Catch a Leaking Drain or Vent Before the Walls Close Up With a 24-Hour Fill Test

The Problem

Every drain line and vent stack in a house is a joint that could leak, and once drywall goes up, those joints are invisible. A slow leak inside a wall can run for years before it shows up as an odor, a stain, or rot — by which point tracing it back to the source means opening up finished walls.

The Technique

1. Before the plumber leaves the job — new construction or a reno where drain lines were opened up — ask them to fill the entire drain and vent system with water up high, through the highest accessible point (a vent stack or an open fixture drain).
2. Mark or note the water level once it's filled.
3. Let it sit for 24 hours.
4. Check the level again the next day. No change means every joint held. A drop in the water level means a leak somewhere in the system — better to find it now than after the walls are closed.

On new construction, this test is typically mandatory as part of the plumbing inspection. On a renovation, it's easy for a plumber to skip since it's not always required — so it's worth asking for directly rather than assuming it happened.