Quick Answer:
Intermittent toilet backups usually mean a partial obstruction in the main line that shifts under different flows. Track exactly when backups happen and which fixture was used first, run a simple dye test from an upstairs flush to check cross-flow, open the cleanout during an event to watch what moves, then snake the main and arrange a camera inspection. Once you know the cause — grease ball, roots, or a broken section — pursue targeted cleaning or repair.
Why This Happens
A partial clog doesn’t block the pipe all the time. It may sit against a wall, be soft enough to deform, or move when flow direction or pressure changes. Normal examples are grease balls that roll, root masses that loosen and shift, or a pipe joint that allows a bit of material to gather and then pass. That explains why one flush sometimes goes through and another sends sewage back.
If the pattern is gradual or gets worse, think about how this connects to other problems like a **Partial clog worsens over time**. If the backups seem tied to an upstairs toilet or shower, also consider the scenario of a **Partial clog only upstairs** when deciding what tests to run first.
Step-by-Step What to Do
1. Track when backups occur
Keep a simple log for a few days. Note the date and time, which fixture was used immediately before the backup (toilet, shower, washing machine, etc.), how many people were using water at the time, and whether any heavy-use appliance ran. Patterns—like always after the upstairs flushes—are key clues.
2. Run a dye or food-color test from upstairs
Flush the upstairs toilet or run a tub briefly with a few drops of food coloring in the bowl. Check lower fixtures (toilets, tubs, floor drains) for the colored water. That shows whether water is crossing into other drains and helps locate where flows are connecting during an event.
3. Open the cleanout when an event occurs
Locate the main-line cleanout outside or in your basement and, during a backup event, open it carefully to observe the flow. Use gloves, eye protection, rags, and a bucket. Watching the cleanout will show whether the obstruction shifts, whether water is slowly seeping past it, or whether solids intermittently pass. If you can’t reproduce an event, run fixtures to increase flow and try to create the same condition while someone monitors the cleanout.
4. Snake the main and schedule a camera inspection
Use a power auger (or hire someone who has one) to snake the main line. Snaking can break up grease balls or cut through small root tangles. After snaking, arrange a professional camera inspection to see what remains. A camera will identify movable obstructions (grease balls, roots), collapses, offset joints, or lodged debris so you can pick the proper repair.
5. Pursue targeted repairs or cleaning once the source is found
Options depend on what the camera shows: hydro-jetting for grease and sludge, mechanical root cutting plus root treatment, spot repair or pipe replacement for damaged sections. Avoid repeated temporary fixes—address the exact cause discovered by the inspection for a lasting solution.
What Not to Do
- Don’t ignore intermittent backups; they usually get worse and can cause a major sewage return later.
- Don’t rely on repeated plunging as a long-term fix. Plunges may clear a temporary symptom but won’t remove a shifting obstruction in the main line.
- Don’t delay calling a pro when backups recur unpredictably, involve multiple fixtures, or you observe sewage coming back into the house.
When to Call a Professional
- If backups happen unpredictably or affect more than one fixture at once.
- If you see sewage backing up inside the house or significant foul smells that don’t clear.
- After snaking if the problem returns, or whenever a camera inspection is needed to identify the cause.
- If you’re uncomfortable opening the cleanout or handling sewage—call a plumber to do the observation and testing safely.
Safety Notes
- Wear gloves, eye protection, and old clothes when inspecting drains or opening a cleanout. Sewage contains pathogens.
- Have towels, a bucket, and a disposable container ready for any overflow. Keep children and pets away from the work area.
- Avoid using strong chemical drain cleaners on the main line; they are rarely effective for a main-line partial clog and can damage pipes or harm you if splashed.
- If you suspect a collapsed pipe, major roots, or large backups, stop and call a professional rather than attempting major repairs yourself.
Common Homeowner Questions
- Why does it only happen sometimes?
Because the obstruction is partial and shifts with different water flow or pressure, so some uses pass while others trigger a backup. - Will snaking fix it permanently?
Sometimes snaking fixes the immediate problem, but a camera inspection is the only way to know if the cause (roots, grease, broken pipe) has been fully addressed. - Can I use chemical drain cleaners on the main line?
No—chemical cleaners are not recommended for main-line partial clogs and can harm pipes and people; use mechanical cleaning and professional services instead.
More in this topic
For more related fixes and similar symptoms, see Main Line Partial Clogs.