I am adding a bathtub/shower to a half bath, which is on a concrete slab. I have done all of the demolition myself and am now ready to cut the 5-foot long trench in the concrete for the plumber to connect the tub drain to the sewer outlet. How wide should this trench be to allow for enough room for the plumber to work? (My floor cutout for the drain is 9-in X 12-in. I'm guessing the plumber will install a 3-inch drain pipe, right?