Short answer: A clog is likely deep in the pipe when plunging and cleaning the trap do nothing, when more than one fixture is slow, or when water comes up in a tub or floor drain as you flush. One fixture: the branch line. Several fixtures, especially on the lowest floor: the main sewer line, which needs a plumber.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| One sink slow, trap is clear | Clog in that branch line | Hand snake into the wall pipe | If the snake will not clear it |
| Toilet flush makes tub or shower gurgle or back up | Main line partly blocked | Watch the lowest drain while flushing | Yes |
| All drains slow, smell in basement | Main sewer line blockage or break | Check the cleanout area | Yes |
| Clogs return every few months | Roots, sagging pipe or grease buildup | Note how often it happens | Yes, camera inspection |
Branch line vs main line
Each fixture drains into a branch pipe, and branches join the main line that leaves the house. A clog in a branch affects one fixture or one room. A clog in the main line affects everything downstream, which is why the lowest fixtures (basement drains, first-floor tubs) back up first.
What you can safely try
- A hand snake through the fixture or the trap arm for a single slow drain.
- Stop running water in the house if water is backing up into tubs or floor drains.
- Avoid chemical cleaners for deep clogs; they rarely reach and can make the plumber’s job dangerous.
What a plumber does
A plumber clears main line clogs with a powered auger through the cleanout, and can run a camera to see roots, breaks or belly (sagging) sections. Recurring clogs are worth a camera inspection before paying for repeated cleanings.
When to call a professional
- More than one drain is slow or backs up.
- Sewage appears in a tub, shower or floor drain.
- Clogs keep coming back.
- You notice wet spots or sinkholes in the yard along the sewer line.
What it usually costs
HomeGuide (2026) lists sewer line cleaning at about $150 to $800 and a sewer camera inspection at about $125 to $500.
Frequently asked questions
How do I know if my main sewer line is clogged?
Several drains slow at once, gurgling, and water backing up into the lowest drains when you flush are the classic signs.
Can I snake a main line myself?
Renting a powered auger is possible, but it can damage pipes or get stuck. For main lines, most homeowners are better off with a plumber.
Related problems
- Kitchen sink clogged
- Shower drain clogged
- Sewer smell in the basement
- Toilet won’t flush all the way
Sources
- HomeGuide: Plumber cost per hour (2026)
- HomeGuide: Plumbing inspection cost (2026)
- Oatey: Sewer gas odor in your house and what to do about it
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.