Roof Purlins

Quantities, sections and spacing come off the structural drawing before anything is ordered. On the frame, spacing is set out from the eaves with end conditions marked — where a purlin lands on a splice, a header or an opening, that is resolved on the layout rather than at the point of fixing.

Purlins are fixed with the specified fasteners and straightened as the plane forms. Levels are checked across the bay, not just at each member, so the roof reads flat when the sheeting follows.

Over longer spans, purlins need intermediate restraint: sag rods and bridging at the specified centres stop the section rotating and sagging under load. Where lengths run out, splices sit over supports with laps detailed to the manufacturer requirements — never mid-span where the bending is highest.

Bracing in the plane of the roof works with the purlins to hold the rafters. Everything is installed in the sequence the structural design assumes, so the frame gains stability as it grows rather than depending on a final connection.

Fixings follow the loading direction: the upper flange carries the sheeting, so fasteners land where the section is designed to take them. Where purlins bear on masonry or concrete support lines, a bearing pad or cleat keeps the load path clean — no member simply sits on a mortar joint. Corrosion protection on cut edges and on any site drilling is touched up before the plane closes up, because nothing under sheeting gets a second pass.

Before the sheeting crew arrives, we walk the plane: spacing, alignment, splice positions, restraint installed, fixings complete. The sheet layout is then checked against purlin centres so every fastener lands on steel.

Quality is measured rather than eyeballed: spacing is recorded at every bay, the plane is checked with a string line or level across the full width, and any member outside tolerance is corrected before sheeting begins. The final walk is signed off jointly with your inspector, and the record goes into the handover pack beside the fastener and splice details.

Where the purlins tie into new steel, work is sequenced with the steel structures scope; where an existing frame is being re-roofed, the survey decides what stays and what is replaced. Send the purlin schedule and we will price supply and fixing together.