The most common way to connect a new drain, vent, or supply line to an existing line is to install a tee fitting. To do so, shut off water to existing supply pipes and drain the lines. Flush all toilets and caution others not to use drains. After opening a drain line, make sure no one uses a sink or faucet that drains into it. Seal any open drain lines with a rag to block fumes. If joining pipes of different materials, make sure the transition fitting conforms to local code.
Typically it doesn't matter exactly where you join to an existing pipe, but the new service must be precisely located. So it's usually easier to start pipe runs at the new location and travel toward the existing pipes rather than vice versa.
It usually takes less than two hours to connect new to old once pipes are run. Prep for the job by running new pipes from the new service location to the existing pipe; installing the last pipe a little longer than it needs to be so you can cut it to length when you make the connection to the old pipe.
To join a new plastic drainpipe to an old steel pipe, run new pipe into the room. Prime and glue two 5-inch pieces of pipe to a tee fitting. Temporarily run pipe—longer than it needs to be—so it comes near the old pipe. (When running pipe across a stud wall, you may need to notch-cut some of the holes, using a reciprocating saw.) Dry-fit the tee assembly onto the new pipe and hold it next to the existing pipe. Mark the existing pipe for cutting. You may need to cut the opening larger than the tee assembly to accommodate the neoprene sleeves on the banded couplings.
Support the pipe above and below with riser clamps so it cannot drop or sway as you work, and so the PVC fitting will not have to bear the weight of the drain. You probably will need to install a new stud or two as well as blocking for the upper clamp.
Slide a no-hub fitting onto each end of the old pipe, slide back the banded coupling, and fold up the neoprene sleeves. Position the tee assembly. Fold the neoprene sleeves over the assembly and slide the metal bands over the sleeves. Tighten the nuts with a hex-head driver.
Cut the new pipe to exact length and test that it fits into the tee fitting; you may need to loosen the nuts and rotate the fitting slightly. Prime and glue the pipe to the fitting.
To tap into existing copper lines, shut off the water. With a tubing cutter cut an opening in each pipe that is about an inch shorter than a tee fitting. Dry-fit the tees. If the pipes are rigidly installed, remove a clamp or two so you can pull the pieces apart slightly.
Dry-fit the pipes that will be inserted into the tees and draw alignment marks. Disassemble, wire-brush the fittings and pipe ends, brush on flux, and sweat the joints.
Even though there are specialty primers and cements intended for joining PVC pipe to an old ABS line, local codes may not permit connecting in this manner. Instead, use a no-hub fitting, which has a neoprene sleeve and metal clamps, to hold it firm. Some municipalities may require that the fitting be accessible for future repairs.
To tie a new copper line into an old galvanized line, follow the steps for installing a new galvanized tee fitting. Screw a galvanized nipple into the tee and connect the copper line to the nipple using a dielectric union.