Storm Drainage
Keep storm water separate
Storm drainage is a separate system: roof drains, leaders, area drains, and storm building drains and sewers. Connecting storm water to the sanitary sewer is prohibited, and sanitary waste may not discharge into a storm system. Sump pumps and foundation drains discharge to grade, a storm system, or an approved dry well - never to a sanitary sewer unless the code expressly allows it.
Sizing
Storm systems are sized from the roof area and the local design rainfall intensity (inches per hour), using the code's tables or rainfall map for the applicable design storm. A useful arithmetic check: GPM ≈ roof area (sq ft) × rainfall (in/hr) × 0.0104. Where the code requires secondary or overflow drainage for large roofs or where ponding would be hazardous, the secondary system is sized independently and must discharge where it will be visible.
Components and details
Roof drains need strainers, proper flashing, and a secure connection to the leader. Leaders and horizontal storm drains need cleanouts, adequate slope, and support. Backwater valves are required where the storm drain can be subject to backflow. Area drains and trench drains in parking or vehicle areas often need a sand or oil interceptor upstream. Traps are generally not required on storm inlets, but the discharge must not create a cross-connection to potable water.
Worked example
A 2,000 sq-ft roof in a region designed for 3 in/hr: 2,000 × 3 × 0.0104 ≈ 62 gpm. Select and count roof drains and leader sizes from the code table for that flow, then verify the horizontal storm drain carries the combined flow of all connected roof areas.