I had a Bathtub installed and they said for the warranty it had to be level. However, now I see the floor is not level and there is a 1/4" gap between the floor and the bottom of the tub on one end of the tub toward the center of the house. How do I bring the floor up to level to keep water from getting from the floor when it is spilled from seeping under the tub and getting the subfloor wet.