Why Does a P-Trap Leak Sewer Gas Smells and How Can I Fix It?

Paige Bennett
Written by Paige Bennett
Updated May 9, 2022
closeup of pipe with p-trap under sink in bathroom next to toilet
Photo: Kasipat Phonlamai / EyeEm / Getty Images

If you smell sewer gas in your home, a dry P-trap could be the culprit

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When a plumbing trap, or P-trap, is leaking, you’ll know it right away. The smell of sewer gas might cause you to crinkle your nose and run to call a plumber. But what is a P-trap, what causes that sickening stench, and how can you prevent P-trap leaking from happening in the first place?

What Is a P-Trap?

Without a P-trap, you’d be smelling those awful sewer gases all day long. All plumbing fixtures include P-traps, and these U-shaped pipes hold water to prevent sewer gases and pests from coming up the pipes and into your home. P-traps can also collect hair, food scraps, and other items that fall down the drain to prevent them from going further into the pipes and clogging your plumbing system.

You can open up your under-sink cabinets and see the P-trap right away; it’s a U-shaped pipe and can be either PVC or stainless steel. The P-trap needs to have water in it to keep smells and flammable gases at bay, so if it runs dry, you’ll notice.

Causes of a P-Trap Leak

Why is the P-trap dry and leaking gas smells? You’ve cleaned and ran the exhaust fan to no avail. Maybe you have a guest bathroom that hasn’t actually had the honor of hosting guests in months, or the summer heat is so intense that the water in your pipes is evaporating faster than the plants in your yard are wilting.

There are a number of causes of this smelly problem.

1. Hot Climates

If you live somewhere dry, the water in the P-trap can evaporate, but even in locales that aren’t particularly dry or hot, the water can evaporate over time. When the water evaporates, there is no longer a barrier keeping the gases from coming up through the pipes and into your home. 

In a household setting, these gases aren’t in high enough quantities to cause major issues, but long-term exposure to sewer gases like methane, sulfur dioxide, and ammonia can lead to headaches, nausea, dizziness, and unconsciousness.

2. Low Usage

If that guest bathroom sink and toilet aren’t getting much use, there’s a higher chance that the P-trap will go dry and begin allowing smelly gases to travel up through the pipes and drains. Plumbing fixtures that are used frequently will keep a P-trap wet and functioning as intended.

3. P-Trap Overload

On the flip side, a P-trap that is overloaded can also release those sewer gas smells. P-traps can’t handle wastewater of more than two to three sinks or other plumbing fixtures. More than that and it will overload, leading to the P-trap leaking sewage odors, plus other issues like bubbling water coming up the pipes.

4. Lack of Ventilation

One of the top issues for dry, smelly P-traps, aside from evaporation, is insufficient ventilation. Plumbing systems have ventilation pipes to refresh the air in the system, leading smelly gases out and drawing fresh air in. You can typically see these pipes sticking out of roofs. Older homes might not have these vent pipes, or debris can clog the vent pipes and prevent proper ventilation.

woman uses kitchen faucet to fill glass with water
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How to Repair a Leaking P-Trap

If you notice a sewer gas smell, it’s best to call a professional. Many times, your nose can confuse sewer gas smells with other smells, so odors are a top sign that your plumbing needs professional repair. A professional can help determine if the issue is a P-trap leak or something even more serious. But if you know for certain the P-trap is dry, here are some simple steps to take to rectify the issue.

Pour 0.5 gallon to 1 gallon of cool water down the drain to make sure the P-trap is wet. In winter or dry climates, P-traps can evaporate in less than one month, so be sure to pour water into the drains or toilets every few weeks.

If the smell is particularly bothersome, you can also add one cup of vinegar or bleach, which will also remove any insects that can enter the P-trap after it runs dry. The bleach or vinegar can also help make sinks shine.

Cost to Repair a Leaky P-Trap

Plumbing repairs don’t come cheap, but they are worth it to prevent catastrophic and costly damage down the road. The average cost for professional P-trap replacement is $200 to $300 for parts and labor.

Preventing P-Trap Leaking

Once you’ve resolved that smelly P-trap, you might fear the day those sewer odors revisit your household. What if the guest bathroom starts smelling like a sewer just as the in-laws are unpacking in the guest bedroom? With these tips, you can prevent your P-trap from leaking and those accompanying nasty smells in the future.

Run Water

One of the easiest ways to prevent P-trap leaking is to regularly run water through the pipes to prevent evaporation. This won’t be hard for regularly used sinks and toilets, but be sure you run water in those guest bathrooms once every few weeks to keep the P-trap wet and properly functioning.

Avoid Overload

Dishwashers or multiple sinks in the same area spell trouble for a lone P-trap. Appliances that release a lot of wastewater, like dishwashers, can overload a P-trap. If you have an upcoming renovation, keep this in mind during planning. Otherwise, avoid running the multiple water-using appliances and fixtures that share a P-trap at the same time.

Keep the P-Trap Clean and Ventilated

Several obstacles can clog vent pipes and lead to sewer gas odors in your home. Check the vent pipes for snow or debris and clear it as needed. If you live in an older building and don’t have vent pipes, consult a professional to install exhaust fans and proper ventilation. It might be time to remodel your bathroom or kitchen.

P-traps can also become clogged, so check for and clear out clogs if you notice slow draining. Make checking for clogs part of your regular kitchen or bathroom disinfecting routine.

Stop Smelly Sewer Gases From P-Trap Leaks

Catching a whiff of sewage in your home is never pleasant, and left untreated, it could cause costly damage and negative health impacts to your family. If you smell sewer gases, hire a local plumber to assess and resolve potential dry or even clogged or leaking P-traps to keep your plumbing system and loved ones healthy.

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