Mark the center of the new drain line onto the top of the floor joist that is in the way.
When you cut the drain hole in the floor where you are installing a new tub, you might discover that a floor joist is in the way, keeping you from assembling the tub drain line. It is necessary to avoid cutting a floor joist that a tub is installed over because of the weight of the tub when it's filled with water and a human. Installing a bathtub drain when a joist is in the way requires boring through the joist.
Measure the height of the tee from the top of the bathroom floor to the horizontal house drain line with a tape measure. Measure the distance from the house drain line tee to the new drain opening location for the tub. For each foot of distance between the tub drain and the house drain, there must be a quarter-inch of slope to allow proper water flow. For example, if the drain tee is 4 inches from the floor and the tub drain is a foot away, the bottom of the drain pipe at the tub should be 4¼ inches off the floor.
Mark the necessary slope measurement on the floor joist you have to go through for the bathtub drain. Measure up 1 inch from the mark. This is the center of the hole for the new drain line.
Drill a hole at the center mark of the floor joist with a 2-inch hole saw and an angle drill. An angle drill is one that has a chuck that bends to a 90 degree angle.
Slip the piece of PVC drain line through the hole to ensure it fits without obstruction. Once the hole is cut into the floor joist, the drain line is ready for assembly.