Traps

• The drain trap on a sink, shower, tub or washing machine holds water at all times. The water in the trap prevents sewer gases from escaping into the house.
  
  

Trap Location

• Toilets incorporate the trap into their design. Other fixtures, such as sinks and tubs, put the trap under the drain. The trap is the U- or P-shaped piping under a sink or other fixture.
  
  

Venting

• The trap lies between the vent and the fixture. Most vents must be installed within 10 feet of the trap. Vents generally utilize 2- or 1 1/2-inch PVC for the pipe. The main vent stack is usually 3- or 4-inch PVC.
  
  

Vent Operation

• Vents allow water to drain freely, similar to dipping a straw into water, holding your finger over the end and taking it out. The liquid stays in the straw until you remove your finger. You vented the straw.
  
  

Wet Venting

• A drainpipe run into the vent of another fixture is wet venting. This is allowed in some cases, but if used incorrectly it will cause problems, such as pulling all the water out of a trap, allowing sewer gases to escape into the house.
  
  

Main Vent

• Houses usually have one main vertical pipe called a sewer stack. All the drainpipes and vent pipes run into this pipe, which exits the house through the roof and exhausts sewer gases.