Key takeaways

  • Set out from the ridge and work down both slopes together — working one side first pulls the frame out of square.
  • Lap Z purlins over the rafter in the direction that sheds water, so the lap never traps moisture.
  • Tolerance stacks: if the frame is out of square, purlin spacing will be wrong no matter how carefully you set out.
  • Never load a purlin before the cladding is fixed across at least two bays — purlins are braced by the sheet.

Before you start

Check two things first: that the frame is square and plumb, and that the purlin lengths match the drawing. Purlins are cut to a fixed length in the factory, so an out-of-square frame cannot be corrected at the purlin stage — it will simply show up as uneven overhangs and gaps at the ridge.

Confirm the lap direction on the drawing. For Z purlins on a roof, the lap should always be arranged so water runs over the lap rather than into it. Getting this wrong is easy and expensive to correct later.

Set-out and fixing sequence

Set out from the ridge and work down both slopes simultaneously. Working one slope to completion first pulls the frame sideways as the purlins stiffen the structure on that side, and the second slope then has to be forced into position.

Fix purlins to the rafter through a cleat or directly through the web with the specified bolts. Do not substitute self-drilling screws for structural bolts on long spans — they have far lower shear capacity, and purlin connections are designed for bolted shear transfer.

Where purlins lap over a rafter, bolt the two sections to each other and to the rafter. A common mistake is to bolt only the outer section, which defeats the purpose of the lap because the two sections cannot transfer load between them.

Lapping Z purlins correctly

The lap is the whole reason Z purlins exist. Two lapped Z sections nested over a rafter form a continuous beam, which is why the bending moment at mid-span drops by roughly half compared with a row of single spans.

For the lap to work, the bolts must transfer shear between the two nested sections along the lap length — typically 300–400 mm each side of the rafter. Bolt pattern and lap length come from the engineer's design, not from what looks adequate on site.

Check that the sections nest properly. If the purlin has been formed out of tolerance or has twisted during transport, the lap will not close and the bolts will pull the sections into line at the cost of local distortion.

Tolerance and the mistakes that matter

Purlin spacing tolerance is usually specified as ±5 mm. In practice the bigger problem is cumulative drift: if each purlin is 3 mm out in the same direction, by the twentieth purlin you are 60 mm out and the roof sheet no longer lands on the purlins it should.

Measure from a fixed datum every time rather than from the previous purlin. Chaining measurements is the single most common cause of cumulative error on a purlin roof.

Do not walk on or load purlins before the cladding is fixed over at least two bays. Purlins are braced laterally by the cladding; an unbraced purlin is far weaker than the design assumes, and it will twist under a person's weight.

Frequently asked questions

What is the correct lap length for Z purlins?
Typically 300 mm to 400 mm each side of the supporting rafter, but the exact figure comes from the structural design. The lap length is what allows shear to transfer between the two nested sections, so shortening it on site invalidates the continuous-beam assumption.
Should I fix purlins with bolts or self-drilling screws?
Use the specified bolts for the purlin-to-rafter connection. Self-drilling screws are acceptable for cladding but have much lower shear capacity than bolts, and purlin connections transfer significant shear — especially at laps, where the whole continuous-beam action depends on it.
Which direction should Z purlins lap?
Arrange the lap so water runs over it rather than into it. On a roof that normally means lapping downhill-overshed. Check the drawing, because getting it wrong means the lap becomes a moisture trap and a corrosion point.
How much tolerance is acceptable in purlin spacing?
Typically ±5 mm on individual spacing, but control the cumulative drift as well. If you always measure from the previous purlin, small errors accumulate — measure from a fixed datum instead.
Can you walk on purlins before the roof is sheeted?
Not safely, and not without changing the structural behaviour. Purlins rely on the cladding for lateral restraint; an unbraced purlin twists under load well below its design capacity. Fix sheets across at least two bays before loading anything.
Why do my purlins not sit flat on the rafter?
Almost always bow or camber from coil set that was not removed during forming. If the machine's leveling unit is set too light, purlins arrive with a curved web and will not sit flat. This is a machine setting issue, not a site issue, so raise it with your supplier.
How do I handle purlins at the ridge and eaves?
The end purlins take different loads from the intermediate ones and are usually set out to match the cladding overhang rather than the regular spacing. Follow the cladding layout drawing, and double-check the eaves purlin position before fixing — moving it later means moving the gutter.