Cantilever Slab Design: Calculation Procedure and Worked Example

A cantilever slab projects beyond its line of support with no beam, column, or wall beneath the free edge, so it carries every load in bending with tension on the top face. Balconies, canopies, chajjas, and sunshades are the usual cases. The member is statically determinate, which keeps the moment calculation simple, but anchorage, deflection, and stability need care like any bending element. The behavior is the opposite of slab foundations, which rest on prepared subgrade over their full length and carry load by bearing rather than bending.

This article works through the Eurocode 2 design of a 1.5 m cantilever slab for an office building, from load calculation to bar selection, then covers deflection, cracking, stability, and the site checks that keep detailing errors out of the structure.

How a Cantilever Slab Carries Load

The projection acts as a lever arm fixed at the support line. The primary reinforcement resists the hogging moment and the bottom face goes into compression, so the main steel sits near the top surface, the reverse of a simply supported slab. The support, whether a beam, wall, or column, must resist the shear at the face and the full fixing moment, and rotation at the support increases the deflection at the free end.

Tension on the Top Face

Because the load hangs off the support, the slab bends concave downward and the upper fibres go into tension. Top steel therefore controls the design. Placing the main bars at the bottom, a mistake that still appears on sites, leaves the concrete to carry tension alone and cracks open at the balcony root within months.

Why Top Steel Governs

In a 1.5 m projection the moment at the support equals the total load times half the span, with no second support to share it. The bars must also extend back into the supporting span far enough to develop their anchorage, covered later. The tension-on-top behavior is the direct opposite of concrete slab foundations, where bending is secondary and the soil provides continuous support.

Common Cantilever Slab Applications

• Balconies and terraces on apartment and hotel floors
• Canopies and porches over entrances and shopfronts
• Chajjas and sunshades shading windows without blocking daylight
• Sign gantries and equipment plinths on rigid projecting bases

Loads and Design Actions

The calculation starts with the actions on the projection. A 175 mm slab contributes 4.375 kN/m² from self-weight alone (0.175 m × 25 kN/m³), and screeds, finishes, parapets, and partitions add further permanent load. Office floors typically carry 2.5 to 3 kN/m² of imposed load, while balconies often need 4 kN/m² or more; this example uses 4 kN/m², common for accessible balconies.

Load Combinations at Ultimate Limit State

EN 1990 combines permanent and variable actions with partial factors. At the ultimate limit state the design load is n = 1.35 gk + 1.5 qk, and favourable permanent actions drop to 1.0 or 0.9 when stability is checked.

Serviceability Combinations

Deflection and cracking are checked on serviceability combinations. The quasi-permanent combination, gk + ψ2 qk with ψ2 = 0.3 for offices, gives 4.375 + 0.3 × 4 = 5.58 kN/m², the value that drives the steel stress in the deflection and crack checks.

One-Way Action and the Design Strip

The 1.5 m projection spans in a single direction, so the calculation uses a 1 m wide design strip and the reinforcement is expressed per metre of width. The member behaves like an inverted one-way slab, and the same one-way slab design procedure applies with the span taken as the projection length.

Worked Example: Cantilever Slab Design to Eurocode 2

The example is an office balcony projecting 1.5 m from a supporting beam, with a 175 mm slab, C25/30 concrete (fck = 25 N/mm²), B500B reinforcement (fyk = 500 N/mm²), and 25 mm cover. The architectural design and building envelope design process fixes the parapet, cladding, and partition loads on the projection, so those decisions should be locked before the structural calculation starts.

The moment relation is direct: a cantilever takes the load times the span squared divided by two, M = n L² / 2, because the member is statically determinate. A simply supported slab of the same span carries only an eighth of that moment, M = n L² / 8, which is why cantilevers need more steel and depth.

Design Data

Step-by-Step Calculation

1. Self-weight: gk = 0.175 × 25 = 4.375 kN/m²
2. Ultimate load: n = 1.35 × 4.375 + 1.5 × 4 = 11.91 kN/m²
3. Design moment at the support: M = n L² / 2 = 11.91 × 1.5² / 2 = 13.4 kNm per metre width
4. Effective depth: d = 175 – 25 – 10/2 = 145 mm
5. Bending coefficient: K = M / (b d² fck) = 13.4 × 10^6 / (1000 × 145² × 25) = 0.0255
6. Limiting value: K’ = 0.21 for no moment redistribution (δ = 1). K is below K’, so compression reinforcement is not required
7. Lever arm: z = d [0.5 + √(0.25 – K/1.134)] = 141.7 mm, capped at 0.95 d = 137.8 mm
8. Steel area: As,req = M / (0.87 fyk z) = 13.4 × 10^6 / (0.87 × 500 × 137.8) = 224 mm²/m

Minimum Reinforcement Check

Eurocode 2 requires As,min = 0.26 (fctm/fyk) b d, not less than 0.0013 b d. With fctm = 2.6 N/mm² for C25/30, As,min is 196 mm²/m, below the 224 mm²/m from bending, so the design moment governs. T10 bars at 300 mm centres provide 262 mm²/m, satisfying both limits.

Reinforcement Detailing and Bar Selection

Bar Spacing and Area Provided

The chosen bars must respect the maximum spacing rules, which for slabs limit main bars to the smaller of 3 h and 400 mm. A 175 mm slab allows up to 400 mm, so 300 mm spacing sits well inside the limit.

The full required area goes across the whole width because a cantilever carries its moment in a single band. This differs from flat slab construction, where the moment is shared between column and middle strips and the reinforcement is distributed accordingly.

Anchorage Into the Back Span

The top bars must develop their full strength beyond the face of the support. For T10 bars in C25/30 with good bond, the basic anchorage length is about 470 mm, roughly 47 bar diameters, so the bars should extend at least that far into the back span. In practice the top mat runs a quarter to a third of the back span, with laps placed beyond the point of contraflexure.

Laps and Distribution Steel

Where the top mat is lapped with a second length of bar, the laps are staggered and kept away from the support face where the moment is highest. Distribution steel across the width should be at least 20 percent of the main area, which T10 at 300 mm in the secondary direction exceeds. Chair bars or spacer stools hold the top mat at the correct cover during concreting.

Deflection, Cracking, and Stability Checks

Deflection Control

Eurocode 2 sets a basic span to effective depth ratio of 8 for cantilevers. The actual ratio for this slab is 1500/145 = 10.3, above the basic value, so a modification factor must be justified. The service stress in the steel, estimated from the quasi-permanent load combination and the ratio of required to provided steel, is about 174 N/mm². That gives a factor of 310/174 = 1.78 and an allowable ratio of 8 × 1.78 = 14.3, so the deflection check passes. Many codes treat the effective span of a cantilever as twice the projection, equivalent to limiting free-end movement to projection/125.

Crack Control

Balconies and canopies exposed to weather need a crack width limit of 0.3 mm. For a steel stress around 174 N/mm², the maximum bar spacing in Table 7.3N is in the region of 280 to 300 mm, which puts T10 at 300 mm at the limit. Where the element faces driving rain or de-icing salts, tightening the spacing to 250 mm cuts the risk of water reaching the steel.

Stability and Overturning

A cantilever must not tip about its support. When the slab projects from a wall or beam, the fixing moment transfers into the supporting element and the structure behind it. When the projection sits on a short back span, check overturning with favourable permanent actions at 0.9 gk against the full 1.35 gk + 1.5 qk on the cantilever, with the back span weight on the restoring side. This is a different problem from slab-on-ground design, where the subgrade carries the load and overturning is rarely the governing case.

Construction Practice and Quality Control

Site Checks Before Concreting

• Confirm the top mat sits at the correct cover, since bars pushed down during concreting cut the effective depth
• Check that the top bars extend the full anchorage length into the back span
• Confirm the spacing matches the bar schedule
• Prop the formwork so the projection stays level and does not sag under its own weight

Common Failures and How to Avoid Them

1. Main bars placed at the bottom instead of the top, leaving the concrete to carry tension
2. Top bars stopped at the support face without anchorage into the back span
3. Formwork struck before the concrete reaches strength, allowing creep and long-term deflection
4. Missing distribution steel, which lets longitudinal cracks open along the slab
5. Construction loads stacked on the projection while the concrete is still young

Curing matters as much as the steel. Cantilevers deflect permanently under their own weight, and early striking or rapid drying adds to that movement. Keep formwork and props in place until the concrete reaches the design strength, and cure the top surface, which carries the critical tension steel, for at least seven days.

The same load path and detailing logic applies to stair landings, small platforms, and the edge elements of larger floors. Where parts of a building sit directly on the ground, the design method changes completely, and slab-on-ground design elements follow different rules for joints, reinforcement, and subgrade preparation.