How to Install a Back Trap

"Back trap" is an old name for a type of plumbing technology that prevents wastewater from flowing back from the sewer into a building's drainage system in the event of a backup or flood. The modern version of the back trap is the backwater valve. These valves are often recommended or required for buildings that have drains below the nearest upstream manhole or overflow outlet of the public sewer line, and they may also be a good idea in flood-prone areas. Their installation requires some plumbing expertise.

Ruler
Normally open backwater valve corresponding to the specifications of your drainage system
Excavation or demolition equipment, if necessary
Cleaning rags
Marker or crayon
Pipe cutter or hacksaw
Pipe cement and primer (if necessary) corresponding to pipe material

Instructions

- 1
  Research the local ordinances in your area to determine whether a backwater valve is necessary and allowed. Investigate permit requirements and obtain any mandatory clearances for installation.
- 2
  Measure the diameter and determine the material of your main drainage pipe. Purchase a backwater valve that corresponds to these specifications.
- 3
  Determine the best location for the valve in the main drainage line. The valve should be installed downstream of the house drain sewer trap and upstream of the air vent opening. It should be easily accessible for inspection and clean out.
- 4
  Perform any necessary excavation or demolition to gain access to the main sewage line at the determined location.
- 5
  Clean the outside surface of the drainage line with rags.
- 6
  Align the valve above the pipe at its determined position along the line. Make sure that you orient the valve in the correct direction: the access cap should be on top, and there will usually be a drop in grade from the upstream end to the downstream end of the valve.
- 7
  Mark edge lines onto the drainage pipe at both ends of the valve, then remove the valve and mark lines 1 inch from each edge line toward the other edge line. These inner lines will be your cutting lines.
- 8
  Cut through the pipe at the two inner lines and discard the pipe segment. Clean the cut edges of the drainage line to remove any shavings from the cutting.
- 9
  Apply any necessary primer and the pipe cement to the ends of the pipe.
- 10
  Fit the valve onto the ends of the pipe, twisting it a few times to make sure that it is fully seated, and then orient it into its proper position, determined in Step 6. Allow the cement to dry and set before testing.

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