Quick Answer:
If you step into the basement after a rainstorm and the floor drain is overflowing, it often means the sewer or storm outlet outside is surcharged, not necessarily that the trap or short-run outlet is broken. Start by removing the grate and looking for obvious blockages, then do a simple bucket test to see if water flows away or surges back. Check whether the backup is coming from inside the house or from outside, and verify any backflow device on the drain is intact before calling a plumber.
Why This Happens
Heavy rain can overwhelm municipal sewers, private storm lines, or your yard drainage, forcing water back up through the lowest opening—the floor drain. If nearby drains, gutters, or downspouts are clogged or the ground is saturated, the system can’t accept more water and your basement can show the effects. In many cases the drain and trap are fine but the system beyond the house is under pressure. Occasionally the drain mechanism (a check valve or flap) is damaged or debris prevents it from seating.
For more on outside causes and site drainage, see Rain causes basement flooding.
Step-by-Step What to Do
Tools and simple items to have ready
- Work gloves
- Flat-head screwdriver or the tool needed to lift the grate
- A container or 1-gallon measure for the bucket test
- Flashlight
- Towels or a mop for cleanup (do not stuff them into the drain)
1) Remove the grate and check for visible debris
Carefully lift the grate using the correct tool and gloves. Look into the trap for leaves, sticks, plastic bags, or other solids that can block flow. If you see debris within reach, remove it by hand or with a grab tool. Keep removed material out of the drain path so water can flow.
2) Do a bucket test
Pour a measured amount of water—about 1 quart to 1 gallon at a time—into the drain and watch what happens. Two outcomes are common:
- If the water disappears steadily, the drain can accept flow right now.
- If the water surges back or immediately rises, the line beyond the house is likely surcharged or the backflow device is failing.
Repeat the test a couple of times if needed, but don’t pour large volumes while the drain is overflowing.
3) Inspect where the backup is originating
Determine whether the source is inside or outside:
- Check other low fixtures (floor drains in other rooms, basement sinks) for signs of backup.
- Look outside at downspouts, yard drains, or a nearby sewer manhole (if safe to view). If outdoor drains are full or the yard is flooded, the problem is likely external.
- Note whether the sump pump is running and handling rain that way.
4) Verify any backflow preventer is intact
Many floor drains have a check flap or backflow valve. With the grate removed, inspect the flap or device to see if it is closed, cracked, or jammed with debris. Do not force a stuck valve; if you can gently clear debris and the flap returns to position, that may restore protection. If the valve is broken or missing, note this for the plumber.
What Not to Do
- Avoid pouring caustic chemicals into an overflowing floor drain or stuffing towels into it; these make retrieval and repairs harder.
- Do not stand in pooled basement water if electrical panels, outlets, or appliances are wet—shut off power to the area if it is safe to do so.
- Call a pro when sewage smell is present or overflow persists after clearing visible debris.
When to Call a Professional
- There is any sewage odor or you see sewage-contaminated water—this requires immediate professional cleanup.
- Overflow continues after you remove visible debris and do the bucket test.
- You find a damaged or missing backflow preventer, or the valve won’t operate.
- Multiple drains inside the house are backing up at once—this often means a main sewer or municipal issue.
If the problem looks external (street flooding, full yard drains), contacting your municipal public works department in addition to a plumber can help identify larger system issues. For seasonal patterns and timing, read about Drain floods only in spring.
Safety Notes
- Wear gloves and eye protection when handling drain debris.
- Do not use electrical tools around standing water unless power is off and the area is clearly safe.
- If you suspect sewage contamination, avoid skin contact and keep children and pets out of the area until it is cleaned by professionals.
Common Homeowner Questions
-
Q: Is the trap broken if water backs up during rain?
A: Not necessarily—backups during storms are often due to external surcharge, though a damaged trap or valve can make the problem worse. -
Q: Can I clear the drain myself?
A: You can remove visible debris and do a bucket test, but stop and call a plumber if you smell sewage or the overflow keeps happening. -
Q: Will drain-cleaning chemicals fix this?
A: No—avoid pouring caustic chemicals into an overflowing drain; they won’t fix an external surcharge and can complicate repairs.
More in this topic
For more related fixes and similar symptoms, see Basement Drain Backs Up Only After Rain.