Plumbing Vent Smoke Testing for Commercial Roofs

A plumbing vent does two jobs. It lets air in behind flowing water so drains run freely, and it carries sewer gas up and out through the roof. The code puts it plainly: vents must keep every trap seal from seeing a pressure difference of more than 1 inch of water column. When a vent cracks, comes apart in a ceiling or gets blocked, the system cannot do either job, and gas can find its way into a room.

Sewer gas contains hydrogen sulfide, which smells like rotten eggs and which ATSDR says people can smell at very low concentrations. A vent leak is a classic reason a building smells but every trap tests fine. See what a plumbing vent is and a blocked vent causing sewer smell.

We use liquid-smoke equipment that forces air into the vent system, not a smoke bomb with a shop vacuum. The smoke fills the network and shows its path. We watch every vent terminal on the roof and work the interior.

What we look for, in order:

1. 1Smoke at every roof terminal. If one vent shows no smoke, that run points to a blockage, a disconnect or a cap.
2. 2Smoke at roof penetrations. Smoke at the flashing, or at the base of a stack, shows a leaking seal where gas can enter an attic or ceiling.
3. 3Smoke in the building. In a ceiling, a plumbing chase, a wall or a mechanical space, which marks a crack, open joint or bad connection in the vent or drain.
4. 4Smoke around a fixture. Often a failed seal or an open trap. Drains are filled with water first, so a trap that still shows smoke is a real leak.
5. 5Smoke drifting toward an intake. The code does not allow a vent terminal within 10 feet horizontally of a door, openable window or air intake unless it is 3 feet above it. A vent too close to one can bring odor inside with no leak anywhere.

The code's own smoke test for new plumbing closes the roof stack openings once smoke reaches them and holds a small pressure for 15 minutes. That is how a new system is proved tight. For an existing building with a smell, we are diagnosing, so we watch the whole system for any smoke that does not belong.

Large commercial roofs hold many vent penetrations, and other trades can disturb them. Rooftop equipment work, roof repairs, patching and tenant improvements are all chances to crack, cap or disconnect a vent. A restaurant adds kitchen vents and an interceptor vent. A multifamily building has stacks serving many units, so one failure can cause smells in several. Because vent piping often runs through ceilings and chases you cannot see, smoke is the practical way to find the one bad joint.

The roof visit is part of the test, so we need roof access and a clear route. Tell us before we arrive about hatches, ladders and any roof area your landlord or roofing warranty restricts. If the roof is under warranty, tell your roofer so a test does not surprise anyone.

If the drains gurgle and run slowly, the problem is probably a stoppage, not a vent. If only one fixture smells, start with its trap. If a vent terminal is close to an air intake, the fix is to move or raise the vent, and smoke simply confirms the problem. Plumbing is our work, not HVAC, so if the odor travels in ductwork with no vent leak, that is an HVAC question. For the right test for your situation, see when you need a smoke test for a sewer smell and what it means when smoke shows up somewhere unexpected.

Call with the building type, where the smell is and when it shows up. HQ offers 24/7 emergency service, free estimates and an upfront price before any work starts. See smoke testing, commercial sewer smoke testing and commercial plumbing. If a vent is damaged, a camera inspection can check the drain below it.