Venting a drain system is an absolute necessity; vent pipes maintain pressure to allow siphonage and prevent sewer gas from entering a structure. While building codes and standard practice require plumbing vents, there are alternatives to routing vent pipes through roof penetrations. Although the alternatives are not applicable or permissible under all circumstances, an understanding of plumbing vents and roof vent alternatives can help you investigate your options and make the right decision.
Plumbing vents are part of a structure's drain, waste and vent (DWV) system. Vents connect to drain and waste lines, such as cast iron or plastic pipes, and typically extend upward through a structure's roof. Vent pipe sizes and locations are closely regulated by building authorities. In general, vent pipes must occur within a set number of feet from each plumbing fixture, and the diameter of the vent pipe generally corresponds to the size of the drain system or amount of fixtures.
Air Admittance Valves
Air admittance valves are the most common alternative to traditional roof vent pipes. Located near a plumbing fixture's P-trap, air admittance valves are suitable for installation beneath both kitchen and bathroom cabinets. Often attached directly to a drainpipe or drainpipe fitting via pipe glue or pipe threads, air admittance valves typically rise no more than 6 inches above the P-trap's finished height. Air admittance valves consist of a metal or plastic cylinder and a spring-like, pressure activated air valve. The valve remains closed until water rushes through the drain. As water passes through the drain, the valve briefly opens to admit air and encourage siphonage. Although considered effective, many local building codes restrict the use of air admittance valves.
The term "auxiliary vent" refers to a vent pipe that joins an existing vent stack rather than extending to the roof. During renovations or remodels, plumbers and do-it-yourself builders can install auxiliary vents in lieu of creating new penetrations through a structure's roof. Auxiliary vents typically join the drain line near a plumbing fixture's P-trap and extend to the nearest vent stack, which provides air to the auxiliary vent.
Standard Vent Alternative Installation
Common among renovations of old buildings, an alternative to routing a standard vent pipe through a roof is running the vent pipe along the structure's exterior wall. To accomplish this, plumbers simply route the pipe through the wall and anchor the pipe to the structure's exterior with straps or brackets. Although considered unsightly, exterior wall vents relieve plumber's of removing interior drywall and accessing a structure's attic. The termination point of the vent pipe varies according to local code; if the pipe extends a set distance above windows, some building authorities allow a pipe to end below a structure's eave line. Alternatively, some building codes require wall-mounted plumbing vents to extend above the structure's eave line.