Balconies, sunshades and projections are the members most often built wrong, because they behave opposite to an ordinary slab.
Why the Steel Goes on Top
An ordinary slab supported on both sides sags in the middle, putting its bottom in tension. A cantilever is fixed at one end and free at the other; it bends downward at the tip, putting its top in tension. The main bars must be at the top, with enough anchorage back into the supporting slab or beam to hold the cantilever down.
Typical Details
| Item | Practice |
|---|---|
| Main steel | Top, along the span, anchored back at least the development length |
| Distribution steel | Across the main bars |
| Depth | Deeper at the support, can taper to the edge; IS 456 basic span/depth = 7 |
| Span in houses | Commonly 1 – 1.5 m; longer needs design |
| Props | Kept longer than for slabs |
| Edge | Drip groove underneath |
Design live loads come from IS 875; residential balconies are designed for at least the room they serve, with a minimum of about 3 kN/m².
Common Mistakes
| Mistake | Consequence |
|---|---|
| Main steel at the bottom | Cantilever cracks at the support and droops |
| Short anchorage | Bars pull out; the balcony tips |
| Top steel trodden down during the pour | Loss of effective depth |
| Props removed early | Permanent sag |
| Heavy planters or tanks added later | Overload |
The Short Version
Steel on top, anchored deep, props left longer, and water sent away from the wall. Check the top bars before every cantilever pour.
