Roof purlins and their relationship with the covering

First identify what the member supports. In a timber roof, a purlin may provide intermediate support to rafters. In a hall, it spans between frames or trusses and receives covering loads. Not every beam visible beneath a roof serves the same function.

Check the force path into the main structure and the required restraint against rotation and instability. A large purlin section does not replace suitable connections or bracing. Assess the member together with its actual restraint conditions.

Material selection relates to span, loads and the building method. Timber requires checks of grade, moisture and joints; steel requires checks of section shape and thickness. A cold-formed thin-walled profile and a heavier beam of equal height do not have identical properties.

Consider fire requirements, moisture and any aggressive environment. The covering type alone does not determine the purlin material. Each option needs appropriate protection and fixings suited to the actual substrate.

The sheet must span safely between purlins with the required stiffness. T18 or T35 are not automatically suitable for every hall; larger spans may require structural profiles. Rib height remains only one of the relevant parameters.

Compare tabulated data at matching thickness, orientation, span count, support width and deflection criterion. The largest number in a table is not universal approval. A multi-span sheet and one supported on only two purlins can behave differently.

Check an overhang beyond the final support separately from the span between supports. That sheet portion has different loading and deflection conditions. Establish eaves requirements before delivery rather than deriving them from a table for an ordinary span.

Sandwich panels have their own load tables; bare trapezoidal-sheet data do not describe their behaviour. Core, facings and temperature effects matter. For dark colours and temperature differences between panel faces, check the conditions specified by the manufacturer.

Provide the required bearing width and level surface, especially at panel end joints. Do not force a panel against a crooked structure to conceal an error. Agree purlin spacing, panel lengths and joint locations before ordering.

Select screws for wind loading and the support material. In steel, drilling capacity and substrate thickness matter; timber requires appropriate anchorage. The screw length visible above the panel does not confirm correct engagement in the purlin.

Do not assume that any screwed-down sheet will brace the entire hall. Using the covering as a diaphragm requires design and suitable connections. Stability during erection must also be provided separately before the final bracing arrangement is complete.

Gather support spacing, widths and levels, plus openings, rooflights and services. Roof area alone is insufficient for selection. Include locations where higher local loads may occur, such as beside a taller part of the building.

Check geometry and complete connections before covering. Reassess the structure whenever the covering profile or thickness changes. Economy should be judged for the coordinated roof assembly, not by independently buying the cheapest purlin, sheet and fastener.