The short version
- What a trap does
- Holds a few inches of water that blocks sewer gas from rising out of the drain
- Why it fails
- Evaporation when the drain sits unused, especially in dry, heated indoor air
- Where to look
- Floor drains in restrooms, utility rooms, mechanical closets, back hallways and warehouse floors
- Quick test
- Pour water into the drain and see whether the smell fades over the next few hours
- Lasting fix
- A routine refill schedule, a trap seal insert, or a trap primer installed by a licensed plumber
Why commercial spaces are prone to this
Homes have few floor drains, and the ones they have get used. Commercial buildings are different. A typical suite in a Westminster office park or strip center might have a floor drain in each restroom, another in a janitor closet, one in a mechanical room near a water heater, and sometimes a drain left behind by a previous tenant's buildout. Many of those drains never see water from one month to the next.
When the water in the trap evaporates, there is an open path from the building sewer into the room. The result is the rotten-egg or swampy smell people describe as sewer gas. Front Range air is dry for much of the year, and forced-air heat in winter dries it further, so a trap can empty faster than people expect. That is why the smell often appears after a long weekend, a holiday closure, or in a suite that has been vacant between leases.
How to confirm a dry trap
This takes a few minutes and costs nothing beyond a bucket of water.
List every drain
Walk the suite, including back rooms, and note each floor drain, floor sink, mop sink and rarely used fixture such as a second restroom or an eyewash station.
Smell each one
Get close to each drain grate. A dry trap usually smells strongest right at the opening rather than evenly through the room.
Refill slowly
Pour a gallon or so of water into each drain. A splash of mineral oil on top of the water can slow evaporation in drains that sit unused.
Wait and recheck
If the smell fades over a few hours and stays gone, the trap was the likely path. Write down which drain it was and the date.
Watch for a return
If the odor comes back within days even though the trap has water, the cause is probably elsewhere: a cracked line, a failed seal, or a venting problem.
Keeping traps sealed
For drains that are easy to reach, a recurring task on the janitorial checklist works: refill every floor drain on a set schedule and note it on a log. For drains that are forgotten or hard to reach, a licensed plumber can discuss a trap primer, which adds small amounts of water automatically, or a trap seal insert, which lets water down but resists gas coming up. Some buildings already have primers that have stopped working, so ask the property manager whether one is installed before assuming it is missing.
If a drain truly serves no purpose, a plumber can advise on whether it can be properly abandoned. Covering a drain with tape or a mat is not a fix and can hide a problem from the next person who needs to find it.
Questions to bring to the property manager
- Which drains in this suite have trap primers, and when were they last checked?
- Are there drains under flooring or behind casework that are not obvious from the room?
- Has the space been vacant recently, and was anyone refilling traps during that time?
- Who is responsible for plumbing repairs under the lease, the tenant or the landlord?
- Have other tenants on the same line reported sewer odors?
Common questions
- Can an odor treatment fix a dry trap smell?
- No. Once the trap is refilled and the gas stops entering, the room usually clears with normal ventilation. Treatment only makes sense if odor has soaked into porous materials over a long period.
- Why does it smell on Mondays only?
- Low use over the weekend gives traps time to evaporate and lets odor build up without the air handler running on its occupied schedule. Refilling on Friday afternoons is a simple test.
General information for Westminster, not a quote or professional advice. In an emergency, call 911.