One of the most repeated hardships I hear while traveling the country is, “I draw-in my roof drainage systems last because they are large-diameter pipe and sloping. It is always the first thing I need to figure out — how to reroute when changes come in on the job.”

Common problems from having to pitch pipe on traditional gravity storm drainage:

1. Pipework can only slope so far before entering livable space.

Result: Physically required vertical drops throughout the design.

2. Pipework cannot discharge to only one location. 

Result: Added civil connections required to meet the storm drainage discharge locations.

3. Overflow downspout nozzle locations are not always achievable with limited chase. 

Result: Sacrifice design to route discharges where gravity and space will allow.

4. Not enough room in a chase to get pipe to outside of building footprint. 

Result: Increase building elevations or civil trenching depth.

None of these have anything to do with the performance of a gravity drain; they have to do with the physical limitations of traditional gravity storm drainage itself.

Siphonic drainage allows the pipework to run with no pitch.

It operates by using the natural height of the building in order to create negative pressure. The design of the siphonic drain eliminates air from entering the pipework, which, in conjunction with the negative pressure, pulls the water off the roof versus gravity drainage, which needs slope and air to drain properly.

A hotel is a perfect example to show the benefit of being able to run storm piping with no slope.

The typical hotel design has roughly five overflow downspout nozzles on the outside of the building. Those downspout nozzles also require five vertical drops on the side of a three-story building, if not taller.

With siphonic drainage, the designer has the ability to collect all five drains at the top level, routing everything to only one vertical riser. This routing eliminates the need for four vertical risers, as well as four civil connections to the building, all while keeping the exact same drain locations. 

This design produces material savings from all the pipe, fittings, couplings, hangers, insulation and manholes no longer needed. The labor savings are even more substantial, eliminating the need to bore holes in each floor, set the fire caulking, install the pipework and secure the bracing, plus trench, install and rebury the civil connections headed to the building. Again, all these savings are repeated for each of the four eliminated runs.

Siphonic storm drainage delivers immediate cost savings to any project, but even more impactful is the long-term benefit to the building owner, as can be displayed in the hospital industry.

Every 10 to 20 years, hospitals update their buildings, given new technology. They do not merely paint a few walls; rather, they completely move hallways creating new floor layouts. As a result, the existing gravity drain piping needs to be reconfigured so it isn’t sticking below the ceiling in a new corridor. This rerouting generally requires engineering and redesign along with the labor to
implement.

The best practice for siphonic drainage is to collect all the roof drains at the top level, then route the piping to a vertical riser next to an elevator shaft or stairwell — any chase that will never be moved. This, in effect, removes any concern of the storm drainage needing rework during future remodels and delivers a cost-savings benefit on the front end of the project to the hospital owner.

You’ll still have 99 other RFIs, but siphonic roof drainage won’t be one.

Brennan Doherty is the national specifications manager at MIFAB Inc. He is published in the newest ASPE Design Handbook V2 for introducing siphonic drainage to the guide. In addition, he has given presentations both nationally and regionally for ASPE, Tilt-Up, MCAA and PHCC. Doherty educates owners, engineers, design/build contractors and architects across the country on the technical, design and cost-savings benefits of siphonic roof drainage.