Smoke at an open roof vent can be expected. Unexpected smoke indoors or through the ground can identify a path from the drain system, but the escape point does not establish the exact defect. Fill traps, document the result and have the crew confirm the source before repairing it. A negative test can miss hidden leaks.
Roof vent: normal
A drain system is designed to breathe through its vents. When smoke goes in, it should come out the roof vent stacks. If no smoke shows at a vent, that stack may be blocked, capped or disconnected below, which is worth looking into. See what a plumbing vent is.
Unexpected escape points: investigate the source
Use this quick map.
| Where smoke appears | Possible cause to confirm |
|---|---|
| A floor drain or sink | Dry trap. Fill it and retest |
| Around a toilet base | Failed seal or flange |
| Wall or ceiling cavity | Cracked pipe or bad joint |
| Mechanical room or crawl space | Open cleanout, broken vent or cut pipe |
| Yard, planter or parking lot | Cracked or broken lateral. RJN Group says smoke in a yard "could mean a drain on the property is connected to the sewer lateral or that the lateral has breaks or cracks in the pipe" |
| Where two suites meet | Shared-line or wall defect |
Rule out the easy one first
Dry traps produce smoke, but they are also easy. Fill them, wait, and retest. Smoke that still appears with water in the trap points to something worse. Superior Signal calls smoke indoors after proper preparation "an open fault, where sewer gas has been entering the home for months or years," and advises ventilating and calling a licensed plumber.
What to do
- 1Ventilate the area and keep people out if they have breathing conditions.
- 2Mark the spot, photograph it and note the time.
- 3Have the plumber confirm the path and exact cause before opening finishes or selecting a repair.
- 4Retest after the repair so you know the gas path is closed.
What it means for you
A smoke finding documents an air path under the test conditions. It does not by itself prove who caused a defect or who is legally responsible. Written records of the finding and any confirmed repair can help resolve a maintenance dispute, and tenant vs landlord responsibility is often decided by that record. If nothing shows at all, when a smoke test is the right tool explains what to try next.
For a site-specific test plan, see HQ's plumbing smoke-testing service.