How is the required size of stormwater drains and sewers for roofs usually determined?

The required size of stormwater drains and sewers for roofs is determined primarily by the effective roof area. The effective roof area takes into account the portion of the roof that contributes to runoff, which includes considerations such as the slope of the roof and any obstructions that might affect how water flows off the surface. By using the effective roof area, plumbing designers can accurately estimate the volume of water that the drainage system will need to handle during a rain event.

This approach ensures that the drainage system is appropriately sized to prevent overflow or flooding, particularly during heavy rainfall. By considering the effective roof area, designers can also take into account factors such as roofing materials and configurations, which impact how quickly water is collected and directed to the drains.

While total surface area, rainfall intensity, and building height are important factors in overall drainage calculations, they do not specifically address the direct contribution of the roof to stormwater runoff in the same way that the effective roof area does.