Storm Drainage
Where the water goes
Storm drainage from roofs, courtyards, and subsoil drains must be kept out of the sanitary sewer unless the building is served by a combined sewer approved by the authority. Roof leaders, conductors, area drains, and footing drains are part of the storm system and are sized separately from sanitary drainage. Sump pumps serving foundation drainage discharge to storm drainage, to grade, or to an approved location - not to the sanitary sewer.
Sizing
Roof drain and leader sizing is based on the cubic feet per second or gallons per minute of rainfall for the project location and the roof area served. The rainfall rate used is the design storm intensity for the site, not an average. Plan area, not sloped surface area, is normally the basis, adjusted for pitch where the tables require it. Larger roof areas are divided among multiple drains, and drains are placed at low points or saddled so water does not pond.
Secondary drainage
Where a roof has a parapet or where the primary drainage could be blocked by ice or debris, secondary or overflow drainage is required - usually overflow scuppers or a second set of roof drains set higher than the primary ones. In Michigan this matters: ice and snow load the roof and freeze the primary drains.
Rules that show up as questions
Storm pipe below the frost line, storm leaders may not serve as vents, and a roof drain must have a strainer. A storm drain may not connect to a foundation drain carrying subsurface water without proper design. Subsoil drains are perforated or open-jointed pipe surrounded by gravel and filter fabric.
Worked example
Roof plan area = 40 ft × 60 ft = 2,400 sq ft. At a design rainfall of 4 in/hr, the drain must handle 2,400 sq ft of roof - the table then sets the leader diameter and roof drain size.