Siphonic Roof Drainage: Rocket science? I don’t think so

By Mike Richmond

Since the first major construction in the U.S. incorporating siphonic roof drainage (SRD), the Boston Convention and Exhibition Center, was completed in 2004, many questions have been asked by contractors, owners, architects and engineers, and many a furrowed brow has been seen on the so called experts and specialist manufacturers and distributors.

Why has there been (and continues to be) such scepticism about the use of what is, on the face of it, a simple piped rainwater disposal system?

Many of the readers of this article will have at least heard of the system if not participated in one of the many seminars or presentations that have taken place in the last five years or so. Hopefully, some will have been involved in one of the many projects that have embraced the system.

Those directly involved in the market will be aware of SRD’s wider use in recent times. Something has triggered this recent change in attitude. There are a number of possibilities for this change:

• The market decision makers have been browbeaten by the various system proprietors.
• The decision makers embrace innovation elsewhere, why not this?
• The recent hard times have given the decision makers time to listen to the proprietors.
• Cost savings are now as important, if not more important, than they once were.
• The great selling tool is the fear of loss.
• It actually … works.

As far as I’m concerned, too many articles have already tried to explain the complex mathematics and science that makes SRD tick. Let us assume that the smart guys have fulfilled that task, and let us look into the practical workings that bring benefits to builders and owners.

Commercial

The first comment usually made is that, if the pipe is smaller, it must be less expensive. This is correct, but far from the end of the matter. Involving SRD at the outset of a project can significantly reduce the amount of pipework, as well as reducing the size, and can reduce the extent of civil works below the building or below ground externally.

Many projects are presented as a gravity design, particularly in buildings where the pipe system is located in and above the ceiling location. In office buildings, schools and medical buildings, for example, the extent of the routing is governed by the available ceiling void. This usually forces torturous extended routing in order to find suitable drop locations. With SRD, you simply pick your elevation, connect the line of drains in the most direct route and connect it to the most convenient downpipe discharge location, either within the building or on its perimeter. Remember, the pipe runs truly level and is smaller in diameter.

What are the downsides? Field changes and modifications cannot be left in the hands of the installer; these must be referred back to the designer for verification. An installer’s first project is quite rightly met with uncertainty and some trepidation. After a cautious start, all in my experience have come back with significant praise of the system’s simplicity and the question “Why have we not been doing this for the last 10 years?”

Constructability and coordination

SRD is a piped rainwater system. In most cases, it uses the identical materials to a gravity system. In some cases, it can even use the same roof drain (with slight modifications), so there is little to be afraid of with respect to the basic construction. The big winners are as follows: 1) Smaller pipe means less cost, less space required, less load on the structure, less weight for the installer and 2) Level installation makes it much easier to install and simpler to coordinate with other trades — set up a laser level to the crown of the pipe — same bracket lengths throughout.

Environmental

The big green word. Less material use is the obvious gain. Controlled flow discharge is a further benefit, and SRD’s simplicity of co-ordination and flexibility gives rise to benefits when used in conjunction with rainwater harvesting.

In summary

What are we all so worried about? I met one plumbing engineer who is adamant that SRD is far too risky, the physics do not work, that the flow patterns are too dynamic around peak. It is a disaster waiting to happen. Last time I met him he was 2,000 miles from home. I asked, “I presume you walked here then. Surely flying is too risky!”

Tens if not hundreds of thousands of SRD systems have been installed worldwide, I suspect that the United States has hundreds, shortly thousands, of installations. It does not rain here any differently than it does in the rest of the world.

Mike Richmond “joined forces” with one of the U.S. drain manufacturers in 2007. He has been involved directly as a design-build SRD contractor for the better part of 18 years, covering the UK, Europe, the Caribbean and, more recently, the U.S. and the Middle East. Richmond has been involved in thousands of different projects.