Short definition
A stack vent is the extension of a drainage stack above the highest connected horizontal drainage branch — the part of the stack where no water flows under normal use. It runs upward from there through the building and exits the roof, where it admits atmospheric air to the DWV system and releases sewer gas above doors, windows, and HVAC intakes.
What it is
Below the highest fixture, the drainage stack carries water and air. Above the highest fixture, that same pipe carries only air — and at that point it’s the stack vent. In typical US single-family construction, the stack vent and the visible roof vent are the same continuous run of pipe; you can usually see the upper end as a 2- to 4-inch pipe sticking up through the roof shingles, capped with a vent cowl.
UPC 906.2 requires the terminal to extend at least 6 inches above the roof — 12 inches in snow regions. Some Washington jurisdictions amend that for high-snow areas.
Why it matters to a homeowner
Most homeowners encounter the term during a re-roof or a remodel:
- “Extend the stack vent during the re-roof” — making the rooftop terminal taller for code or snow compliance.
- “Stack vent terminal too low” — an inspector flag during permit final; needs an extension piece.
- Frost closure on a cold morning — the stack vent is narrowing internally with condensation freeze, common east of the Cascades.
Common variants / not the same as
- Stack vent vs. vent stack. Stack vent is the upper extension of the drainage stack. Vent stack is a dedicated dry vent run paralleling a drainage stack — used in tall buildings and some commercial work.
- Stack vent vs. roof vent. Roof vent is specifically the visible above-roof terminal. Stack vent is the entire pipe between the highest fixture and the roof.
- Stack vent vs. branch vent. Branch vents tie into the stack vent or vent stack inside the building. The stack vent goes to the roof.