I have plumbed a 2 bed bungalow, leaving out 1 1/2 wastes under concrete floor for shower and basin as part of first fix. I am now back on site for 2nd fix. The job is in the countryside so all waste goes to a septic tank. The builder ran all outside wastes which includes a 4 inch sewer pipe under footpath to tank. He didnt however run up a vent pipe over eaves. To save breaking up footpath could I run up 1 1/2 vent using a strap on boss to main 4 inch. in ground