Z purlins for sandwich roof panels: spacing, flange width and connections

Panel documentation may also account for facing thickness, colour and serviceability criteria. The purlin check requires the properties of the actual section and its support conditions. Both sets of calculations must describe the same roof. When using load tables, check their assumptions against the proposed arrangement.

Only part of the Z section supports the panel. The bearing width must meet the requirements of the panel and the connection detail and accommodate the specified screw positions. Contact with the steel alone is insufficient. The connection must transfer the design forces in the relevant directions throughout its service life.

Z sections of equal depth can have different flange widths. Compare the section drawings before making a substitution. Check any end laps between roof panels as well. The bearing requirements at a lap can differ from those at an intermediate support beneath a continuous panel.

For example, a 200 mm deep Z section formed from 2 mm steel may have flanges 67 and 60 mm wide. Identify which flange faces upwards when checking bearing width, and show the orientation on the erection drawing. The nominal section depth gives no measure of the available bearing. Use the full section drawing, including the bends, to determine the usable flat width.

Where the structural system calls for it, Z purlins may overlap above a primary support. The structural design specifies the lap length and bolts. This connection allows the purlins to act together. A roof panel joint above a support has separate requirements for bearing, sealing and fixing.

Draw both details together in a section through the support. Bolt heads, changes in level and bracket parts must leave the intended bearing surface clear, with access for the installer. Resolving clashes on site by cutting the panel to fit can create unnecessary cuts and potential leaks.

Select the panel fastener for the type and thickness of the supporting steel. Check the drilling capacity, the conditions for thread engagement and the roof panel thickness at the fixing point. A single long screw cannot cover every combination of purlin and panel.

The cladding documentation and design loads determine the number and positions of fasteners. They must resist wind suction in service as well as hold the panels during erection. Specify the complete fixing assembly when selecting galvanised sections so that accessory costs can be included when comparing quotations.

The roof panels can restrain the purlin if their properties and fastenings provide the interaction assumed in the design. The degree of restraint depends on the panel and can differ under downward loading and wind uplift. The design should identify the structural model and the bracing required.

Bracing may consist of channel sections with suitable connections or another proprietary arrangement. Keep the specified bracing in the bill of materials even when roof panels are to be installed. Check stability separately for the erection stage while the cladding is incomplete. The erection plan must provide the restraint the cold-formed sections need at that stage.

Before laying the panels, check the position and alignment of the supports in the roof plane. Forcing a panel down with the fasteners to compensate for excessive unevenness can affect the facings, joints and seals. Resolve discrepancies before covering the area.

Inspect the galvanised surfaces, remove swarf and dirt, and assess any damage in accordance with the material requirements. Use tapes and seals specified for the detail. An extra layer can change the bearing level or how a fastener tightens, so adding one does not necessarily improve weathertightness.

Channel and sigma sections may suit other purlin designs, but their connections must be checked individually. Hat sections have separate applications in framing and battens. Any change in section type or dimensions requires a check of the structure and of the panel fixing surface. Approve the change before ordering the sections and panels.

Assess the roof as an assembly of purlins, panels, fasteners and flashings. A price per metre of purlin covers only one of those items. Well-chosen Z sections can provide a lightweight supporting structure. Coordinating the connections with the sandwich panels and showing them clearly on the drawings reduces rework on site.