Every fixture that has a trap MUST have a vent.

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how does he goes about reventing to handle the kitchen tap in

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He may run a vent to the attic and connect to a existing roof vent or if AAV's are allowed in your code he could install a automatic vent just off the trap. (see image) Good, Luck, Tom

Thanks for your reply above. He installed the AAV in the kitchen and we are still having the same issues. Since everything drains into one main line in the basement he wants to know if he can simply tap into this main line with a vent line to the outside of the house? Everything (kitchen, shower, laundry, toilet) are situated back to back within 10 ft of each other and all drain into this main line. If so, he is thinking a 2" vent line, is this big enough? Thanks, Lauren