Vent Terminal Height, Frost Closure, and Location (IPC 903)
Three independent rules land on the same pipe, and satisfying two is not compliance.
Example
You enter
- Height above the roof (in) 6
- Anticipated snow accumulation (in) 18
- Roof used as a promenade, deck, or similar? no
- 97.5% outdoor design temperature (°F) -10
- Vent diameter through the roof (in) 2
- Size increase made this far inside the thermal envelope (in; 0 = none) 6
- Horizontal distance to the nearest door, openable window, or intake (ft) 4
- Height above that opening (ft) 1
You get
- Required height (in) 24
- Height deficit (in) 18
- Min diameter (in) 3
- Min increase depth (in) 12
Checked against published worked examples from: International Code Council.
Details, formula, and sources
ScopeHEIGHT (IRC P3103.1.1; IPC 903.1.1 lets the jurisdiction set it): 6 in above the roof or 6 in above the ANTICIPATED SNOW, whichever is greater -- 18 in of snow makes it 24, not 6 -- and 7 FT where the roof is used for anything but weather protection. FROST CLOSURE (903.2): where the 97.5% design temperature is 0 degF or less the extension must be at least 3 in, and the increaser must sit at least 1 ft INSIDE the thermal envelope, which is the part everyone misses -- a fitting made up in a cold attic frosts shut exactly like the small pipe it replaced. LOCATION (903.5): never directly beneath, and not within 10 ft horizontally of any door, openable window, or air intake -- of this OR AN ADJACENT building -- unless it is 3 ft above. Vent SIZE from fixture units is separate.
Formula or tablerequired height = 84 in where the roof is used as a promenade, deck or similar, otherwise max(roof minimum -- IRC 6 in, or the number the jurisdiction wrote into IPC 903.1.1 -- , snow accumulation + 6 in); frost closure applies when the 97.5% outdoor design temperature <= 0 degF, requiring diameter >= 3 in and any size increase made >= 12 in inside the thermal envelope; location passes when the terminal is >= 10 ft horizontally from a door, openable window, or air intake OR >= 3 ft above it.
Source and editionIRC P3103.1.1 (vent terminal height; IPC 2027 903.1.1, like 2021, leaves the number to the adopting jurisdiction), IPC 903.1.2 (7 ft for a roof used as a promenade, deck or similar), IPC 903.2 (frost closure), and IPC 903.5 (vent terminal location relative to doors, openable windows, and air intakes of the building or of an adjacent building).
Free accessAll three are short prescriptive provisions reproduced in state and municipal adoptions and code-viewer sites. No table is reproduced; the snow accumulation and the design temperature are local figures entered by the user.
What governsEstimate. AHJ and licensed professional govern.
API field namesheight_above_roof_in, snow_accumulation_in, roof_other_use, design_temp_f, vent_diameter_in, increase_inside_envelope_in, horizontal_to_opening_ft, height_above_opening_ft, required_height_in, height_deficit_in, min_diameter_in, min_increase_depth_in
- Height 6 in above the roof or 6 in above anticipated snow, whichever is greaterIRC P3103.1.1 (IPC 903.1.1: jurisdiction number, default 6)
- Roof in use 7 ft where the roof is used as a promenade, restaurant, bar, sunbathing or observation deck, or similarIPC 903.1.2
- Frost trigger 97.5% outdoor design temperature of 0 degF or lessIPC 903.2
- Frost sizing not less than 3 in, with the increase made not less than 1 ft inside the thermal envelopeIPC 903.2
- Location 10 ft horizontally, or 3 ft above, from a door, openable window, or air intake of this or an adjacent buildingIPC 903.5
- Not checked vent size from fixture units, the branch below, flashing, wall terminations, fuel-gas ventingstated scope limit