Standing water only in one drain

Floor drain with a puddle in only one drain location in the room

Quick Answer:

If only one drain holds standing water while you mop, it’s usually a local issue: a clogged trap or the short lateral serving that fixture, a disturbed flange or pitch, or a foreign object. Start by removing the local grate and probing the trap for hair, grease, or foreign objects. If that doesn’t clear it, snake only that lateral a short distance, then run a controlled flow into the single fixture to watch how it drains. Also inspect the drain flange and the pipe pitch for any recent disturbance and compare construction with nearby drains.

Why This Happens

When a single drain shows standing water, the problem is typically near that fixture rather than in the building main. Common causes:

  • Debris in the trap: hair, grease, soap scum, or small objects that block the P-trap.
  • Partial obstruction in the short lateral that connects the trap to the branch line.
  • Disturbance from recent work (tile, flooring, remodel) that changed the flange, seal, or pitch — see Standing water after remodel for details to watch for.
  • Damaged or misaligned trap or flange after an impact or repair.

Step-by-Step What to Do

1. Remove the grate and probe the trap

Tools: flat-head screwdriver or wrench, flashlight, disposable gloves, bucket, needle-nose pliers or a hooked probe.

  • Lift the local grate or unscrew the drain flange carefully. Keep a bucket underneath to catch water.
  • Use a flashlight and probe the trap for hair, grease ropes, paper, or small toys. Pull out visible material with pliers or your probe.
  • Clean out what you can by hand or with a small flexible brush. Replace the grate and test drainage.

2. Snake only the short lateral a short distance

Use a small hand auger sized for the fixture (not a long mainline machine).

  • Feed the auger only into the lateral from the trap side a short distance (typically a few feet) to clear localized blockages.
  • Avoid pushing a long cable far down the line — you want to address a local obstruction rather than assume a mainline clog.
  • Withdraw the cable slowly and remove any pulled debris. Run a quick water test.

3. Run a controlled flow into that single fixture

Tools: a 1–5 gallon bucket or jug of water, stopwatch optional.

  • Pour a measured amount (for example, one bucket) into the fixture and watch how fast it drains.
  • Note whether water stalls in the trap, backs up slowly, or clears with sputtering — each pattern gives a clue about where the resistance is.
  • Repeat after each cleaning step to see if drainage improves.

4. Inspect the drain flange and pipe pitch for recent disturbance

Look for signs the drain was moved or the slope changed.

  • Check the flange for loose screws, cracked plastic, or a broken gasket that can let debris slip into odd places.
  • Observe the pipe pitch: a reversed or flattened slope prevents proper flow. Recent floor or fixture work may have altered the pitch.
  • If you smell sewer gas or see gaps, the seal may be compromised and needs repair.

5. Compare to nearby drains for differences in construction

Check how adjacent drains are built and how they perform.

  • Note whether nearby drains have deeper traps, different flange types, or different slope. Differences can explain why only one is affected.
  • If nearby drains are clear, the issue is almost certainly local to the one you’re working on.

What Not to Do

  • Do not assume a whole-house problem and immediately use full-length mainline tools when the issue is isolated to one drain.
  • Avoid pushing a long sewer machine or snake through the line when a short lateral or trap cleaning will likely fix it.
  • Don’t pour strong chemical drain cleaners into a standing-water situation repeatedly; they can damage finishes and piping and often won’t remove solid obstructions.
  • Do not attempt invasive access that requires cutting finished surfaces; call a pro when access needs cutting or major work is required.
  • Call a professional if localized snaking fails, if you see signs of pipe collapse, or if you can’t access the trap safely.

When to Call a Professional

  • If short, local snaking does not clear the drain after a couple of attempts.
  • If you see crumbling, sagging, or collapsed pipe material or persistent sewage odor suggesting structural failure.
  • If the drain requires cutting into finished flooring, tile, or cabinetry to access the pipe.
  • If standing water spreads to other fixtures, or if backups are recurrent and not fixed by trap cleaning.
  • If you’re uncomfortable performing any of the steps or if the work exposes hazardous waste (sewage), enlist a licensed plumber.

Safety Notes

  • Wear gloves and eye protection when probing drains; debris and bacteria can cause infection.
  • Ventilate the area and avoid mixing chemical cleaners. Do not use chemicals right before mechanical snaking — residues can splash back.
  • Support and secure heavy grates before lifting; edges can be sharp and traps can be heavy when full.
  • If you encounter raw sewage, major structural damage, or a situation beyond hand tools, stop and call a professional.

Common Homeowner Questions

  • Why is only one drain backing up? A local clog in the trap or short lateral, a misaligned flange, or recent disturbance near that drain is the usual cause.
  • Can I fix this with chemical cleaners? Not reliably. Chemicals rarely remove hair or solid objects and can harm pipes and finishes; mechanical removal is usually better.
  • How far should I snake before calling a plumber? Try a short hand auger limited to the lateral length (a few feet past the trap). If that doesn’t work, stop and call a pro.

More in this topic

For more related fixes and similar symptoms, see Standing Water in Floor Drain.

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