3.4 Vent Terminals, Frost Closure & Prohibited Vent Uses

Why Wisconsin is strict about vent terminals

Warm, moist air leaving a vent in a Wisconsin January condenses and freezes at the pipe opening. A small terminal can frost shut, and then every trap downstream loses its protection from the 1-inch pressure limit of SPS 382.31 (3). Wisconsin's terminal rules answer three questions: how high, how far from openings, and how big.

Height above the roof (SPS 382.31 (16) (a) and (b))

Every roof penetration must be made watertight with an approved flashing. The code doesn't specify flashing material, so any approved product that makes the penetration watertight complies.

Clearances (SPS 382.31 (16) (d))

Worked example. A vent must penetrate a flat roof 6 feet from an openable penthouse window whose top is 5 feet above the roof. It is inside the 10-foot horizontal zone, so the terminal must rise at least 2 feet above the opening, to 7 feet above the roof. Moving the penetration 10 feet or more from the window also complies, as long as the terminal still clears the 8-inch minimum height.

Notice the parapet rule: a vent close to a parapet doesn't need to be 5 feet away if it extends 2 inches above the top of the parapet wall.

Wall terminals and exterior piping (SPS 382.31 (16) (e) and (f))

• A vent must terminate at least 10 feet horizontally from any lot line.
• A vent extended through a wall must terminate beyond the soffit, turn downward and be screened. It must also meet all the clearance rules in (d).
• Drain or vent piping may not be placed on the outside of an exterior wall of any new building; it must be inside the building.

Frost closure (SPS 382.31 (16) (g))

Two numbers to remember: 2 inches minimum terminal and an increaser located at least 6 inches inside the building. A 1-1/2 inch stack vent from a small addition, for example, must be increased to 2 inches before it leaves the building. The increaser must be at least 6 inches inside, so the larger pipe passes through the roof structure. The ch. SPS 382 Appendix has explanatory material on this rule. In the code text, the measurement is simply "inside the building."

Worked example. A house's only stack vent is 3 inches (the main-stack minimum). It already exceeds 2 inches, so no increaser is needed. A separate 1-1/4 inch individual vent that goes through the roof on its own must be increased to 2 inches, with the increaser at least 6 inches inside the building. Tying that vent into the 3-inch stack vent instead avoids the second roof penetration entirely.

Penetrations through grade (SPS 382.31 (16) (h))

A vent that penetrates grade, such as from an exterior sump, must terminate at least 12 inches above finished grade with a vent cap or return bend. Earth-covered roofs are the exception and use the 7-foot rule above.

Prohibited uses (SPS 382.31 (16) (c) and (18))

• Vent terminals may not be used as flagpoles, antenna supports or similar purposes.
• A vent system may serve only the plumbing system.
• Boiler blowoff basin vents may not join a vent serving a sanitary, storm or chemical waste system.
• Chemical waste vents may not join a sanitary or storm vent system.
• Steam vents from sterilizers, degreasers, pressing machines and other equipment that discharges steam into the vent may not join sanitary, storm or chemical waste vents.

Exam strategy

Terminal questions usually give one measurement and ask whether it complies. Read the clearance table carefully. An air intake needs 10 feet with no "above" alternative. A window or door allows either 10 feet horizontally or 2 feet above. A power exhaust needs only 5 feet. And for frost closure the answer is always 2 inches and 6 inches inside the building. Wisconsin code does not use 3-inch frost-closure terminals or an "inside the thermal envelope" test.