C300 Steel Roof Purlins: The 300 mm Backbone for Long-Span Roofs

C-section steel purlins are the horizontal members supporting roof cladding between rafters. Below is data from an actual LC300 purlin system drawing.

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Basic Specs

 

Item	Value
Purlin type	C-section (LC300×80×20)
Heights	300 mm
Flange width	80 mm
Lip size	20 mm
Wall thickness	2.5 mm (LT1) / 3.0 mm (LT2)
Material	Q355B
Tie rods	Φ12 round bar, Q235B
Knee braces	L63×5 angle, Q235B
Spacer tubes	D32×2.5, Q235B

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Purlin Schedule

 

Name	Size	Thickness	Material	Use
LT1	LC300×80×20	2.5 mm	Q355B	Standard purlin
LT2	LC300×80×20	3.0 mm	Q355B	Heavy load purlin
WLT1–WLT6	Φ12	—	Q235B	Sag rods
WXL1–WXL3	Φ12	—	Q235B	Diagonal tie
WYC	L63×5	—	Q235B	Knee brace
WCG1–WCG4	D32×2.5	2.5 mm	Q235B	Spacer tube

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Why C300 Purlins

1. 300 mm depth — deep section spans longer distances, fewer purlins needed
2. Q355B strength — higher yield than Q235, allows longer spans between rafters
3. C-section efficiency — lip stiffeners prevent flange buckling under load
4. Galvanized option — cold-formed galvanized for corrosion resistance
5. Complete system — purlins + tie rods + knee braces + spacers, all in one package

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Advantages

1. Longer spans — 300 mm deep purlins span 6–9 m between rafters
2. Lightweight — cold-formed steel is light yet strong per kg
3. Fast installation — bolted to cleats, no welding on site
4. Versatile — fits metal panels, sandwich panels, FRP sheets
5. Cost effective — standard cold-formed sections, low material cost

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Disadvantages / Limitations

1. Needs sag rods — 300 mm deep purlins still need Φ12 tie rods at midspan
2. Thin wall dents — 2.5–3.0 mm wall damages easily during handling
3. Corrosion at cuts — cut edges lose galvanized coating, need touch-up
4. Not for heavy loads — C300 only for roof, not floor or crane loads
5. Fire rating — thin steel needs fire protection for rated buildings

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Typical Applications

1. Large-span factory roofs — 6–9 m rafter spacing, C300 purlins
2. Warehouse & logistics centers — long clear spans, light roof
3. Steel structure workshops — standard purlin system
4. Aircraft hangars — huge roof area, uniform purlin grid
5. Exhibition halls — column-free space, long-span purlins

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Installation Notes

• Purlins bolted to steel cleats on rafter top flange
• Φ12 sag rods installed at midspan every other purlin
• L63×5 knee braces connect purlin to rafter for lateral stability
• D32×2.5 spacer tubes at support points for stiffening

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