Building components · Frame Members
Floor Joist
Explains the floor joist as a repeated spanning member, distinct from the heavier beams that commonly carry it.
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 floor joist is
Floor joists are the parallel members, spaced at regular intervals, that span between walls or beams and form the structure of a floor. Load applied to the floor is carried by whichever joists are beneath it and delivered to the supports at their ends.
Because they repeat, they behave partly as a group: decking or boarding fixed across them distributes load between neighbours, and members between them stop them twisting.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a beam. A beam is a heavier member that commonly supports joists; joists repeat at close spacing and a beam generally does not.
- It is not a ceiling joist. A ceiling joist carries only the ceiling below and, in a roof, commonly ties the feet of the rafters; a floor joist carries floor loading.
- It is not a timber I-joist. That is a specific engineered product with its own material entry; a floor joist is the position, and an I-joist is one of several things that can fill it.
- It is not a deck joist, which does the same job outdoors in a different exposure environment.
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.
- floor joists
- joists
- floor members
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Span and spacing, which together determine what each member carries and how far.
- The bearing at each end, including how much of the joist actually sits on its support.
- Lateral restraint between joists, without which slender members can twist.
- What is cut into or through them for services, since notches and holes change what remains.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Floor deflection and bounce are serviceability matters that occupants notice long before anything approaches a structural limit.
A joisted floor transmits both airborne and impact sound, and how it is built up above and below determines how much.
Services routed through a joisted floor are a common source of unauthorised notching, which is structural alteration by another name.
What a joisted floor contributes to separation between storeys depends on the whole build-up rather than on the joists alone.
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.
Joist End to Masonry Bearing
Where a floor joist is supported by a masonry wall, and where structure meets a wetting risk.
Raises: Structural transfer · Moisture · Thermal continuity · Buildability
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.
- Ground Floor
- Intermediate Floor
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Narrower forms of this element
Clarifications
Commonly misunderstood
- That notching for pipes and cables is a trade detail. Where and how much can be removed is a structural constraint.
- That a bouncy floor is unsafe. Deflection and strength are different criteria; a floor can be well within strength limits and still feel poor.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What span and spacing does this floor assume, and does the proposed use match it?
- What bearing do the joists have at each end?
- Where can services pass through the joists without affecting them structurally?
- What restraint is provided between joists, and is it intact?
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, spacing, section, notching allowance or capacity is stated here.
- Cutting, notching or drilling structural members is structural alteration and requires qualified advice.
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 →