How to Install a New Bathtub and Drain Assembly

Begin by ensuring your safety and preparing the work area. Shut off the main water supply to your home. Open the faucet at the tub and a lower-level faucet to drain the lines. Use a utility knife to score the caulk line between the old tub and the wall surround. Remove the faucet handles, escutcheons, and tub spout. Disconnect the drain linkage at the overflow plate. Next, you must expose the tub's flange by removing the wall material (tile or surround) at least 6-8 inches above the tub deck. A pry bar, hammer, and reciprocating saw may be needed. Finally, disconnect the tub's drain from the P-trap, which may require cutting the pipe. With a helper, carefully pry and lift the old tub out of the alcove.

With the old tub removed, thoroughly clean the alcove of all debris using a shop vac. Carefully inspect the subfloor for any signs of water damage, rot, or mold. Replace any damaged sections of plywood. Use a 4-foot level to check if the subfloor is level. If it is not, you may need to apply a self-leveling compound. Also, inspect the wall studs for any damage. Reinforce or replace any studs that are not structurally sound. This is a critical step to ensure a stable and long-lasting installation.

It is vastly easier to assemble the drain and overflow components onto the new tub before it is placed in the alcove. Unbox the new tub and the new waste and overflow kit. Apply a rope of plumber's putty under the flange of the main drain fitting (the drain shoe) and insert it from inside the tub. From underneath, place the rubber gasket over the threads and screw on the drain tee. Tighten it securely using a tub drain wrench or pliers. Next, attach the overflow tube to the drain tee and to the overflow elbow. Fit the overflow gasket between the tub and the overflow plate and screw the plate on from inside the tub. Do not overtighten any connections.

To provide solid, level support, most alcove tubs require a stringer board (or ledger board). Refer to your tub's installation manual to find the required height from the subfloor to the underside of the tub's flange. Mark this height on the studs at both ends of the back wall. Use a 4-foot level to draw a perfectly level line connecting these marks across all the studs. Cut a 2x4 to the length of the back wall and secure it to the studs with screws, aligning the top edge of the board with your level line. This will support the weight of the tub's rim.

Many fiberglass and acrylic tubs require a mortar bed for support to prevent flexing and cracking. Check your tub's instructions. If required, mix a bag of mortar mix with water in a bucket to a thick, pancake-batter consistency. Trowel a 1-2 inch thick layer of the mortar onto the subfloor where the base of the tub will sit. With a helper, carefully lift the new tub, angling it to get the pre-assembled drain through the hole in the floor. Slide it into the alcove, resting the back flange on the stringer board. Gently press the tub down into the mortar bed. Use your level across the tub deck in both directions (lengthwise and widthwise) to ensure it is perfectly level. Use wood shims under the feet if necessary to achieve level before the mortar sets.

Once the tub is perfectly level and set (allow mortar to cure if used), you must secure it to the wall framing. Most tubs have an integrated flange with pre-drilled holes that rests against the studs. Using a drill, drive 1-1/2 inch galvanized roofing nails or stainless steel screws through these holes into the studs. If there are no pre-drilled holes, drill them yourself about every 8-12 inches along the flange. Do not overtighten the fasteners, as this can crack the acrylic or fiberglass flange.

Now you will make the final plumbing connection. Working from your access point, dry-fit the tailpiece from the new tub's overflow tee to the existing P-trap. You will likely need to cut a piece of PVC or ABS pipe to the correct length to join them. Use a pipe cutter or hacksaw for a clean cut. Once you are certain of the fit, apply PVC/ABS primer to the outside of the pipes and the inside of the fittings, followed immediately by the cement. Push the pieces together with a slight twist and hold for 30 seconds to allow the solvent weld to set. Ensure the P-trap is properly aligned to hold water.

Before closing up the wall, it is absolutely essential to test for leaks. Use a test plug or a tightly packed rag to block the drain from inside the tub. Fill the tub with 3-4 inches of water. From the access panel, use a flashlight to carefully inspect every joint you made: the drain shoe, the overflow gasket, and all cemented connections. Look for even the smallest drips. After 15 minutes, if there are no leaks, remove the plug and let the tub drain completely. As it drains, watch the connection at the P-trap to ensure it is also watertight. If you find a leak, you must drain the tub and fix the connection.

With the plumbing confirmed to be leak-free, you can now complete the installation. Install the new tub spout and faucet handles onto the valve body in the wall. Once all plumbing fixtures are installed, you can proceed with closing the wall. This typically involves installing cement backer board, followed by your chosen tile or a solid surface surround. After the walls are finished, the final plumbing step is to apply a thick bead of 100% silicone caulk in the gap where the tub deck meets the finished wall. Smooth the bead with a wet finger or caulk tool for a clean, waterproof seal.