For a new building, architect will be determining the location of the roof drains on the roof plan. Right next to each roof plan there will be an overflow drain for in case of clogging the roof drain. The overflow drain is similar to the roof drain but with a 2” high External water dam.
The plumbing engineer should design the piping system including the roof drains to run the piping horizontally for each roof drain or combine the multiple roof drains by a common horizontal pipe to the exterior walls and run vertically (called vertical leader or down ****) down to the ground level and dump to the planters, or connect to the site storm drain piping system (by Civil) and then connect to the street public storm drain piping system.
The rain fall rate for each area is determined by the National Weather organization. The standard tables in UPC (Uniform Plumbing Code) or state code based on the rainfall rate are showing the maximum area which each pipe size can carry horizontally or vertically (starts with the minim 3” pipe).
The following items should be considered when establishing the design criteria:
- Pattern of roof drainage slopes and roof drains & overflow drains location
- Vertical wall heights at the roof (like adjacent building, stairs dog house, etc.) and parapet heights
- Scupper locations for the small upper roofs (like stairs dog house or similar).
- Pipe location in the ceiling, wall and chase.
Roof drains and pipe sizing are based on the collection areas (square footage), the slope of the pipe and the rainfall rate, then refer to the pipe sizing table in Uniform Plumbing Code (UPC) or State Plumbing Code book. By measuring the area covered by each floor drain and obtaining the square footage and referring to the sizing charts (tables) in UPC can determine the size of the main pipes and vertical leaders.
Overflow drains and their associated piping is sized the same way as roof drains. Drains should be piped separately from the primary drainage system and should discharge independently to the outside of the building. The vertical leaders can be leaved about an inch above the grade around the building.