I recently relocated a shower in my master bath and now I need to tie the 2" drain into the 1 1/2" vent again. The vent stack is only 3 feet away but I can't go up above the subfloor anymore). So I only have 5 inches of space above the drain line to work with. Is 5 inches enough of a drop for a vent before I add a 90 and run it over to the original stack?