Dry P-Trap: The #1 Cause of Sewer Smell (And How to Fix It)
Top TLDR:
A dry P-trap is the most common cause of sewer smell in a home—and the easiest to fix. When the water seal inside the curved pipe evaporates from disuse, sewer gas travels straight up through the open drain and into your living space. Polk County homeowners can resolve most dry P-trap odors in under two minutes by running water in every unused fixture; if the smell returns or doesn't clear, contact S&S Waterworks at (863) 362-1119.
If your home suddenly smells like sewage, stop and think about it for a moment before calling a plumber or tearing apart your bathroom: when did you last use the drain closest to where the smell is strongest? In the majority of household sewer smell cases, the answer explains everything. A dry P-trap—one that has lost its water seal through simple evaporation—is the leading cause of sewer gas odor in residential homes, and it's a problem that often takes less than two minutes to fix.
What a P-Trap Is and Why It Matters
A P-trap is the curved, U-shaped section of pipe located directly beneath every sink, shower, tub, and floor drain in your home. If you look under a bathroom or kitchen sink, you'll see it—a bent loop of pipe that holds a small reservoir of standing water at all times.
That standing water isn't incidental. It's the entire point. The water creates a physical barrier—a liquid seal—between your home's living space and the sewer system below. Without it, sewer gas has a clear, unobstructed path from the municipal sewer line or septic system straight up through the drain opening and into the room.
Your plumbing system produces sewer gas constantly. Hydrogen sulfide, methane, and ammonia are byproducts of organic waste decomposition in sewer systems. These gases are why your drain smells faintly when you lean close to it—and why a dry trap turns that faint smell into a room-filling odor almost immediately.
Why P-Traps Dry Out
P-traps dry out for one simple reason: evaporation. When a drain isn't used regularly, the water in the trap slowly evaporates until the seal is gone. This isn't a malfunction—it's just physics. But certain conditions accelerate the process significantly.
Infrequent use is the primary factor. Drains that go weeks or months without water flowing through them are almost guaranteed to develop dry traps. In most homes, that includes:
Guest bathrooms that only get used when company visits
Basement and garage floor drains that exist for emergencies but rarely see water
Utility room sinks and laundry room drains, particularly after appliances are moved
Vacation homes or seasonal properties that sit empty for extended periods
Rarely used bathtubs in homes where showers are the daily preference
Florida's heat makes this worse than in most states. In Lakeland and throughout Polk County, ambient temperatures stay high year-round. A P-trap that might hold its water seal for three or four weeks in a cooler climate can evaporate completely in under a week during a Florida summer. If you have guest fixtures you're not actively maintaining, dry traps will happen regularly—it's just a matter of when.
Low humidity conditions during periods of air conditioning can accelerate evaporation indoors as well, compounding the seasonal effect.
How to Know If a Dry P-Trap Is Your Problem
The good news about dry P-traps is that they produce a very recognizable pattern of symptoms—one that's distinct from more serious plumbing issues.
The smell is near a specific, identifiable drain. Unlike vent stack problems or main sewer line damage, which spread odor throughout the whole house, a dry trap typically produces a smell that's concentrated near one fixture.
The affected drain hasn't been used in a while. Think back honestly. If the smell is coming from a guest bathroom no one has used in two weeks, or a utility sink that's been dormant since the washer was moved, the answer is almost certainly a dry trap.
The smell disappears quickly after running water. This is the clearest confirmation. Run water into the drain for 30–60 seconds. If the smell fades within a few minutes of refilling the trap, a dry trap was your cause. If it doesn't improve—or returns within a day or two—the problem is something else, and further diagnosis is needed.
How to Fix a Dry P-Trap
Fixing a dry P-trap is genuinely as simple as it sounds.
Step 1: Run water in every infrequently used drain. Spend 60 seconds at each fixture—sink, shower, tub, floor drain—running water to fill the trap. Include drains you've been meaning to use but have been avoiding. Cover every drain in the house that doesn't see daily use.
Step 2: Don't forget floor drains. These are the most commonly overlooked traps in any home. Basement floor drains, garage floor drains, and utility room drains receive almost no regular water flow and dry out faster than fixture drains. Run a bucket of water down each one.
Step 3: Wait 10–15 minutes, then reassess the smell. If the odor clears, you've confirmed and solved the problem.
Step 4: For drains that dry out repeatedly, add mineral oil. Pouring a small amount of mineral oil (about a cup) on top of the water after filling a floor drain creates a barrier layer that dramatically slows evaporation. The water seal stays in place; the oil just prevents it from evaporating. This is particularly useful for floor drains that you can't feasibly run water through every week.
Prevention: Keeping P-Traps Full Year-Round
Once you know how dry traps work, prevention is straightforward. The goal is to make sure water moves through every drain in your home on a regular basis.
Establish a weekly habit for unused fixtures. Run 30 seconds of water through guest bathroom sinks, showers, and tubs every week. This takes under five minutes to do for your entire home and eliminates dry traps as a recurring problem. During hot Florida summers, consider doing this twice a week for drains that tend to evaporate quickly.
Use mineral oil on floor drains. Any floor drain that gets little regular water flow—basement, garage, utility room—benefits from an oil layer. Refresh it every few months or whenever you notice the drain may have dried out.
Before long trips or vacancy periods, flush all drains. If you're leaving a property empty for two weeks or more, run every drain thoroughly before you leave. Add mineral oil to floor drains. When you return, run all drains again before assuming any odors that developed during your absence are something serious.
Track your guest bathroom use. If you have a bathroom that only sees guests occasionally, build a reminder to run the water periodically. A phone reminder set every two weeks costs nothing and prevents the scenario entirely.
When It's More Than a Dry P-Trap
A dry P-trap is the most common cause of sewer smell, but it isn't the only one. If you've refilled every trap in your home and the smell persists—or returns within a day despite regular water use—the cause is something that requires professional diagnosis.
Wax ring failure produces a persistent, steady bathroom odor that doesn't respond to drain flushing. The wax seal between the toilet base and the floor flange can compress or crack over time, allowing sewer gas to escape continuously. Signs include a toilet that rocks slightly when you sit, soft flooring around the base, or a bathroom that still smells like sewage even after all drains are flushed. Wax ring replacement requires removing the toilet and assessing the flange—a job for a licensed plumber.
Vent stack blockage creates a whole-house odor, not a single-fixture smell. If the sewer gas smell is spread throughout multiple rooms and is accompanied by gurgling drains after flushing, the vent that normally releases gas out through the roof has been blocked by debris or a nesting animal. Understanding when a plumbing issue requires professional help is important here—vent stack clearance requires roof access and specialized tools.
Main sewer line damage produces odors alongside other symptoms: slow drains throughout the house, possible sewage backup, wet areas in the yard, or unusually green grass along the sewer line path. Main sewer line issues in Lakeland and Polk County are frequently caused by tree root intrusion, pipe corrosion, or ground settlement—all of which require professional video inspection to diagnose accurately.
Drain line buildup in kitchen drains and garbage disposals produces odors from decomposing organic material rather than sewer gas itself. These smells are often triggered by running hot water and can be addressed through professional drain cleaning, including hydro jetting to fully clear buildup from pipe walls rather than just punching through a blockage.
If you've ruled out the dry P-trap and aren't sure what you're dealing with, identifying the exact source of the sewer gas smell is the right next step before attempting any further repair.
The Two-Minute Fix Is Worth Trying First
The reason dry P-traps are worth covering in detail isn't because they're complicated—it's because homeowners don't always think of them. It's easy to assume a sewage smell signals something serious and expensive. Often it signals a drain that hasn't seen water in ten days.
Before calling anyone, run water in every unused fixture in your home. Check the guest bath, the basement floor drain, the utility sink, the vacation bathroom. Give each drain a full minute of water. Wait. If the smell clears, you've solved your own plumbing problem in under five minutes with zero cost.
If it doesn't clear, that's important diagnostic information—and it means it's time to call in a professional.
S&S Waterworks Serves Polk County Homeowners
S&S Waterworks provides drain odor diagnosis and elimination for homes throughout Lakeland, Winter Haven, Auburndale, Bartow, and surrounding Polk County communities. When the problem is more than a dry trap, our technicians diagnose it accurately using video inspection technology and resolve it with the right method—not the most expensive one.
Every service call includes upfront pricing before work begins, a technician profile sent before arrival, and real-time status updates. Our complete plumbing services cover everything from drain cleaning to slab leak detection and repair.
Call us at (863) 362-1119 or schedule your appointment online. If a dry P-trap is your problem, we'll tell you that too—and save you the service call cost if we can help you solve it over the phone.
Bottom TLDR:
A dry P-trap is the number one cause of sewer smell in homes—and fixing it takes nothing more than running water down the affected drain for 60 seconds to restore the water seal. In Polk County's warm climate, P-traps in guest bathrooms, floor drains, and utility sinks can evaporate in under a week, making this a recurring issue without regular maintenance. If the sewer smell doesn't clear after refilling all traps, contact S&S Waterworks at (863) 362-1119 to diagnose the actual source before the problem worsens.