What is a Cantilever?
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A cantilever is a beam or slab that is fixed (supported) at only one end and free at the other, projecting out into space. A balcony, a sunshade over a window and a canopy over a gate are all cantilevers.
Why the main steel goes on top
This is the single most important — and most often misunderstood — fact about a cantilever. When a load pushes down on the free end, the cantilever hogs: the top face stretches (tension) and the bottom face compresses. Because concrete is weak in tension, the reinforcement must go where the tension is — at the top.
Put the main steel at the bottom, as an untrained mason might by habit, and the member has almost no capacity: the steel sits in the compression zone doing nothing, while the tension face is plain concrete. This is the mechanism behind real balcony collapses.
Common cantilever examples in a house
- Balconies
- Chhajjas / sunshades over windows
- Canopies and porches
- Projecting staircase landings
Why cantilevers are unforgiving
A cantilever has no continuity to help it — the whole moment is resisted at the one fixed end. So it needs more depth than a simply supported member of the same span (roughly span/7 rather than span/12), the top steel must be correctly placed and anchored well back into the supporting member, and the formwork props must stay longer — IS 456 keeps cantilever props on longest. Getting any of these wrong is dangerous, which is why cantilevers beyond about 1.2 m should be designed, not thumb-ruled.
Frequently asked questions
What is a cantilever? A beam or slab fixed at one end and free at the other, projecting outward — such as a balcony or sunshade.
Why is the main steel in a cantilever at the top? Because a cantilever hogs under load: the top face is in tension and concrete is weak in tension, so the reinforcement must sit at the top.
What happens if cantilever steel is placed at the bottom? The member has almost no capacity — the steel is in the compression zone and the tension face is plain concrete. This causes balcony and sunshade collapses.
What are examples of cantilevers in a house? Balconies, sunshades (chhajjas), canopies, porches and projecting staircase landings.
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CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.