Tapping into Drain and Supply Lines
Learn how to tie new plumbing fixtures into existing drain and supply lines when remodeling.
For most modest to moderate remodel jobs, you will tap into an existing drain/vent line rather than run new pipes all the way from the main drain and out through the roof. Consult with a plumber or with your building department to make sure that your new project will be drained and vented properly. The following pages show some of the main methods for tying into existing lines.
T fittings are commonly used to tap into an existing drain and vent pipe. If the new plumbing will use smaller pipes than the existing plumbing, you can use a reducer T or install a reducer bushing in the T. Measure for cutting into the pipe carefully, so the new lines will be at the correct heights and can be sloped correctly. An important consideration is whether there is sufficient leeway for the existing pipe to be moved up or down—or side to side—to permit you to install the T. If it is solidly in position, you may need to use flexible fittings. Flexible fittings are also often the best solution when changing materials—for example, when tapping into steel pipe with PVC.
Working in tight quarters
Sometimes you need to tie into a pipe that is in a tight spot, as in this case where a new drain line must be installed for a whirlpool tub. Here you may have a choice: tear open a wall, floor, or ceiling to give yourself plenty of room, or work more surgically.
Tapping into supply lines
When supplying water to a new fixture, you need to tap into the existing pipe, usually with a T fitting. Don’t forget to shut off water to the existing pipes before cutting or unscrewing them to tap in. If the pipes are movable after cutting, you can often simply install a T fitting. But if they are solidly in place, you will need to use repair couplings as well.
If you are transitioning from galvanized pipe to copper, codes often require that you install a dielectric union, which has plastic innards that prevent ionization that can corrode the joint.