Building components · Frame Members
Trimmer
Explains trimming as the redistribution of load around an opening, and why the members alongside an opening are affected too.
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 trimmer is
When an opening is formed in a floor or roof — for a stair, a chimney, a rooflight — the members crossing it have to stop. A trimmer runs across the ends of those interrupted members and carries them, delivering their load sideways into the members alongside the opening.
Those side members then carry more than they did before, which is why trimming an opening affects three sets of members: the interrupted ones, the trimmer, and the ones alongside.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not an ordinary joist or rafter. It runs at right angles to them and carries their ends rather than spanning between supports in the same direction.
- It is not a lintel. A lintel carries a wall over an opening; a trimmer carries floor or roof members around one.
- It is not a noggin. A noggin restrains or backs; a trimmer carries structural load redistributed around an opening.
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.
- trimmers
- trimming joist
- header
- opening trimmer
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How many members it carries and how much each of them brings.
- The members alongside the opening, which receive the trimmer's reactions and are commonly doubled as a result.
- The connections, since load has to pass from the interrupted members into the trimmer and out into the side members.
- The size of the opening, which sets what has to be redistributed.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Trimming redirects load around an opening, and the members alongside receive more than they carried before.
Connections at each end of a trimmer carry concentrated loads, which is why hangers and similar connectors are commonly used rather than simple bearing.
Deflection of a trimmer shows in the floor around an opening, commonly at a stairwell where it is very visible.
Openings between storeys are also breaches in separation, which is a design matter beyond the structural one.
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.
Trimmer to Trimming Joist
Where a trimmer delivers the load of interrupted members into the joists alongside an opening.
Raises: Structural transfer · Movement · Fire continuity · 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.
- Intermediate Floor
- Superstructure
Clarifications
Commonly misunderstood
- That forming an opening only affects the members cut. The members alongside pick up the redistributed load and commonly need to be stronger.
- That a small opening needs no trimming. Whether it does depends on what it interrupts, not on how it looks.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What members does this trimmer carry, and where does that load go?
- Have the members alongside the opening been assessed for the additional load?
- How are the connections at each end of the trimmer made?
- Does the opening affect fire separation between storeys as well as structure?
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, connection or capacity is stated here.
- Forming or enlarging an opening in a floor or roof 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 →