If a 10,000 square foot roof has 5 drains, what is the tributary area per drain?

• A. 2,000 sq ft.
• B. 5,000 sq ft
• C. 1,000 sq ft
• D. 20,000 sq ft

Answer: (A)

Tributary area per drain is found by dividing the total roof area by the number of drains, assuming the drainage is evenly distributed. So, 10,000 square feet of roof divided by 5 drains gives 2,000 square feet per drain. This represents the portion of roof each drain would “handle” under uniform spacing, which is a common assumption when evaluating drainage capacity and related loads. The other numbers come from misapplying the division: 5,000 would result if there were only two drains, 1,000 if there were ten drains, and 20,000 would occur if you treated the whole roof as feeding a single drain.