P-Trap Plumbing Problems: What Goes Wrong and How to Fix It

The P-trap is the curved pipe under every sink, shower, and floor drain. It’s a simple component that does an important job: holding a water plug that blocks sewer gases from entering the home. When P-traps fail — through leaks, clogs, or drying out — the symptoms range from puddles under the sink to sewage smell throughout the house. Here’s what goes wrong and how to fix it.

How the P-Trap Works

The P-trap holds water in the curved section to block sewer gases.

Every fixture drain (sink, shower, bathtub, floor drain) connects to the drain system through a P-trap. The curved “P” shape retains a small amount of water at the lowest point of the bend. This water plug physically blocks sewer gases — hydrogen sulfide, methane, and other gases — from coming back up through the drain.

What connects to a P-trap:
– Kitchen and bathroom sinks
– Bathtubs (built into the drain assembly)
– Showers (built into the floor drain)
– Floor drains in basements and utility rooms
– Laundry tubs

Problem 1: Leaking P-Trap

The most common P-trap failure — water drips from under the sink.

Leaks occur at the slip-joint connections (the threaded nuts where pipe sections join) or from cracks in the trap body itself.

Causes:
– Slip-joint nuts loosened from vibration or thermal movement
– Worn or hardened rubber washers inside the connections
– Cracked plastic trap body (from overtightening, impact, or age)
– Corroded metal (chrome) trap with pinhole leaks

Fix:
– Loose connection: hand-tighten the slip-joint nut plus 1/4 turn
– Worn washer: remove nut, replace washer ($1–$2), reassemble
– Cracked or corroded trap: replace the P-trap assembly ($5–$15 for PVC kit)

Problem 2: Clogged P-Trap

Hair, grease, and debris accumulate in the trap bend.

The curved section of the P-trap is a natural collection point for debris that washes down the drain. Hair, soap scum, food grease, and toothpaste buildup accumulate over time, eventually restricting or completely blocking the drain.

Signs of a clogged P-trap:
– Slow draining sink or bathtub
– Water backing up in the sink
– Gurgling sounds as water drains

Fix:
1. Plunger first: Use a cup plunger on the sink drain to dislodge soft clogs
2. Remove and clean: For persistent clogs, unscrew the P-trap slip-joint nuts, remove the trap, clean it out over a bucket, and reinstall
3. Drain snake: Feed a hand-powered drain snake through the drain to break up clogs in the trap or further into the drain line

What not to use: Chemical drain cleaners. They’re corrosive, may not reach the actual clog location, and can damage trap washers and pipe surfaces.

Problem 3: Dry P-Trap (Sewer Smell)

When the water plug evaporates, sewer gas comes up through the drain.

P-traps in infrequently used drains — a guest bathroom sink, a basement floor drain, a vacation home drain — can dry out if water isn’t run regularly. Without the water plug, the trap is open to the sewer system and sewer gases enter the home.

Signs:
– Sewage smell from a specific drain
– Smell is worse in a room that’s rarely used

Fix: Run water down the drain for 30 seconds to refill the trap. For floor drains, slowly pour a quart of water directly into the drain. If the smell persists after refilling, the issue may be a different drain, a venting problem, or a cracked trap.

Prevention: For infrequently used drains, run water once a month to maintain the water plug.

Problem 4: S-Trap vs. P-Trap (Code Issue)

S-traps are prohibited by current plumbing code but exist in older homes.

An S-trap is a drain configuration where the trap curves down and then back up in an “S” shape before connecting to a vertical drain. S-traps can siphon the water out of the trap as water drains, breaking the seal and allowing sewer gas in.

How to identify: Under the sink, if the drain goes down, curves up, then down again into the floor — that’s an S-trap. If it goes down and then horizontally into the wall — that’s a P-trap.

Fix: S-traps in older homes often function adequately if the drain venting is sufficient. If you’re renovating, replace with a proper P-trap and wall drain connection. A plumber should assess whether an existing S-trap is causing recurring sewer smell issues.

FAQ

Q: Why does my P-trap keep leaking?
A: Recurring leaks after tightening the slip-joint nuts indicate worn rubber washers that need replacement, or a cracked plastic trap that needs complete replacement. The fix is simple — a replacement P-trap kit costs $5–$15.

Q: Why does my drain smell even though the P-trap is intact?
A: If the trap is dry (infrequently used drain), run water to refill it. If the trap holds water and still smells, check other nearby drains for the odor source. A venting problem can also cause intermittent sewer smell regardless of trap water level.

Q: How do I unclog a P-trap?
A: Remove the P-trap by unscrewing the slip-joint nuts (have a bucket ready), clean out the debris, and reinstall. For clogs further into the drain line, use a drain snake through the wall stub-out.

Q: Do P-traps need to be replaced?
A: Not on any fixed schedule — they fail when they crack or corrode. PVC P-traps last 15–20+ years. Chrome traps may need replacement sooner as corrosion develops. Replace when leaking or visibly damaged.

Q: What causes a P-trap to dry out?
A: Lack of use — if water isn’t run through the drain, evaporation slowly removes the water plug over weeks. In a consistently air-conditioned or heated home with low humidity, evaporation can empty a trap in 2–4 weeks. Run the drain once a month for infrequently used fixtures.