The waste vent runs from your main drain line to prevent back pressure in the main sewer line. This pipe can be interrupted at any point up to 4 feet from the vent shaft that generally exits your home from the roof or the side of the home. At these points you can cut and run secondary drain lines into the main shaft.
You will want to ensure a water tight seal to prevent leakage of raw sewer into your homes walls or sub walls and ensure proper waste drainage. You want to take extreme precautions to avoid objects or construction waste falling down the drain line as it can cause clogging and back water issues
Step 1 - Measure the Grade of the Pipes
Before you can insert your added drain lines into the main waste vent, you will have to first check the incline or grade on your pipes. A good first rule is that you do not want a line going into the waste vent any closer to the vent cap outside your home than 4 feet. This will prevent issues with drainage, it is recommended to have this amount of air distance between the drain systems, and the outside of your home. Once you have selected a location to insert your new drain line, the rest is down hill.
Step 2 - Cut into the vent Shaft
You will want to cut the waste vent pipe at the location where your new drain line is going to enter. When doing this you will want to cut 1/2 inch less out of the shaft to allow the two female ends of your t-bend to fit snugly into place. Use a liberal amount of sealant to secure the fitting over the top and bottom of the vent shaft to prevent it from future leaks when it is in use. Make sure that you incoming drain line also matches up to the connection before it dries, or you are going to have serious issue reversing the effects of it positioning.
Step 3 - Secure the new Drain Line
Once you have the t-bend in place and glued properly for a tight water seal into the existing waste vent, you can now run your new drain line into the side of the new outlet. Again, you want to ensure that you are creating a water tight seal to prevent leakage from the new joint going forward. Remember that your drain line coming in needs to have the proper trap in place on the sink, tub or shower end, to prevent fumes from coming back through this line into your home. You can clean excess glue away using ether, or even rubbing alcohol and a rag with little effort. Once everything is in place, go ahead and test the system for leakages, before you seal up walls and work spaces that would limit access to the pipes. It is best to simply let the water run and drain and check the joints for any visible problems in the new system.