The storm drain or rain water leader is sized at what diameter?

• A. 8 inches
• B. 4 inches
• C. 6 inches
• D. 12 inches

Answer: (A)

Sizing a storm drain or rain water leader is about ensuring the vertical pipe that carries roof runoff can convey the peak flow without overflowing. The diameter is chosen based on the tributary roof area and the rainfall intensity, so the water moves down the leader smoothly and doesn’t back up into the gutter or spill over the top. Eight inches is a common, practical size because it provides enough cross‑sectional area to handle typical roof sizes and heavy rain events without excessive risk of overflow. Smaller leaders, like four or six inches, may be sufficient for smaller roofs or milder climates but can overflow during a heavy downpour. A twelve inch leader is usually reserved for very large roofs or extreme conditions and is not typical for standard residential work.