What is a tributary area?

• A. The total roof area that drains to all outlets.
• B. The area of the roof closest to the drainage outlet.
• C. The region around the HVAC equipment on the roof.
• D. The portion of the roof that drains to a specific outlet.

Answer: (D)

Tributary area is the portion of the roof surface that drains to a particular outlet. In a roof drainage system, water is collected and directed toward individual drains or outlets, and the roof is divided into areas whose runoff contributes to each specific outlet. This area is determined by the drainage paths across the roof, so water from parts of the roof that funnel toward that outlet—regardless of how close they are—are included. The entire roof can be thought of as the sum of all tributary areas for all outlets. This concept matters when sizing drains and predicting how rainfall will be collected and removed. The other ideas aren’t correct because one describes the whole roof draining to all outlets, another emphasizes proximity to the outlet rather than drainage paths, and the HVAC region is unrelated to roof drainage.