Updated November 2, 2026
How it works
- The trap. Its water seal stops sewer gas and rodents coming in from the street sewer.
- The two plugs on top. Cleanouts on the house side and the sewer side of the trap, for clearing the trap and the line past it.
- The fresh-air inlet. Air enters here, runs through the building drain and leaves through the vent stacks on the roof. That flow keeps the drain ventilated and the trap seals steady.
What the NYC Plumbing Code requires
| Part | NYC Plumbing Code |
|---|---|
| House trap | Required on every building drain (703.7). Near the foundation wall, inside the street line, on the sewer side of every connection except a sewage ejector, an oil separator or a leader on a combined system. Same size as the building drain (1002.6). |
| Access | Outside the foundation wall or below the cellar floor: a manhole with a cover, or two handhold extensions with cleanouts at the cellar floor or grade, no more than 18 inches above the centerline of the drain (1002.6). |
| Fresh-air inlet | Connected within 4 feet upstream of the trap and carried to the outside air. Open end at least 6 inches above grade, with a perforated metal plate or a return bend, inside the street line. At least half the diameter of the building drain and not less than 3 inches (703.7.1). |
What you see, and what it usually means
| What you see | What it usually means | What to do |
|---|---|---|
| Water backing up at the cellar floor drain or the lowest fixture; every drain slow | A stoppage at the house trap or in the sewer past it | Stop running water and call. Do not open a trap plug on a full line: it empties the line onto the cellar floor. |
| Water or sewage at the fresh-air inlet outside | The drain is full to grade, blocked at the trap or past it | Same. Clear it, then camera the line to the street. |
| Sewer smell in the cellar near the trap | A missing or loose plug, a lost trap seal, a cracked trap | New plug and gasket, check the trap, camera if it comes back. |
| Air or a whiff at the inlet outside while a lot of water goes down | Air moving out of the building drain | Normal now and then. A steady smell, or one with nothing running, needs a look. |
| Rodents in the cellar | An open plug, a broken trap or a broken drain | Plug, repair, camera. |
| Inlet cap missing, buried or paved over | The drain has lost its air supply | Restore an open end at least 6 inches above grade, with a perforated plate or a return bend (703.7.1). |
How it is cleared and repaired
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Find where the stoppage is
In the trap, or past it in the house sewer.
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Clear it
Cable for roots and solids, hydro jetting for grease and scale.
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Camera the line to the street
A belly, roots at a joint, a broken hub, a collapsed section. You see the recording.
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Repair what failed
New plugs and gaskets, a new trap when the old one is cracked or will not hold a seal, a proper inlet termination. A break past the property line is street work with its own city permits. We tell you before it comes to that, with the camera recording in hand.
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Run water and show the result
You see the line drain.
Questions
Can the house trap be removed so it stops clogging?
No. The NYC Plumbing Code requires a house trap on every building drain (703.7). The fix is the cause: grease, roots past the trap, a belly in the sewer, a broken section.
Can I seal the fresh-air inlet to stop a smell?
No. The code wants it open to the air at least 6 inches above grade, with a perforated plate or a return bend (703.7.1). A sealed inlet starves the drain of air. A steady smell means the trap or the drain needs a look.
Where is my house trap?
Usually in the cellar near the front wall, where the drain leaves for the street, with two plugs on top. If it sits outside the wall or below the floor, it is in a manhole or has two extension pipes with cleanouts at the floor or grade (1002.6).
Does a house trap need upkeep?
Keep the plugs tight and the inlet clear. In a building with a restaurant, grease that gets past the interceptor collects at the trap. A camera inspection shows what is building up before it backs up.