How to Install a Bathtub Concrete Base

Measure the floor underneath the tub for length and width. Cut the 1/4-inch plywood underlayment to fit the floor under the tub. Cut a hole in the underlayment to match the one in the floor for the tub drain. Nail the underlayment to the subfloor with ringed floor nails placed 6 inches apart along the edges and in rows across the middle.

Prepare the space for the tub according the manufacturer's directions. Each manufacturer has different requirements. Set the tub in place to check the fit, but do not step into it. Mark the tub feet on the underlayment when it is in place. The drain kit must be connected and ready for attachment to the tub. The tub framing should be complete and adequately support the tub rim.

Measure the minimum amount of water required to mix one bag of concrete according to the directions. Pour the water into the mixing tub. Open the bag of concrete and pour it into the tub.

Mix the concrete thoroughly with the square shovel. An elastic consistency similar to peanut butter or cake frosting is desired. Add 1/2 cup of water to the mix if it is too dry. A ball of concrete made in your hand will hold together by itself when the mix is correct.

Pour the concrete mix onto the plywood underlayment and spread it from side to side, but not into the drain access or where the tub feet will be. Keep it in the middle where it will support the bottom of the tub.

Set the tub into the opening and push it down with your hands. Use the level on the tub rim to help level the tub from side to side and front to back. Take the tub out if necessary to move the concrete, but put it back immediately. Secure the tub to the framing with clips or nails as directed by the instructions.

Allow the concrete 24 hours to cure and harden. Do not step into the tub or attempt to move it once it is settled into the concrete.