Roof Drain Systems: Design Considerations to Keep in Mind

A roof drain system’s primary function is efficient water removal, but in most cases it must also offer aesthetic appeal and on-deck workspace. There are various approaches that can accomplish this balancing act, but each offers its own advantages / disadvantages depending on a number of factors.

Before anything else, you should always consult the local plumbing codes before starting any roof drain system project. From there, consider the following:

  • Structural design of the roof and roof load. Roofs inevitably serve as temporary rainwater reservoirs. Normally, a flat roof with a 30 lb./sq. ft. load will limit the temporary water depth up to 3 inches.
  • Slope of the roof. The greater the slope, the less water build-up, and thus roof load. The location and degree to which a roof pitches may require less drains in one area and more in another.
  • Maximum Pipe Flow Rates. All pipes have a set maximum flow rate. Using this controlled variable, engineers can properly align with size leaders and storm sewers.
  • Wind. Depending on the layout of the building, wind gusts and swirls can restrict water from normal flow patterns.
  • Roof Deflection. As stress is added to a roof, the roof itself will bow and bend creating low spots than can affect water flow. Consult with the building architect on how much of an effect this will have.

General Roof Drain System Guidelines

Following are some basic design guidelines to consider as part of your project, courtesy of Jay R. Smith MFG. Co.

  • Use at least two drains where possible.
  • Limit roof area to 25,000 sq. ft. per weir opening.
  • Install a drain at most 50 ft. from a roof edge.
  • Install a drain at most 50 ft. from the edge of a valley.
  • Keep the distance between drains to at most 200 ft.
  • Provide adequate flashing for parapets, openings, walls, joints, skylights, etc.
  • Limit the number of parapet walls.
  • If you have parapet walls, make sure an overflow scupper is installed at the anticipated maximum water depth.

If you have any questions, contact our Drain Wizard at 800-635-0384 or