Does the peppermint oil test find a sewer leak?

It is an older scent-based troubleshooting method. A plumber introduces peppermint scent through suitable vent access and looks for unexpected odor inside, as described by Engineer Fix. The mixture and access procedure are not a universal code-approved recipe. The 2024 International Plumbing Code's drainage and vent final test, Section 312.5, describes a smoke test and does not describe a peppermint test (2024 IPC). Ask your building department what it accepts before you count on it for an inspection.

The hot water vaporizes the oil, and the scent travels the drain and vent pipes and leaks out where the pipes leak. A strong smell at one drain can mean a dry trap. A strong smell near a wall may mean a cracked pipe.

• Weak or unclear results. Engineer Fix says a strong smell suggests "a more extensive problem like a cracked pipe or a failed connection," but if the sewer smell continues after a negative test, the source is still unknown (Engineer Fix).
• Contaminated testers. The person who handles the oil will have it on their hands. Use a helper for the sniffing (House Digest).
• Outdoor leaks. A lateral leak in a yard or lot produces no smell you can catch inside.
• Big buildings. In a large building a strong scent can make a small leak hard to place.

A small building with one suspect vent, no roof access issues, and a plumber who wants a quick cheap check. For a commercial building with many drains and vents, smoke is easier to read because you see where it goes.

Do not spend a weekend on peppermint if the smell is costing you tenants. Fill traps, then call for a smoke test, or compare all three tools in smoke test vs camera vs peppermint. To see how smoke works, read what a plumbing smoke test finds.

For a site-specific test plan, see HQ's plumbing smoke-testing service.