I'm looking at the venting requirements for my tub, toilet, and bathroom sink. This is on a concrete slab and has a main vent at the end of the main sewer line. All of the fixtures are about 3 feet from the main sewer line. The fixtures are about 12 to 8 feet from the vent stack but it's going to be difficult to tie into the vent stack. Do I need to add an air admittance valve to each fixture? If so how in the world could I add one to the toilet? Thank you.