Why does my commercial building smell like sewer gas?

In a commercial building the number one source of a sewer smell is a trap that has dried out from sitting unused. Think about how many drains a building has that rarely see water: floor drains in mechanical rooms and electrical rooms, mop sinks in a janitor closet, a low-use restroom on a slow floor, drains in a basement or a seldom-entered storage area, and floor drains tucked under equipment. Each one has a trap, and a trap with no water in it is an open pipe to the sewer main.

The water seal does not last forever when nothing refills it. The seal slowly evaporates, and once it drops too low, sewer gas flows straight into the room. In Phoenix the dry air and heat speed this up. A trap that might hold its seal for a couple of months in a humid climate can dry out in a few weeks here, especially with building HVAC pulling dry, conditioned air across the floor.

Plumbing code sets the trap seal at a specific depth for exactly this reason. The International Plumbing Code, Section 1002.4, states that "each fixture trap shall have a liquid seal of not less than 2 inches and not more than 4 inches." That few inches of water is the entire defense against sewer gas. The smell is the proof the seal is gone.

The first test is simple and free. Pour about a quart of water down each floor drain, mop sink, and rarely used fixture, then flush every toilet and run every faucet you can find. If the smell fades within a day, a dry trap was the cause.

Pouring water in works once, but staff will not remember to refill a drain in a mechanical room every few weeks. For drains that keep drying out, code has a built-in answer: a trap seal primer.

A trap seal primer is a small valve that automatically feeds a little water into a trap to keep the seal full. The International Plumbing Code, Section 1002.4.1, requires this protection where a trap is at risk of losing its seal. The code allows either a trap seal primer valve that meets ASSE 1018 or ASSE 1044, or a barrier-type trap seal protection device that meets ASSE 1072. A barrier device is a flexible insert that sits inside the drain and opens to let water pass, then seals shut to block gas, so it works even with no water supply.

For a property manager, this is the difference between a smell that comes back every month and one that is solved. The plan is straightforward: refill the dry traps today, then have a plumber add primers or barrier devices on the handful of drains that the building will never keep wet on its own. Those are usually the mechanical-room and low-use-restroom floor drains.

If the traps are full and the smell is still there, the problem is somewhere else in the system. Work through these in order.

• A blocked or broken vent. Your drain system has vent pipes that run up through the roof so air can enter and water flows smoothly. When a vent is clogged by a bird nest, debris, or grease, draining water creates suction that pulls the water out of nearby traps. The result is gurgling drains and a smell that returns no matter how often you fill the traps. For how venting works, see what a plumbing vent is.
• A failed toilet wax ring. The wax ring seals a toilet to the drain flange in the floor. When it fails, or the toilet rocks loose, sewer gas leaks out at the base and you may see water there too. This is common on older or heavily used commercial toilets.
• A grease or FOG problem in a kitchen. If the building has a restaurant or break-room kitchen, a neglected grease trap produces a heavy, rancid smell that water will not fix. The U.S. EPA's grease program points to a common service rule: clean an interceptor before more than 25 percent of it fills with grease and solids. For a deeper look, see why a restaurant smells like sewer.
• A cracked sewer line. Roots, shifting soil, or old pipe can break the line under the slab and let gas escape into the building, sometimes with no visible water. If the smell is constant, building-wide, and immune to the water test, this is the likely reason.

One case to rule out fast: a rotten-egg smell can also mean a natural gas leak, not sewer gas, because gas utilities add an odorant that smells like sulfur. If the smell is near gas equipment, hisses, or comes with headaches or dizziness, treat it as gas. Leave the building and call 911 and Southwest Gas at 877-860-6020 before doing anything else.

A sewer smell is not just unpleasant, and it is a real reason to act fast. The rotten-egg odor in sewer gas is hydrogen sulfide. According to the ATSDR, people can smell hydrogen sulfide at very low levels, well below where it causes harm, which is why even a faint smell is worth chasing down.

There is a catch that makes the smell unreliable as a warning. The ATSDR notes that "exposure to hydrogen sulfide may cause irritation to the eyes, nose, or throat." It also explains that at higher concentrations a person loses the ability to smell the gas, so the nose stops being a guide right when the level gets serious. The takeaway for a building owner is plain: do not assume a smell that "went away" means the problem is gone. It can mean the level rose past the point where you notice it. Low-level exposure also brings headaches that staff may blame on a long day.

For a food business the stakes include a failed inspection. The FDA Food Code, which Arizona health departments follow, requires the plumbing system to be kept in good repair and the premises kept free of pests. A dry floor drain breaks both rules at once, because the same open trap that lets gas in also lets drain flies and roaches up from the sewer. Inspectors read a persistent sewer smell as a sign of a system nobody is maintaining.

Work from cheapest to most involved, and you will solve most cases in the first step.

1. 1Map and flush every trap. Walk the whole building, including mechanical rooms, janitor closets, basements, and slow restrooms. Pour a quart of water into every floor drain and mop sink, flush every toilet, and run every faucet. Wait a day.
2. 2Note where the smell is. If it cleared after flushing, a dry trap was the cause, and the spots that stayed smelly are your candidates for trap seal primers. If a smell is strongest at one toilet base, suspect the wax ring. If it is strongest near a kitchen, check the grease trap.
3. 3Watch for vent signs. Gurgling drains or a smell that returns within days points to a blocked vent. This needs a plumber on the roof.
4. 4Camera the line. If the smell is constant, building-wide, and survives full traps, the cause is likely a vent failure or a cracked line under the slab.

Call a commercial plumber once the free steps run out. If the smell survives a full building flush, comes back fast, or covers the whole building, the source is past what pouring water can reach. A plumber can scope the vents, pressure-test the system to find where gas escapes, and run a sewer camera down the line to see a crack or a deep blockage directly instead of guessing.

A camera inspection is also what turns a recurring mystery smell into a fixed problem with a paper trail, which matters if you manage the building for an owner or answer to a health inspector. The same visit can confirm which drains need trap primers and whether a wax ring or a broken line is involved.

The bottom-line move for a property or restaurant owner: do not mask a sewer smell with air fresheners, because the odor is telling you a trap or line has lost its seal. Flush every trap today, since that is free and solves most cases, then have a plumber install trap seal primers on the drains that keep drying out and camera the line if the smell holds. Catching a blocked vent or a cracked line early costs far less than a sewage backup or a failed inspection.