In reinforced concrete (R.C.C.) slab design, when should a roof slab be treated as a two-way slab rather than a one-way slab? Consider the practical rule that compares the longer span to the shorter span.

Introduction / Context:
Slab behavior depends on how bending moments distribute across two orthogonal directions. Distinguishing between one-way and two-way action is essential for choosing correct reinforcement and estimating moments and deflections accurately.

Given Data / Assumptions:

• A rectangular slab with different spans in two perpendicular directions.
• Support conditions are comparable on all edges (typical room slabs).
• Rule of thumb for classification is based on span ratio.

Concept / Approach:
When the longer span (Ly) is not much larger than the shorter span (Lx), load distributes significantly in both directions. A common practical criterion is: if Ly/Lx < 2, the slab is analyzed as a two-way slab; otherwise, for Ly/Lx ≥ 2, it is treated as one-way spanning along the shorter direction.

Step-by-Step Solution:
Identify Lx (short span) and Ly (long span).Compute span ratio R = Ly / Lx.If R < 2 → adopt two-way slab design (reinforcement in both directions for flexure).If R ≥ 2 → adopt one-way slab design (main steel along short span, distribution steel orthogonal).

Verification / Alternative check:
Moment coefficients and strip methods in common design aids reflect significantly higher two-directional moments when Ly/Lx is below 2, which validates the practical rule used by designers for preliminary categorization.

Why Other Options Are Wrong:

• It supports live loads in both directions: All slabs carry area loads; this statement does not define two-way action.
• Slab continuous over two supports: Continuity impacts end moments but not the one-way vs two-way classification.
• Slab discontinuous at edges: Edge condition affects distribution but is not the primary criterion.

Common Pitfalls:
Confusing continuity with two-way action, or using panel aspect ratio without checking support conditions. Also, ignoring torsional reinforcement at discontinuous edges can lead to cracking even in two-way panels.

Final Answer:
When the ratio of the longer span to the shorter span is less than 2.