Building components · Frame Members
Roof Strut
Explains the strut as the element that gives a purlin its support, and why the wall beneath it matters.
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 strut is
A strut carries load from a purlin down to something that can take it — typically a loadbearing wall or a binder below. It is the reason a purlin can span less than the whole building and therefore be a modest member.
Struts are commonly inclined rather than vertical, which means they deliver both a downward and a sideways force at their base, and the structure beneath has to accept both.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a column. A strut is part of a roof's internal support arrangement and typically stops at a wall or binder rather than continuing to a foundation.
- It is not a bracing member. Bracing resists sideways movement of the whole structure; a strut carries vertical load from a purlin.
- It is not a collar or a tie. Those work in tension against spread; a strut works in compression carrying load down.
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.
- struts
- roof strut
- prop
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What it lands on, which must be a wall or member able to accept the load — and that wall being loadbearing is frequently the assumption that gets overlooked.
- Its angle, since an inclined strut delivers a horizontal force as well as a vertical one.
- The connection at both ends.
- Whether the wall beneath is itself supported all the way down.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A strut is a load path from the roof to a wall in the middle of a building, and that path is easy to miss when planning alterations below.
Struts commonly cross loft spaces diagonally and are among the first things a conversion wants to remove.
A strut landing on a wall that has moved, or on a floor rather than a wall, transfers its load somewhere unintended.
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.
- Superstructure
Clarifications
Commonly misunderstood
- That a wall in the middle of a house carrying no floor is non-loadbearing. It may be carrying the roof through struts.
- That struts can be removed once a new beam is installed at ridge or purlin level. What the strut was carrying, and where that load now goes, has to be resolved.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What does this strut land on, and is that wall loadbearing all the way down?
- What horizontal force does the inclined strut deliver at its base?
- If the strut is removed, what carries the purlin instead?
- Are the connections at each end of the strut sound?
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 angle, section, spacing or capacity is stated here.
- Removing struts, or the walls they bear on, is structural alteration 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 →