The Purpose of Venting Drains

Soil Stack

• The primary component of a DWV's waste drain is its soil stack. The soil stack is a pipe that transfers the wastes from a house's plumbing system to the sewage system. Soil stacks are where all the drains in the house terminate in a vertical stack. Most soil stacks start in a house's basement or underneath the house and extend out the top of the house. The top of the soil stack has a roof vent for allowing sewer gases to escape from the house's plumbing system. Every appliance and sink in a house must be vented. As a result, houses typically have multiple roof vents.

Pipes

• The Uniform Plumbing Code (UPC) requires DWV drainpipes and vents to have minimum diameters. According to homebuilding expert Don Vandervort, the Code bases a DWV drainpipe's minimum diameter on the amount of fixture units in the plumbing system. A fixture unit is equal to 1 cubic foot of water drained in a 1.25-foot-diameter pipe. Proper drainpipe diameters prevent the system from leaking or clogging. Vents usually have smaller diameters than drainpipes. The UPC also says the DWV system's drain traps and vents must within a maximum distance of each other; this distance is known as critical distance. If vents and drain traps are too far from one another, sewer gas may get into the house.

Traps

• Plumbing traps are another component of a house's DWV system. Traps block sewage gases from entering the plumbing system. Without traps, gases have full access to a house's plumbing system and emit from drain openings in faucets and showers. One of the most common plumbing traps is the P-trap. P-traps are usually found underneath sinks and work by retaining a small amount of water after a sink's use. The small amount of water blocks gases. P-traps are also known as U-traps. Other types of plumbing traps are S-traps and J-traps; the names usually derive from the traps' shapes.

Faulty DWVs

• A common mistake made in a DWV system is faulty designs in the system's traps. Some traps are made too long for the DWV pipes' diameters and other traps do not have vents. Also, some homes have DWV systems with vents that end in the attic. A DWV vent pipe must end outdoors, where sewer gases are able to leave a house completely. When inspecting your DWV's soil stack, make sure debris does not plug its roof vents. This could result in gases being trapped within your house.