Calculate the required drainage capacity for a roof drain on a 40' x 60' warehouse roof in Hilo (annual rainfall 126 inches). What minimum drain diameter is required?
Correct Answer
C) 8 inches
For high rainfall areas like Hilo, roof drains must handle 1 inch per hour per 100 sq ft. A 2,400 sq ft roof requires 8-inch minimum drain diameter to handle the 24 GPM flow rate.
Why This Is the Correct Answer
The roof area is 40 × 60 = 2,400 sq ft. Hilo is classified as a high-rainfall area requiring drainage design for 1 inch per hour per 100 sq ft. At 2,400 sq ft, the required flow rate is approximately 24 units, necessitating an 8-inch minimum drain diameter to handle the resulting flow (approximately 24 GPM or higher depending on tables used).
Why the Other Options Are Wrong
Option A: 10 inches
A 10-inch drain is oversized for this application. While it would handle the flow, it exceeds the minimum requirement and would not be the correct minimum answer. Exam questions about minimum size have a specific correct answer.
Option B: 6 inches
A 6-inch drain is insufficient for a 2,400 sq ft roof in a high-rainfall area like Hilo. A 6-inch drain handles smaller roof areas and lower rainfall intensities than those found in Hilo.
Option D: 4 inches
A 4-inch drain is far too small for a 2,400 sq ft roof in Hawaii's highest-rainfall city. This diameter is suitable only for very small roof areas or low-rainfall regions.
Memory Technique
Step 1: Area = 40 × 60 = 2,400 sq ft. Step 2: Hilo = high rainfall zone. Step 3: Bigger area + heavier rain = bigger drain. For 2,400 sq ft in a high-rain zone → 8 inches. Think '8 is great for the wet plate (2,400 sq ft)'.
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