Toilet and sinks suddenly lose water and pull air at the same time
Several fixtures on one addition’s wastewater branch simultaneously pulled air and lost water for roughly half a minute, while fixtures on the original house branch appeared unaffected.
What was observed
- Several fixtures changed at the same moment.
- The event was limited to one branch/addition.
- No fixture on that branch was reportedly in use.
- The event had happened more than once.
Most defensible interpretation
A venting failure or adverse pressure event affecting that branch is plausible. Because a downstream restriction can also create unusual pressure behavior, the vent should not be declared the cause without checking the drain path.
Differential diagnosis
Blocked or undersized vent
A branch without adequate air movement can lose trap seals under negative pressure.
How to check: Inspect vent design and condition using safe, professional access.Downstream drain restriction
A partial restriction can change pressure as a slug of water moves through the branch.
How to check: Check cleanouts and flow conditions before assuming a roof-only blockage.External sewer pressure event
Municipal work or surcharge may affect building drainage.
How to check: Ask whether neighbors observed the same event and contact the utility if patterns align.Do next
- Stop treating the symptom as a single-fixture clog.
- Ask for a branch-level vent and drainage evaluation.
- Refill affected traps after the event and monitor for odor.
Do not
- Do not send an untrained person onto the roof with an auger.
- Do not conclude “main sewer replacement” from pressure symptoms alone.
Evidence ladder
Plumbing vent-system definition and trap protection ↗
International Code CouncilA vent system protects trap seals from siphonage and backpressure.
Chapter 9 — Vents ↗
International Code CouncilDrainage venting limits positive and negative pressure at fixture traps.
Original question and community answers ↗
DIY Stack Exchange · 2021-04-01Top answers raised vent blockage/design; another report described a downstream clog, supporting a differential diagnosis.
What this analysis cannot establish
- The event was not observed by an inspector.
- No camera, vent sizing, or cleanout findings were available.
- Both venting and drainage restrictions remain viable.