Plumbing Vent TrapsAuthor: Pacific Northwest Inspections Group, LLC Date: 25-Nov-2013. Category: Plumbing Add to Favorites
A trap - the bend in the drain below every plumbing fixture - creates a water seal that blocks sewer gases from escaping into the house. A vent prevents the trap water from siphoning away. Vents maintain equal air pressure on both sides of the trap seal. This keeps negative pressure from building up so that a slug of water running down the drain doesn’t pull the trap water out with it.
- Trap Rules The following guidelines apply to all fixture traps:
- No fixture trap outlet may be larger in diameter than the drain to which it is connected.
- No plumbing fixture can be double trapped.
- A slip joint is allowed anywhere on the inlet side of the trap. On the outlet side, the slip joint may not be allowed above the top of the water seal. In any case, any trap made with slip joints must be accessible through a minimum 12x12-in. opening.
- Each fixture trap must have a water seal that is at least 2 in. deep and not more than 4 in. deep.
- Most codes require a horizontal leg on the discharge side of the trap that’s at least two pipe diameters long. This leg must be sloped 1/4 in. per foot from the top of the water seal toward the vertical discharge. Code limits the horizontal distance before the trap arm tees into a vent.
- Most codes limit the length of the vertical drop from the fixture inlet to the trap. For sinks, lavs, showers, and bathtubs, the maximum drop is typically 18 to 24 in.; check with local code.