Can a plumbing Vent Terminate in The Attic?

Plumbing traps are protected by plumbing vents. They don’t speed up the draining process; in fact, they have the opposite effect. Plumbing vents appear to be one of the least understood aspects of a home by first-time home buyers. These are the pipes that protrude from the roof and run through the attic and the rest of the house. A plumbing vent is required for all residential plumbing fixtures. Inside the attic, vents are frequently linked together, resulting in fewer roof penetrations.



Plumbing vents prevent traps from being siphoned, to repeat. They also prevent trap back-pressure, but the focus today is on siphoning. It’s a common misconception that properly vented plumbing fixtures drain faster, but this is not the case. An incorrect analogy is to compare it to dumping a soda bottle upside down. You watch the water glug out as air replaces it, causing it to slowly drain. Water drains quickly once a hole is drilled in the top because air can replace the water as it drains.


Because the top side of every plumbing fixture is wide open, this analogy falls flat. A toilet’s top is exposed. A sink’s top is exposed. The bathtub’s top is open. To re-create the soda bottle analogy, block off the top of the plumbing fixture and then try to drain the water. I can’t think of any scenario in which this could occur.


Every plumbing fixture has a trap, which prevents sewer gas from entering the building, as I mentioned in last week’s blog post. When a large amount of water drains through a plumbing fixture, it can cause a siphon effect, which can pull water straight out of the plumbing trap. I included a quick video clip of water being siphoned out of an unvented drain in my blog about S-traps, leaving the trap with far less water than it should have had.


I created a video to demonstrate the difference between a vented drain and an unvented drain. For simplicity, I used clear tubing, but the physics are the same. The unvented drain drained about 2 seconds faster than the vented drain. This is due to the fact that the water that had left the fixture was assisting in the removal of water. There is no pull with a vented fixture. Instead of water, the vent allows air to be drawn in.


When you hear a gurgling noise after water has been drained from a fixture, it’s air being sucked through the trap. This occurs when there is no vent, the vent is obstructed, or the vent is installed incorrectly. I’ll talk about air admittance valves in next week’s blog post; they’re devices that replace individual fixture vents without having to run pipes through the roof.


If you feel overwhelmed and need help with your Plumbing Repair, give us a call.


Smart Living Home Repair Services

244 Madison Avenue , #1019

New York, NY 10016

(888) 758-9103

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