Steel Roof Purlins: The Unsung Heroes Holding Up Your Roof Panels

Steel purlins are the horizontal secondary members that span between roof trusses or rafters, supporting roof cladding. Below is purlin data from an actual roofing layout drawing.

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Basic Specs

Item	Value
Purlin types	C140 (SL1-1), C90 (SL2 series)
Material	Q235B
Surface finish	Plain galvanized
Purlin spacing	~1,590 mm
Standard length	8,575 mm
Max length	9,400 mm
Total purlins	380+ pcs

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Purlin Schedule

 

Name	Size	Qty	Length	Unit Wt	Total Wt	Wt/m
SL1-1	C140	70	8,575 mm	26.3 kg	1,814.7 kg	3.07 kg/m
SL2-2	C90	10	3,900 mm	8.9 kg	89 kg	2.29 kg/m
SL2-3	C90	218	8,575 mm	19.6 kg	4,272.8 kg	2.29 kg/m
SL2-4	C90	44	9,400 mm	21.5 kg	946 kg	2.29 kg/m
YP-1	C90 (eave)	27	5,875 mm	13.5 kg	363.2 kg	2.29 kg/m
L1 (tie rod)	∅12 tie	400	100 mm	0.2 kg	80 kg	—

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Why C/Z Purlins

Lightweight — 2.3–3.1 kg/m is far lighter than steel angle or channel

Galvanized corrosion resistance — zinc coating prevents rust for 20+ years

Cold-formed efficiency — thin-wall section is strong for its weight

Easy to cut & drill — onsite cutting and drilling is fast and simple

Standard sizes — C90, C140 readily available from stock

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Advantages

Cost effective — cold-formed steel is cheaper than hot-rolled sections for secondary members

Fast installation — bolted to cleats on rafters, no welding needed

Compatible with all roof panels — supports metal, FRP, and sandwich panels

Adjustable spacing — spacing can be tuned to roof load and panel span

Recyclable — galvanized steel is 100% recyclable at end of life

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Disadvantages / Limitations

Thin wall prone to damage — 2 mm wall dents easily during handling

Needs sag rods — long spans require tie rods to prevent sagging

Less durable than painted steel — galvanized coating scratches off easily

Not for heavy loads — C90/C140 only suitable for light roof loads

Fire protection needed — thin steel loses strength quickly in fire

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Typical Applications

Industrial factory roofs — supports metal roof panels on steel rafters

Warehouse & logistics buildings — long-span C/Z purlins for light roof

Canopy & awning structures — small C purlins for canopy roofs

Agricultural buildings — livestock sheds and greenhouses

Portal frame buildings — standard purlin system in pre-engineered buildings

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Installation Notes

Purlins bolted to steel cleats welded on rafters

Sag rods (L1) installed every other purlin row to prevent sagging

Eave purlins (YP) reinforced at roof edge

Galvanized coating damaged during install must be touched up with zinc paint

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