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Bath tubs can settle just as a house can settle. This settling causes unevenness over time. When a bath tub is uneven it does not drain correctly and the plumbing fixtures may become loose causing leaks. Eventually the bath tub will need to be leveled back to as close to original position as possible. There are two methods, depending on whether you have a footed tub or solid base.
Lay the board with the larger side vertical across the length of the bathtub. Set the carpenter's level on the board at the center. Inspect the bubble in the level. If the bubble is centered between two red lines the tub is level. If the bubble is positioned more to one side, where the line passes through the bubble, this is the side of the tub that must be raised. Make notes about which side the bubble is on and how much of the bubble is passed the line. The greater the amount passed the line, the more uneven the tub is.
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Gather readings at various points around the tub to obtain an accurate observation of how uneven the tub is. Use the four corners, sides, front and back to get these observations. If any sides of the tub are higher than other sides use the bottom of the tub to obtain readings.
Use the readings to determine the exact point where the tub must be raised.
Set the level on the location where the tub is uneven.
Have someone lift up the tub at the corner needing to be raised.
Insert wood shims underneath the foot of the tub. Continue building support underneath the foot until the tub is leveled according to the reading of the bubble on the carpenter's level. Repeat this step if another corner also needs to be raised to complete the leveling.
Purchase premixed mortar in a quantity that will bring the floor level underneath the tub. The mortar must be thick enough to place the tub into position and then hold it.
Install the mortar using the manufacturer's directions to prevent cracking or damage to the floor beneath it. Allow the mortar a short time to settle, but not long enough to become permanently set. The mortar must be pliable enough to allow some give to the tub when the tub is set upon it.
Set the bath tub into position so the higher side of the tub sinks deeper into the mortar. Set the level on all sides to ensure evenness. Adjust as needed doing so quickly but carefully.
Let the bath tub sit on the mortar long enough for the mortar to permanently set.