Control flow drains are designed to slow the rate of runoff from roofs during storm events.  These drains typically have an insert with a weir that regulates the flow of water through the drain.  They are available in a variety of heights.  Generally, flat roofs have lower weir heights than sloped roofs.  Some models have adjustable weirs which can be set for each drain / location, and some may utilize copper flanges to inhibit root growth.  All control flow drain systems should be carefully designed by a licensed professional engineer. These manufacturers sell controlled flow drains: