How to Install Vent Loop Plumbing

Mark the location of the vent loop's two pipes on the floor or on the bottom of the island cabinet with a pencil; the pipes must sit parallel with the outlet of the sink's P-trap. Space the pipes as far apart as possible. Attach a hole saw to the power drill. Drill holes through the cabinet, floor and subfloor at the marks.

Measure the distance from the cabinet's bottom or floor to the underside of the counter. Add the distance that the pipes must extend below the floor to this measurement. If the pipes will attach to an existing drain line, the overall dimension equals the distance from the center of the existing pipes to the underside of the counter. If the vent loop is part of a new drain line, the dimension equals the distance from the underside of the counter to the desired position of the main drain line, usually at the center of floor joists. This dimension is the overall height of the vent loop.

Cut 1 1/2-inch drainpipe with a hacksaw and dry-fit an upside-down U-shaped assembly with two 90-degree elbows. The overall height of the U-shaped loop should equal the predetermined height of the vent loop, and the space between pipes conforms to the width between the pipe location marks on the floor.

Insert the open ends of the pipe assembly through the holes in the floor and raise the assembly to its desired position. While holding the assembly in position, measure the distance from the P-trap's outlet to the center of the closest pipe and mark a level line on the pipe's outer wall to indicate the intersection of the P-trap's extension pipe. Remove the assembly from the holes.

Use a hacksaw to cut a piece of pipe to the measured length between the trap's outlet and the vent loop pipe. Cut the loop assembly at the mark to accommodate a sanitary tee fitting. Dry-fit a sanitary tee to the loop assembly and dry-fit a P-trap extension pipe to the sanitary tee. Place the assembly back into position and dry-fit the extension pipe to the P-trap. If everything fits, remove the assembly from the cabinet or floor.

Dismantle the assembly and permanently reassemble it with solvent cement. Replace the assembly in its installation location and join the P-trap extension pipe to the P-trap with solvent cement. Anchor the loop's top pipe to the underside of the counter with plumber's tape, a power drill and screws.

Dry-fit sanitary tees to the open ends of the loop that extend below the floor. Measure the distance between the tees, and use a hacksaw to cut a pipe that runs between and joins the tees. Dry-fit the pipes to the tees. Dismantle the tees and pipe, and reassemble them with solvent cement. Run a pipe from the remaining open tee end to the nearest main drain branch.