Why are sewer gases escaping my drainage system?

November 29, 2018 by Chris Drake

Have you ever entered an older building an noticed an offensive odor coming from the toilet areas? This is likely caused to improperly sealed traps, or traps that have dried out due to prolonged non-use of the fixture. Knowing the basics of trap primer systems can help alleviate potential odor issues within a building.

All plumbing fixtures are required to have a p-trap. The p-trap provides a means to seal the drainage system, preventing sewer gases from discharging through the fixture and into the inhabitable space. The p-trap is often created as part of the fixture (i.e. – toilets) or is created within the drainage connection to the fixture (i.e. – floor drains, lavatories, sinks). For fixtures that are not regularly utilized (i.e. – floor drains, hub drains), a means of providing water-based or mechanical trap sealing is code required.

Trap Seal Types: Water-based vs. Mechanical

Water-based trap primers connect to the water piping and provide a means of keeping a p-trap wet, hence, providing a seal. The City of Austin typically requires PPP Oregon trap primers for floor drains. The primers include a diaphragm that discharges water during random intervals when there is a pressure drop in the domestic water system. Electronic water-based primers release water into the trap via an electronic timer. It is important to locate the trap primer distribution unit so that it remains accessible, as required by the City of Austin.

trap primer
Courtesy of Carnationconstruction.com

Another form of trap protection is through the form of mechanical trap sealing. These devices are placed in the floor drain body after installation, and allow for water to enter the drainage system, while preventing the release of sewer gases. The p-trap in these cases, typically remain dry after prolonged non-use of the fixture it serves. Note that, while mechanical trap seals are permitted in many jurisdictions and are generally code compliant, the City of Austin does not currently allow this type of device. These seals are also a great option for existing installations. It is a simple retro-fit application in most instances and does not require slab modifications to install underslab piping.

mechanical trap seal
Courtesy of SureSeal

What Material Should I use for Tubing?

The selection of underslab trap primer tubing material is often a designer decision. Concrete can often eat through copper tubing over time. Because of this, cross-linked PEX tubing is a practical choice, for both its resistance to corrosion and its durability. The PEX tubing would route from the primer distribution outlet, run underslab, and connect to the side of the floor drain. Designer should specify the ‘trap-primer connection’ option often provided by floor drain manufacturers. When utilizing a hub drain, the primer discharge tubing can simply day-light into the hub above slab.


Keeping a plumbing fixture trap wet and/or sealed is vital (and code required) in order to prevent potentially hazardous sewer gasses from entering your building. These gases are not only offensive but can have negative heath effects for building personnel. If you need assistance with your next project, our mechanical engineers and plumbing designers can help! Contact our MEP department today at (877) 733-3642 or fill out a quick contact form and we’ll be in touch!

Written by Chris Drake

Chris Drake

Chris Drake is the Plumbing Discipline Leader at BIG RED DOG. He received a degree in Aeronautical Engineering from Rensselaer Polytechnic Institute in Troy, NY and is a registered P.E. His professional experience includes the design of plumbing and HVAC systems in the campus, institutional, sports, federal, and manufacturing markets. His is also experienced in design/build, construction administration, and is a member of ASHRAE and ASPE. Chris originally hails from the Finger Lakes region of upstate New York. Outside of work, Chris enjoys exploring Austin and the surrounding hill country, traveling, and supporting the Buffalo Bills.