What are the two types of drainage systems discussed?

• A. Primary drainage and secondary drainage.
• B. Primary drainage and tertiary drainage.
• C. Internal drainage and external drainage.
• D. Surface drainage and subsurface drainage.

Answer: (A)

In roof drainage design, water removal is planned with two levels of pathways: a primary drainage path and a secondary drainage path. The primary path is the main route sized to carry the normal storm water load from the roof to the outlets. The secondary path provides a backup or overflow route so water can continue to be removed if the primary path is blocked, clogged, or overwhelmed. In practice, the primary system handles routine rainfall, while the secondary system kicks in to prevent ponding during heavy rain, ice, or debris situations. This redundancy is why the two types discussed are described as primary drainage and secondary drainage.