C Purlins or Z Purlins: What Is the Difference?

What a purlin does

Purlins are the horizontal members that run across the rafters or trusses and carry the roof sheets. On walls the same sections are called girts and carry the cladding. If the purlins are not straight and level, the roof will not be either.

C purlins explained

A C purlin has a flat web with a flange and a small lip on each side, both facing the same way, like the letter C. It is simple to fix with a cleat at each rafter.

C purlins suit simple spans from one support to the next, which is why they are common on houses, extensions, car porches and smaller sheds.

Z purlins explained

A Z purlin has its flanges facing opposite ways. That shape lets one purlin nest into the next, so they can overlap at each support.

The lap makes a run of purlins behave like one continuous member, which is efficient on long warehouse and factory roofs with many bays.

Why sag rods matter

Long, slim purlins can twist or sag between rafters. Sag rods tie them together at intervals, keep the line straight and help the sheets sit flat. Your drawings will show where they go.

Galvanised or painted?

In humid and coastal air, galvanised or zinc aluminium coated purlins resist rust far better than painted steel, especially where the frame is open to the weather. Treat cut ends and drilled holes so the protection carries on.

Common mistakes to avoid

Most purlin problems come from small details rather than the section itself. Spacing purlins further apart than the sheet allows, leaving out sag rods on long spans and mixing coatings on the same frame are the usual culprits.

Check the roof sheet manufacturer's guidance for maximum purlin spacing, and make sure cleats and bolts match the purlin size on your drawings.

Ordering from drawings

Section, depth, thickness and spacing must follow your engineer's design. Send us the structural drawings or a materials list and we will price every purlin, cleat, sag rod and fixing.