01In short
Every drain has a trap that holds a little water to block sewer gas. When a floor drain or unused fixture dries out, gas comes straight through. Other causes are a loose cleanout cap, a failed vent, a cracked cast-iron pipe, or, in older NYC houses, the fresh-air inlet that serves the house trap.
02Common signs
- Smell strongest near the floor drain or a rarely used fixture
- Smell after a long dry spell or after a vacation
- Smell near the front wall of the cellar (house trap area)
- Smell with gurgling drains
- Damp or stained spots on cast-iron pipes
03Possible causes
| Cause | Clue | Fix |
|---|---|---|
| Dry trap | Floor drain or unused fixture | Refill with water; trap primer or seal |
| Loose or missing cleanout cap | Smell near a cleanout | Replace or tighten the cap |
| Fresh-air inlet / house trap | Smell near the front wall or sidewalk vent | Inspect and service the house trap and inlet |
| Vent problem | Smell plus gurgling drains | Clear or repair the vent |
| Cracked pipe | Damp, rusty cast iron; smell persists | Repair or replace the section |
| Failed toilet seal | Smell around a basement toilet | Reset with a new seal |
A loose or damaged cleanout cap can let sewer gas into the cellar.
04What you can safely check
- Refill all floor drains and unused fixtures with water
- Check cleanout caps are tight
- Look for damp or rusty spots on drain pipes
- Ignore a smell that persists after refilling traps
- Seal a floor drain permanently without asking a plumber: it may be needed for the water heater or flooding
05When to call a plumber
- The smell persists after refilling traps
- You see damp or cracked pipe
- There's gurgling or slow drainage as well
A plumber can smoke-test the drain system to find leaks, service the house trap and vents, and repair cracked pipe. See drain cleaning and pipe repair.
06By building type
- Brownstones & older houses
- House traps and fresh-air inlets are common sources.
- Two-family houses
- Basement floor drains often dry out.
- Suburban houses
- Unused basement bathrooms and laundry drains.
07Local considerations
Many older NYC houses have a house trap with a fresh-air inlet near the front wall; it can smell when the trap is clogged or the inlet is damaged. See the brownstone guide.
08Frequently asked questions
Is sewer gas dangerous?
Strong, persistent sewer odors should be fixed promptly; ventilate the area. If you smell gas from appliances, leave and call the utility.
Why does it smell more in summer?
Traps dry out faster in warm weather, especially rarely used floor drains.
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.