How the odor gets inside

Every building's drain system is vented through the roof. Those vent pipes exist to let sewer gas escape safely above the building and to keep traps from being siphoned dry. On a single-story strip center or flex building, the roof is crowded: vent stacks, exhaust fans, kitchen hoods, skylights and one or more packaged rooftop units, usually called RTUs, each serving a suite below.

An RTU brings in outside air through an intake hood or an economizer. If a vent stack sits close to that intake, or upwind of it under the right conditions, the unit can pull sewer gas straight into the supply ducts and deliver it into the suite. Tenants then smell sewer odor at the ceiling diffusers rather than near any drain, which sends people searching in the wrong place.

Snow can make it worse. A short vent stack can be partly buried in a drift on a flat roof, and wind patterns around parapets and taller neighboring buildings change which way vented gas drifts. Along the US 36 and I-25 corridors, strong westerly winds and occasional upslope easterly storms both occur, so the smell may track the weather rather than the time of day.

Is the rooftop a likely path?

Use what staff have already noticed to decide whether to look up rather than down.

  • IfThe smell is strongest at supply diffusers and fades when the fan is offSuspect something entering the air handler. Ask the HVAC contractor to inspect the intake area on the roof.
  • IfThe smell appears on windy days or with a particular wind directionNote wind direction on the odor log. Shifting gas plumes from nearby vents are a strong candidate.
  • IfThe smell started after a reroof, a new RTU, or a neighbor's kitchen hood installRooftop layout may have changed. Ask the landlord for any recent roof work records.
  • IfThe smell is strongest near a restroom drain and does not follow the fanReturn to the drain and trap investigation first; the roof is less likely.
  • IfMore than one suite smells it at the same timeTell the property manager. A shared vent or sewer issue affects the whole building, not just one tenant.

What a rooftop check should include

Roof access is usually controlled by the landlord. Let the building's HVAC or plumbing contractor do this, not staff.

  • Location of every vent stack relative to each RTU intake and economizer hood
  • Height of vent stacks above the roof surface, and whether any are damaged, capped or buried
  • Whether a stack was cut short or moved during reroofing
  • Condition of the RTU intake filters and screens, and any staining around the intake
  • Nearby exhaust from kitchen hoods, restroom fans or dryer vents that could also be pulled in
  • Photos with the RTU and stack in the same frame, labeled by suite

Common questions

Will closing the outside-air damper stop the smell?
It may reduce it, but commercial buildings are designed to bring in a minimum amount of outside air for occupants. Changing that setting should be a decision made with the HVAC contractor, not a permanent workaround.

General information for Westminster, not a quote or professional advice. In an emergency, call 911.