Wanted to know how to install a "Saddle T" and primarily what they are used for. Do I need a gasket or do I simply use pvc glue and primer to make the seal? Would they work for installation of a new vent for a fixture? Any thoughts on using an "offset toilet flange versus a standard flange. Thanks for the help..

I am running a new PVC drain line for a bar sink in my basement. I have already figured out where to run the drain line so that it has the required pitch from my proposed sink location to one of the main (3") stacks in my house. My plan is to cut into that stack, insert a "y" for the new drain line, and then patch everything back together with PVC slip couplers. I know the height that the drain line is off the floor near the stack, and will need to insert a 90 or 45 to get it to hook onto the y into the 3" stack. I realize that I will also need a short piece of 1 1/2" PVC to fit between my 90/45 and the Y, and my problem is how to determine what length to make that piece. Is there an easy way to figure out how long to cut that filler piece, assuming I know the height of the drain pipe from the floor, and the distance that drain pipe is from my 3" stack? I think this involves a bit of geometry to figure things out, and I was hoping someone could point me in the right direction before I cut into that stack and make things unrepairable. Any ideas? Secondly, is this the best way to plumb in my bar sink? Any other suggestions would be appreciated. Thanks!