Building components · Frame Members
Purlin
Explains the purlin as an intermediate support that shortens the effective span of what it carries.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a purlin is
A purlin runs horizontally across a roof, at right angles to the rafters, and supports them somewhere between eaves and ridge. Its effect is to halve or otherwise divide the distance each rafter has to span on its own.
In portal-framed and industrial buildings the same word describes the horizontal members spanning between frames that carry the roof sheeting directly, and side rails do the equivalent job on the walls.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a rafter. A rafter runs up the slope; a purlin runs across it.
- It is not a ridge member. A purlin sits between eaves and ridge; the ridge is at the apex.
- It is not a tile batten. Battens are small members carrying the covering units directly; a purlin is a structural member carrying rafters or sheeting.
- It is not a beam in the frame sense, although it spans — its defining property is that it provides intermediate support to repeated members.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- purlins
- side rail
- roof purlin
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What supports it, which may be walls, struts down to a loadbearing wall below, or the frames of a portal structure.
- Its own span between those supports.
- How the rafters bear on it, since the connection is where the intermediate support is actually delivered.
- Whether anything below that supports it — a strut down to a wall, for instance — is being altered.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A purlin gathers load from many rafters and delivers it to relatively few supports, which makes those supports significant.
A sagging purlin shows in the roof line, and it commonly indicates that its own support has moved or was never sufficient.
Purlins and the struts that support them frequently cross loft spaces, and they are commonly in the way of conversions.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Purlin to Strut Support
Where a purlin is supported by a strut, and where a roof load path reaches down into a wall below.
Raises: Structural transfer · Movement · Buildability
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Pitched Roof
- Superstructure
Clarifications
Commonly misunderstood
- That a purlin crossing a loft can be removed once new members are added. What supports it, and what it supports, both have to be resolved first.
- That purlin and batten describe the same kind of member. One carries rafters; the other carries covering units.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What supports this purlin, and does any proposed work affect that support?
- What span do the rafters have if the purlin is removed or repositioned?
- Are there struts down to a wall below, and is that wall loadbearing?
- Is there evidence of deflection in the purlin or in the roof line it supports?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No span, section, spacing or capacity is stated here.
- Removing or altering a purlin, or the struts supporting it, is structural work requiring qualified design.
Structural Frame Members
The columns, beams, joists, rafters, plates and bracing that make up a load-carrying frame, identified by the position each occupies.
Browse all frame members entries →