Skip to main content
King County logo

Approved materials

Waste and vent pipes and fittings:
Cast iron, galvanized steel, galvanized wrought iron, copper, brass, Schedule 40 ABS DWV and Schedule 40 PVC DWV are some commonly used approved materials. 

Before you buy materials

Drainage Fixture Unit Value Table (DFU):
Look at the DFU and figure out the size of pipe you will need for the project. Each type of fixture has a different value and minimum size for the waste and vent pipes.

Cleanouts (applies to first floor only):
A "cleanout" is an access to a pipe that will make it possible to clean out the pipe if it becomes blocked. Be sure to use a cleanout made of an appropriate material. You will also need a cleanout for each horizontal change of direction exceeding 135º.

Lay the pipe

Drainage and grading:
Sewage runs downhill. All your waste pipe must be graded at 1/4 inch per foot; otherwise you may have trouble getting the sewage out of your house.

Decide which fittings to use:
It is very important you use the correct plumbing fittings when installing your new system. Using the wrong fittings could cause sewage backups or other problems. See different types of fittings.

  • Elbows: Use a 90º vent elbow (short turn) for venting only above the flood level. Use a 90º medium elbow for wastes going from horizontal to vertical and for vents above the flood level. Use a long turn 90º elbow for directional changes of vertical to horizontal and horizontal to horizontal.
  • Tees: Use a sanitary tee in vertical position only for trap application and horizontal to vertical drop.
  • Wyes: Can be used on vertical and horizontal and in many applications except fixture connections. Also can be used as a cleanout.
  • Combination Tee-Wyes: Can be used in all applications except to a trap arm.
  • Traps: A "p-trap" with a union must remain accessible but a p-trap with a solvent weld may be concealed.


A trap is a fitting or device so designed and constructed as to provide, when properly vented, a liquid seal which will prevent the back passage of air without affecting the flow of sewage or waste water through it. Each fixture must be trapped to prevent sewer gas from coming up into your house. The weir of the trap must be below the opening to the vent. The trap seal shall be no less than 2" or more than 4" and installed true to the water seal.


The sizing of the vents shall be done using the fixture unit's table on the back of this insert. Vents must be graded or be level so there are no drops or sags. For vent termination, each vent pipe must extend through its flashing and end at least six inches above the roof. Vents may be connected together at least 6" above the highest fixture flood level.