Building components · Frame Members
Rafter
Explains the rafter as the inclined spanning member of a roof, distinct from trusses and from the horizontal members that restrain 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 rafter is
A rafter runs up the slope of a pitched roof, from its bearing at the eaves to the ridge. Rafters repeat in pairs across the roof and carry everything above them — battens, covering, and whatever load arrives on the roof surface.
Because they are inclined, the load they carry has a component pushing outward at their feet. Dealing with that outward push is one of the defining problems of pitched roof structure, and it is why rafters are rarely acting alone.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a trussed rafter. A prefabricated truss is a complete triangulated frame delivered as a unit and has its own material entry; a rafter is a single inclined member within a roof built up from parts.
- It is not a purlin. A purlin runs horizontally across rafters to support them mid-span; a rafter runs up the slope.
- It is not a joist. A joist is horizontal and does not generate outward thrust at its bearings.
- It is not a hip or valley rafter, which occupy specific positions where roof planes meet and carry different loads.
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.
- rafters
- common rafter
- roof rafter
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What resists the outward thrust at its foot, whether that is a tie, a ceiling joist acting as one, or a wall designed to take it.
- Whether it is supported part-way up its length by a purlin, which changes its effective span.
- Its bearing at the eaves, including how it is connected to the wall plate.
- What is cut from it for rooflights, dormers or services.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The outward thrust at rafter feet has to go somewhere, and removing what resists it — a ceiling tie, for example — changes the roof's behaviour.
Roof spread showing at wall heads is commonly a sign that the restraint on rafter feet is not doing what was assumed.
Where insulation sits relative to the rafters determines whether the rafter is in a warm or cold environment, which is central to how the roof build-up behaves.
Openings formed in a rafter roof — for rooflights or dormers — interrupt members and require trimming.
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.
Rafter Foot to Wall Plate
Where a rafter lands on the wall plate, delivering both a downward load and an outward push.
Raises: Structural transfer · Movement · Thermal continuity · Buildability
Rafter to Ridge
Where opposing rafters meet at the apex, and where the roof's whole structural action is decided.
Raises: Structural transfer · Movement · Buildability · Thermal continuity
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
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 a ceiling below a pitched roof is only a ceiling. In many roofs the ceiling joists tie the rafter feet, and removing them alters the structure.
- That rafters and trusses are interchangeable words. A trussed rafter roof cannot be altered the way a cut roof can.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What is resisting the outward thrust at the rafter feet in this roof?
- Is this a cut roof or a trussed rafter roof, and what does that mean for any alteration?
- Where insulation is being added, does the rafter end up in a warm or cold zone?
- How are any openings in the roof plane trimmed?
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, pitch, spacing or capacity is stated here.
- Altering roof structure, including cutting trussed rafters or removing ceiling ties, 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 →