Purlin coatings: black steel, galvanized Z275, Magnelis (ZM) or Aluzinc?

How the four material options for cold roll-formed purlins compare — grades, coating mass, cut-edge behaviour and typical exposure — and how to specify them.

Updated October 16, 2026

Cold roll-formed purlins are made from steel strip that is either uncoated ("black") or metallic-coated before forming, in the continuous hot-dip lines covered by EN 10346. The coating is part of the strip, so the choice has to be made at the order — it cannot be changed after the sections are rolled.

The number after Z or ZM is the minimum total coating mass on both faces in g/m². For Z275 that corresponds to roughly 20 µm of zinc on each side.

Black steel (uncoated)

Uncoated strip offers the widest grade range, up to S500, and the lowest material cost. It has no corrosion protection of its own, so it is used where the members will be painted or primed after rolling, or where they stay dry indoors and the specification accepts it. Account for the cost and time of painting when comparing it with pre-coated options.

Pre-galvanized Z275

Z275 is the most common purlin specification in Europe. The zinc layer protects the steel as a barrier and, where it is cut or scratched, sacrificially: zinc corrodes in preference to the steel, so thin cut edges and punched holes are protected in normal atmospheres. It is a sound default for enclosed industrial and commercial buildings.

Magnelis (ZM): zinc–aluminium–magnesium

ZM coatings add aluminium and magnesium to the zinc bath. Coating producers report clearly better corrosion resistance than pure zinc at a similar coating mass, including at cut edges, and better behaviour in ammonia-rich and chloride-laden air. That is why ZM is often specified for agricultural buildings, coastal sites and other aggressive environments, and for higher strength grades up to S450GD.

Aluzinc (AZ): aluminium–zinc

Aluzinc is an aluminium–zinc coating with about 55% aluminium. The aluminium gives a durable barrier and good heat resistance and reflectivity; the zinc share gives some sacrificial action, but generally less at cut edges than a zinc or ZM coating. It is chosen where its appearance or heat behaviour matters, or where the cladding system is specified in AZ.

Choosing by exposure — a starting point

On site: cutting, drilling and welding coated sections

• Order members cut to length and punched at the mill wherever possible, so fewer edges are cut on site.
• Remove drilling swarf from coated surfaces the same day; left on the surface it rusts and stains the coating.
• Welding burns the coating off around the weld and releases zinc fumes — ventilate, and repair the area with a zinc-rich paint afterwards.
• Use fasteners with corrosion protection compatible with the coating and the exposure.

How to write it on an order

State the grade and coating together, for example Z 200×60×15×2, S350GD+Z275 or Sigma 300×80×17×2.5, S420GD+ZM310. For black steel, give the grade only (e.g. S355). Send it with lengths, quantities and hole positions through the quote form; the minimum is 5 t per size and material. The section guide explains how to choose the profile itself.