighs about 11.7 kg.
Can FRP purlins be welded?
No. They are joined with brackets and stainless bolts.
Contact: Santiago Import Export Co., Ltd, Vietnam. WhatsApp +84 935 391 495 (Mr. Quyen), hotline +84 965 437 316 (Mrs. Ly), email salessantiagoroofing@gmail.com.
Updated October 2026.
