My shower has ceramic tile on both the walls and the floor. The space between is caulked. Over 8 years the drain has backed up and the water has seeped through the caulk and damaged ceilings below. The caulk now is crumbling and must be replaced. The space between the floor and wall is 1/2" deep. I was planning to put a 1/8" bead of silicone before putting in the caulk. Is there something I should put in behind for the other 1/4"? If so, what? Any other ideas on fixing this problem?