Installation and construction
What do purlins do in a roof or steel-framed building?
In brief
Purlins are structural members that support other parts of a roof or its covering directly – in a house they work with rafters, while in a hall they work with trapezoidal sheet or sandwich panels. When choosing, it is worth checking not only the covering profile but above all the spacing and width of supports, the span arrangement and the required loads, as these determine the section selection. A higher sheet profile alone does not allow supports to be spaced arbitrarily. The supplier also needs the layer build-up and fixing method, and the documentation should state who is responsible for the calculations and section selection.
Where is the purlin located?
In a timber roof structure, a purlin usually runs parallel to the ridge and supports the rafters. It transfers the load onward to columns, walls or other designated supports. This changes the effective span of the rafters. The position of the purlin therefore affects both the structure and the layout of the loft space.
In a hall, purlins can rest on the main frames and form the substrate for the covering. They are often made of steel. Structural trapezoidal steel sheet or an insulated sandwich roof panel then spans between the purlins. This is a different arrangement from tile-effect metal roofing fixed to a dense grid of timber battens.
How does a purlin differ from a batten?
A batten is a component of the grid beneath the covering, whose spacing depends among other things on the profile and module of the sheet. A purlin has a different role in the load-bearing system. The names should not be used interchangeably when ordering materials or reading a design. Doing so could lead to incorrect selection of cross-sections and fixings.
The function of a given element is determined by the structural scheme. It is not enough to assess its thickness from a photograph. It is worth tracing the load path: from the covering to the supports, then to the main elements and foundations. Such a simple description helps to understand why removing one support can change the behaviour of the entire roof.
What determines the purlin spacing?
Snow load, wind load and the weight of the layers are taken into account, as well as the parameters of the covering elements. The number of spans, the width of the bearing and the direction in which the sheet is laid are important. Load tables apply to specific conditions. A result from one arrangement is not automatically transferred to another.
A deeper trapezoidal profile may have greater stiffness, but the profile name alone does not provide the full answer. Thickness, steel grade and the orientation of the cross-section matter. An insulated panel additionally requires consideration of the core, the facings and the operating conditions. The selection of covering and supports should therefore be carried out together.
What matters besides load-bearing capacity?
The structure should carry the loads and remain within the permissible deflections. Excessive deformation can impair the performance of connections, water drainage or the appearance of the cladding. It is not enough to state that the element will not break. In a roof in service, its behaviour during normal use is also important.
The stability of the purlins and the way they are braced are important. A thin-walled steel profile may require specific ties and interaction with the sheeting. Elements that appear to be merely a fixing accessory should not be overlooked. Their role should follow from the design, also for the construction stage before the full cladding is in place.
What is the significance of the connections?
The fixing of the purlins to the support and of the covering to the purlins must correspond to the loads. In timber, the edge distances of the fasteners, the grain direction and the condition of the material matter. In steel, the thickness of the base material and the screw’s ability to form a correct hole and transfer forces are important, among other things.
Not every sheet-metal screw is suitable for any purlin. A fastener that is too short or incorrectly selected may not provide the required fixing. The design should also account for wind suction, particularly at the edges. The colour of the head and a similar appearance do not confirm the technical compatibility of two fasteners.
What changes during renovation?
New covering may require a different support spacing, a wider bearing or additional fixings. The condition of the existing purlins and their connections must be checked. Corrosion, loss of timber and permanent deformation do not disappear when the sheet metal is replaced. The assessment is carried out before ordering the material that is to work with them.
Additional insulation, installations or equipment may change the roof load. Heavy elements must not be fixed to an arbitrary purlin without verification. Even if the whole roof has a load reserve, a local support or its connection may be overloaded. The specific location of the load is important.
What data does the supplier need?
Prepare the spacing and width of the supports, the length of the roof slope, the span arrangement and the required loads. For structural sheet metal, the layer build-up and the method of fixing are also needed. An enquiry limited to the hall area does not allow the profile to be selected. The supplier can then present only a preliminary material offer. In the documentation, distinguish the centre-to-centre spacing from the clear distance between the edges of the supports.
The order documentation should clearly indicate who is responsible for the calculations and the selection of the cross-section. A catalogue table should not be confused with the design of the entire roof. The completed arrangement must account for the supports, fasteners, bracing and installation conditions. Only such a set describes the actual behaviour of the structure.
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