Building your own bathtub is not an inexpensive or easy process, but it can be done. Tubs can be built from wood, concrete or tile-covered masonry. Probably the easiest is to build a tub with concrete backer board and tile it. A homemade tub will generally cost more than a purchased tub, but can be shaped to fit an unusual space or made longer, taller or wider than standard tubs.
Lay out the size and shape of the tub and build a frame of 2-by-4s. Join the lumber using butt joints and at least 3 screws per joint. The minimum spacing of the framing lumber should be 16 inches. Install side walls at right angles to the floor, but angle the back of the tub so that is comfortable to lean against.
Rough-in the plumbing for the drain and faucet. Install two layers of 19/32-inch exterior grade waterproof plywood on the floor of the tub to provide adequate support. Offset joints at least 6 inches.
Cut a 4.5-inch hole in the plywood where the drain will go. Disassemble the drain and lay the drain base in the hole so the flange of the drain base rests on the plywood. Solvent weld the drain base to the drain pipe. Cover the drain base opening with duct tape to keep materials from falling into the drain.
Fasten concrete backer board to the frame using screws designed for concrete backer board. Space the screws every 6 inches. Cut and fit the backer board as tightly as possible to make it easier to seal the tub. Cover the joints with adhesive and immediately embed backer tape.
Apply Portland cement/sand mortar mix over the floor of the tub so that it slopes toward the drain at 1/4 inch per foot. The mortar bed should be flush with the top surface of the drain base.
Trowel a thick, smooth layer of waterproofing over the entire tub to a minimum of 2 inches above the overflow drain. Allow that to dry, and then add another coat. When the second coat is dry, place the clamping ring for the drain over the bolts and slide the ring counterclockwise until it is tight. Plug the drain; fill the tub with water and let it sit for at least four hours. Drain the tub and repair any leaks before proceeding.
Place pea gravel over the weep holes in the drain assembly so mortar does not completely block them. Determine the finished height of the floor of the tub and screw the drain barrel and strainer into the clamping ring so that the drain barrel will be flush with the bottom of the tub.
Lay a 1 1/2-inch bed of mortar on the tub floor. When it is dry attach expanded metal mesh 2 inches above the water line and cover the sides of the tub with 1/2 inch of mortar. Allow the mortar to cure before installing tile.
Install tile using thin-set mortar. Lay tile on the tub floor first, cutting tile to fit around the drain and to accommodate the slope. Install the cove tile in the corners where the floor meets the wall. Use a straight edge to start and a level to ensure you keep the tile level as it rises up the sides of the tub. When the mortar cures, fill between the tile with grout. Mix 75 percent sanded grout and 25 percent nonsanded grout.
Install the overflow drain and faucet.