The design of siphonic roof drainage systems is growing in popularity in the U.S. thanks to new standards for design from ASPE and ASME.

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.

Design Standard

On March 14, 2005, ASPE initiated Work Group No. 45, assigned with the task of drafting a technical design standard for siphonic roof drainage systems. A draft of the proposed standard was sent to the Main Committee in April of 2005. This review resulted in apparent positive acceptance of the standard, and only minor but helpful comments had to be addressed. This standard establishes the calculation procedures for siphonic piping systems, acceptable pipe materials, performance requirements, and a set of "do's"

Software Support

Even with these standards available, however, the calculation procedure itself can be quite burdensome. Fire suppression sprinkler systems today are typically hydraulically designed using computer programs. The principles of siphonic roof drainage are quite simple and no different than any other piping system, but the sheer number of calculations required to arrive at a satisfactory result requires the assistance of computer software. The software unburdens designers of the "number crunching"

New Products

Perhaps one of the more exciting developments in the siphonic roof drainage market is the pending rollout of a siphonic roof drain product by a major manufacturer of engineered commercial plumbing and drainage products. The drain prototypes are slated for their ASME A112.6.9 performance testing in August and should be available on the market at the same time as the software product. Presently, siphonic roof drains are available only from European suppliers, and then only if the supplier's proprietary software is used to design the piping system. The availability of both non-proprietary design tools and specified products promises to open the siphonic roof drainage market wide open in the U.S. European suppliers of proprietary siphonic roof drainage systems have already discovered that selling proprietary systems will not work in the states.2

A Warning About ASTM F 2021

Plumbing engineers who have researched siphonic roof drainage may have come across an American Society of Testing and Materials (ASTM) standard for plastic siphonic roof drainage systems. Designated ASTM F 2021, this document was first published in 2000 and focuses on high-density polyethylene (HDPE) as a pipe material for siphonic roof drainage systems. Review of this document by experts in siphonic roof drainage has found fundamental technical errors and inadequacies. Under the section "System Calculations,"

Looking Toward the Future

Siphonic roof drainage offers many benefits. System costs are reduced due to smaller pipe diameters. Velocities are higher, making the inner bores self-cleansing. The ability to run the piping overhead without pitch reduces or eliminates the need for pipe trenching and saves money. Pipe inverts leaving a building are up high, making deep trenching on the site unnecessary and saving money. Flexible pipe location and the consolidation of drains to a single vertical pipe reduce column chases both in quantity and size, and save money. European experts predict that siphonic roof drainage could be the standard in large-roof construction. Such applications include factories, warehouses, airports, convention centers, stadiums and "big-box"