01In short
Surface clogs like hair in a tub trap clear easily. When a clog comes back, the usual culprits are deeper: grease building up in a kitchen line or shared stack, scale narrowing old cast-iron pipe, roots in the sewer line, a sag in the line where debris settles, or a drain that wasn't fully cleared last time.
- Cold water
- Hot water
- Drain / waste / sewer
- Vent
- 1Trap: holds a water seal that blocks sewer gas
- 2Branch drain: needs a steady slope toward the stack
- 3Stack: carries waste down to the building drain
- 4Vents: let air in so draining water doesn't siphon the trap seals
- 5Cleanout: access point for clearing a clog
Simplified, not to scale.
02Common signs
- The same drain clogs every few weeks
- Several drains slow at once
- Gurgling from other fixtures
- Kitchen sink backs up when neighbors use water
- Slow drains on the lowest floor
03Possible causes
| Cause | Typical location | Lasting fix |
|---|---|---|
| Grease buildup | Kitchen lines and shared kitchen stacks | Hydro jetting; stop pouring fats down the drain |
| Scale in cast iron | Prewar buildings and older houses | Descaling; replacing badly narrowed sections |
| Roots | Building sewer outside | Root cutting; camera inspection; repair at entry points |
| Sagging line ("belly") | Under floors or yards | Repair or re-slope the section |
| Partial clearing last time | Anywhere | Full cleaning to the main line, then camera check |
| Venting problem | Plumbing vents | Clear or repair the vent |
04What you can safely check
- Which fixtures are affected, and in what order they slow
- Whether it's worse after heavy use or after rain
- Clean hair from tub and sink stoppers
- Keep using chemical drain cleaners: they rarely fix the cause and can damage old pipes
- Pour grease down the kitchen sink
05When to call a plumber
- The same drain clogs more than once or twice
- Several drains are slow
- You suspect the main line or building stack
A plumber clears the line fully, runs a camera to find the cause, and recommends jetting, descaling or repair. See drain cleaning and camera inspection.
06By building type
- Apartment buildings
- Shared kitchen stacks collect grease; buildings often schedule regular jetting.
- Brownstones
- Scale in cast iron and clogs at the house trap.
- Suburban houses
- Roots in the sewer lateral.
07Local considerations
Older housing is common across NYC and the inner NJ suburbs, so cast-iron scale and grease in shared stacks are frequent causes. In Suffolk and other areas with septic systems, slow drains everywhere may mean a full system.
08Frequently asked questions
Is hydro jetting worth it?
For grease and scale, jetting cleans the pipe walls instead of just punching a hole through the clog, so it lasts longer. A camera check first confirms the pipe can handle it.
Who pays for a stack that keeps clogging in my co-op?
Usually the building, but your proprietary lease decides.
09Related
Services that fix this
Related problems & guides
Sources & review
Last reviewed . General information, not legal or engineering advice. Rules change and individual buildings differ.