Quick Answer:

If a sink or floor drain overflows when the washing machine pumps out, start by confirming the timing, then test the local drain and appliance connections. Often this is a partial blockage in the laundry branch or a full-height drain problem, not necessarily a sewer main collapse. Follow the steps below to narrow the cause and decide whether you need a pro.

Why This Happens

The washing machine pumps a large volume of water quickly. If the laundry standpipe, trap, or nearby branch drain is narrowed by lint, soap build-up, or grease, the surge can push water out the lowest nearby opening. When the main sewer has a heavy restriction, you can also see overflow at the lowest house drain during big discharges. Listen for gurgling or slow drains elsewhere — those are clues to where the restriction is.

Step-by-Step What to Do

Step 1 — Confirm it correlates with the washer pump-out

Watch the drain during one normal wash cycle or the spin/drain phase. If the overflow happens precisely when the washer pumps out, that points to a flow/capacity issue with the laundry drain or a downstream restriction that can’t handle the pulse of water.

Step 2 — Check if other fixtures gurgle or back up

Run water in a sink, flush a toilet, and listen for gurgling in nearby drains while the washer pumps. If multiple fixtures gurgle or the toilet water level moves when the washer drains, the problem is likely downstream of where those drains join.

Step 3 — Test the drain with a controlled bucket pour

Pour a measured 5–10 gallon bucket of water down the laundry standpipe (or the drain that overflows) at a steady rate to simulate the washer pump. Do this once and watch the point that overflows. If the same drain overflows with the bucket, the issue is local to that branch or a restriction downstream.

Step 4 — Inspect lint screen and the standpipe

Check and clean the washer lint screen and any lint trap on the standpipe. Look down the standpipe with a flashlight — lint or ball-shaped soap scum often lodges near the trap or upper standpipe and reduces flow. Remove any visible debris by hand or with a small grabber tool.

Step 5 — Note whether the lowest drain is the one overflowing

If the lowest open drain in the house (basement floor drain or lowest sink) is the one that overflows, that raises the chance of a main line restriction. If the overflow is limited to the laundry sink or utility floor drain and other fixtures are fine, it’s more likely a local clog or capacity issue.

What Not to Do

• Do not run repeated wash cycles to “flush” the system — that can make an overflow worse and cause more water damage.
• Don’t snake blindly through the washer standpipe without securing the machine and hose — the cable can catch or damage the trap and may push debris further down.
• Avoid pouring large amounts of chemical drain cleaner into the laundry drain; it often doesn’t reach the clog and can be hazardous.

When to Call a Professional

Call a plumber if:

• Multiple fixtures back up or gurgle when the washer pumps — this suggests a blockage in the main or a shared branch.
• Your bucket test caused the same overflow and simple cleaning didn’t help.
• There is standing water in the yard near the sewer cleanout or sewage odors indoors.

If the issue seems limited to lint and soap buildup, a service visit for a proper inspection and targeted clearing is the safest next step. You can read more about related drainage problems in the article Drain Keeps Clogging Every Few Days and, for odors that appear with other appliances, see Drain Smells Worse When the AC Runs.

Safety Notes

• Turn off power to the washer before working near electrical connections or removing panels.
• Wear gloves and eye protection when reaching into traps or handling debris.
• If sewage is present, avoid contact and call a professional; sewage cleanup can pose health risks.

Common Homeowner Questions

• Q: Could this be a collapsed main sewer line?
  A: It’s possible but uncommon; confirm multiple fixtures back up and call a plumber for a camera inspection if you suspect collapse.
• Q: Will a drain snake fix this right away?
  A: If the clog is local and accessible, snaking can help, but don’t force a cable into the standpipe and avoid repeated DIY attempts if you’re unsure.
• Q: Can I prevent this from happening again?
  A: Regularly clean the lint screen and flush the standpipe with a bucket of hot water after laundry loads; avoid dumping large amounts of lint or greasy items into the drain.