Perhaps one of the hotter topics discussed among the plumbing engineering community is siphonic roof drainage. It is growing from a once "obscure curiosity"

A horizontal drain manifold overhead at an IKEA in Atlanta, GA.
An Engineered System
Siphonic roof drainage is not simply a new product to be specified. It is a technique for sizing drainage piping to allow the drainage system to flow full-bore, to utilize the full cross-sectional area of the piping and to exploit the potential energy available from the roof elevation to the point of discharge. There are many design advantages of this type of engineered system, and they have been discussed in other articles on the subject.1 Siphonic roof drainage is a true "engineered system,"Familiar Engineering
When sizing a centrifugal pump, engineers know that the operating point (i.e., the discharge pressure and flow) is determined by both the pump's performance curve and the system curve of the connected distribution system. Such a distribution system may be a wet pipe sprinkler system, a domestic water system or some sort of process system like an RODI distribution loop. The performance curve of a pump can be obtained by the pump manufacturer. However, the system curve is unique to the pipe system and must be calculated using friction loss formulae and the resistance coefficients of pipe components, such as elbows, reducers and valves. Often, the pressure loss can be estimated using a friction loss chart and one or two "rules of thumb"
A typical baffled siphonic roof drain.
Siphonic Roof Drainage Design
Much like sizing a centrifugal pump, siphonic roof drainage design centers on the balance between the energy available to the system and the energy loss experienced as a result of viscous flow through the pipe system in a full-bore steady state condition. In this case, the height of the building represents the available "potential energy"Drain Standard A112.6.9
In August of 2002, the American Society of Mechanical Engineers (ASME) Committee A112 "Plumbing Materials and Equipment"
A drain tailpiece joining with a horizontal manifold.