Drill a hole into a 1/8-inch thick piece of plywood that will allow you to slide the plywood sheet over the pipe so you can rest the sheet on the floor. Use a sheet at least 12 inches square.
Place the plywood sheet over the pipe and rest it on the floor. The sheet will protect the floor from saw marks left by the reciprocating saw blade.
Place the tip of the reciprocating saw blade against the side of the pipe. The blade on a reciprocating saw is approximately 12 inches long. You can flex the blade by pressing the blade against the floor and then by rotating the saw housing upward to position the blade as close to the floor as possible.
Pull the trigger lightly on the saw and slowly cut. As you pull the trigger, the reciprocating saw moves forward and backward across the surface of the pipe.