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Building components · Frame Members

Cantilever Member

Explains the cantilever as a position defined by having no support under its free end, and by the fact that its back span is part of it.

Form:Linear memberRoles:StructureSupport

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 cantilever is

A cantilever member projects beyond its support with nothing beneath its free end. Everything applied along the projection has to travel back to that support, arriving there as bending and shear rather than as a simple reaction, which is why the worst condition in a cantilever is at its support rather than out in the middle of the projection.

The part most often overlooked is that the member continues inside. A projection is held by the back span behind it and by whatever holds that back span down, so a cantilever is not the piece that can be seen: it is the visible projection together with the length behind it and the hold-down at the far end. Work inside a building can therefore undo a projection outside it. The same position is met reaching out past a gable or a frame line, where the members continuing back inside the roof are what hold the projection down.

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 in the ordinary sense. A beam is supported at each end and its bending is commonly worst between the supports; a cantilever is supported at one end only and its bending is worst at that support, which puts the design attention in the opposite place.
  • It is not an outrigger member. An outrigger is named for reaching outward past the frame to pick up load and return it into the structure, and its outer end may be propped; a cantilever is named for having nothing beneath its free end at all.
  • It is not a support bracket. A bracket is fixed to a face and carries a load back into it over a short projection; a cantilever member is the spanning element itself, continuing past its support as the same member.

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.

  • cantilever
  • projecting member
  • oversailing member

Dependencies

What it depends on

What has to be right elsewhere for this element to do its job.

  • The back span behind the support and what holds it down, since a projection lifts its own tail and needs something to resist that.
  • Whether the support provides continuity rather than merely a seat, because a member simply resting on a support cannot cantilever past it.
  • Deflection accumulating toward the free end, which is greatest exactly where it is most visible and where finishes and guarding meet it.
  • Anything added at the tip, since load at the free end has a greater effect than load anywhere else along the member.

Considerations

Topics that genuinely apply here

Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.

Structural transfer

A cantilever transfers bending into its support rather than a simple reaction, so what it is built into matters as much as the projecting member.

Movement

Movement at a free end is greater than anywhere else along the member, and elements fixed across that edge respond to it rather than to the structure as a whole.

Thermal

A member continuing from inside to outside crosses the insulation line, which is why projecting structural elements are a recurring subject in envelope design.

Durability

Cantilevered elements are commonly the most exposed part of a building, and the condition of the member where it passes through the envelope is the part that cannot be seen from either side.

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.

Materials LibraryReinforced concrete →Cast slabs and beams continuing past their support are among the most commonly met cantilevered elements.Materials LibraryStructural steel →Steel members continuing past a support, or brackets built into a frame, occupy the position in framed construction.Materials LibraryGlued-laminated timber →Engineered timber members are met where a projection is exposed and the member is part of what is seen.Materials LibraryStructural softwood →Joists continuing past a wall to form a projection are the lightest and most common timber form of the position.

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.

Primary supportCantilevered Balcony →The projecting member itself, and the length of it that continues back into the structure.Primary supportStructural Thermal Break →The member the thermal break interrupts, where a projecting element continues through the insulation line.Primary supportConcrete Frame →Slabs and beams continuing past their supporting line, commonly to form balconies and projecting floors.Primary supportSteel Frame →Members continuing past a column line, where the back span and its hold-down are part of the frame.Primary supportTimber Platform Frame →Floor members continuing past the wall below to form a projecting upper storey or bay.

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.

Clarifications

Commonly misunderstood

  • That the cantilever ends at its support. The back span and its hold-down are part of the cantilever, and altering the floor inside can change the projection outside.
  • That a cantilever can be loaded or extended at its tip if it looks substantial. Load at the free end has the greatest effect of anywhere on the member, and the member's behaviour was fixed by the design behind it.

Conversations

Questions for qualified professionals

Questions worth asking someone with the project in front of them. This page cannot answer them.

  • Where is the back span of this cantilever, and what holds it down?
  • Is the support arranged to provide continuity, or only to seat the member?
  • What deflection at the free end does the design allow, and does anything fixed there depend on less?
  • How does the member cross the insulation line, and what does the envelope design assume about that?

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

  • This entry describes a structural position. It states no projection, span, section, deflection limit, capacity or fire rating, and none can be inferred from it.
  • Loading, extending or altering a cantilevered element, including work on the structure behind it, 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 →