Which factor most directly influences drainage performance?

• A. Sump depth and blockage of strainers.
• B. Membrane color.
• C. Substrate roughness alone.
• D. Roof edge height.

Answer: (A)

Drainage performance is all about how quickly water on the roof can be collected and carried away by the drainage system. The most direct control over that rate is the intake at the sump and whether the strainers are clear. A deeper sump allows more water to be stored and routed before reaching the outlet, while clean strainers let water enter the drainage path with minimal resistance. If the strainer becomes blocked, the effective flow area is reduced, increasing headloss and throttling the discharge, which can cause ponding and reduced drainage performance.

Membrane color doesn’t affect how water is collected or discharged. Substrate roughness can influence how uniformly water spreads toward the drain, but it’s not the primary factor that governs how much water can be removed. Roof edge height mainly affects where water would overtop the edge or where it would spill if drainage is overwhelmed, not the rate at which water is removed under normal conditions.