How to Install a New Bathtub and Drain Assembly

Begin by turning off the main water supply to the house. Open a faucet on a lower floor to drain the lines. Next, carefully remove the old faucet handles, escutcheon plates, and tub spout. You will then need to gain access to the tub's plumbing. This typically involves cutting an access panel in the drywall behind the faucet or working from a basement or crawlspace below. Disconnect the drain and overflow assembly from the tub and the house's main waste line using channel-lock pliers or a pipe wrench. Remove the wall material (tile, fiberglass surround) from around the tub flange down to the studs using a hammer, pry bar, and utility knife. Finally, with the tub fully disconnected and the flange exposed, use a pry bar to lift the tub out of its alcove. If the tub is cast iron or difficult to maneuver, it may be necessary to break it into smaller pieces with a sledgehammer (use extreme caution and full safety gear) or cut a fiberglass/acrylic tub with a reciprocating saw.

With the alcove empty, thoroughly clean all debris from the subfloor and studs. Inspect the subfloor for any signs of water damage, rot, or weakness. Repair or replace any damaged sections of plywood or OSB. Use a 4-foot level to check that the subfloor is perfectly level where the tub will sit. If it is not, use a self-leveling compound to create a flat, level surface. Inspect the wall studs for plumb and ensure they are sound. If required by the tub manufacturer, install a ledger board. This is typically a straight 2x4 screwed horizontally to the studs along the back wall. Measure up from the subfloor to the height specified in the tub's installation manual (this height corresponds to the underside of the tub's rim). Use a level to ensure the ledger board is perfectly level before securing it to the studs with long wood screws.

This step is crucial for preventing issues later. With a helper, carefully lift the new, empty bathtub and place it into the prepared alcove. Do not apply any adhesive or fasteners yet. Check that the tub's flange sits flush against the wall studs and rests securely on the ledger board (if one was installed). Place a level across the top rails of the tub, checking for level from front-to-back and side-to-side. If the tub has leveling feet, adjust them now. If not, you may need to use shims under the tub base. With the tub perfectly positioned, use a pencil or marker to trace the outline of the drain opening onto the subfloor. Also, make a mark on the wall studs corresponding to the top edge of the tub's flange. These marks will serve as critical reference points. Once all checks and marks are made, carefully remove the tub and set it aside on a protected surface.

With the tub out of the alcove, it's time to install its drain components. Unpack your new waste and overflow kit. Take the plumber's putty and roll a small amount into a rope about 1/4-inch thick. Press this rope firmly around the underside of the new drain flange (the chrome piece that sits inside the tub). From inside the tub, insert the threaded end of the drain flange through the drain hole. From underneath the tub, place the large rubber gasket over the threads and screw on the drain shoe (the elbow fitting). Tighten this connection using a drain wrench or the handles of channel-lock pliers inserted into the crossbars of the drain. As you tighten, excess putty will squeeze out from under the flange; this indicates a good seal. Next, attach the overflow assembly. From the back of the tub, place the tapered rubber gasket over the overflow opening. From the front of the tub, push the overflow plate/cover through the hole and screw it into the overflow elbow on the backside. Finally, connect the straight waste pipe between the bottom of the overflow tee and the top of the drain shoe, ensuring gaskets are properly seated before tightening the slip nuts. Hand-tighten plus a quarter turn with pliers is usually sufficient.

For a rock-solid installation that won't flex or creak, setting the tub in a mortar bed is essential for acrylic and fiberglass models. Mix a bag of mortar in a bucket with water until it reaches a thick, paste-like consistency, similar to chunky peanut butter. Spread a 1- to 2-inch thick layer of this mortar onto the subfloor in the area where the base of the tub will rest (refer to the footprint of the tub). With your helper, carefully lift the tub (with its drain assembly now attached) and lower it into the alcove. The drain tailpiece should slide into the hole in the subfloor. Press the tub down firmly, embedding it into the mortar bed. Use your level across the top rails to confirm it is perfectly level in both directions. Once level, secure the tub's integrated flange to the wall studs. Use 1-1/2 inch roofing nails or washer-head screws, driving them through the flange into the studs. It's best to pre-drill holes in the flange to prevent cracking. Do not overtighten the fasteners, as this can stress and crack the flange.

Access the plumbing area from below or through the access panel. You will now connect the tub's drain assembly to the house's waste line. This typically involves installing a P-trap. Dry-fit all the PVC pipe and fittings first. Connect the P-trap to the vertical tailpiece coming down from the tub's overflow tee. Then, connect the other end of the P-trap to the sanitary tee in the main waste line. Ensure there is a downward slope of 1/4 inch per foot from the tub to the main drain to ensure proper flow. Once you have confirmed the fit, disassemble the pieces. Apply PVC primer (usually purple) to the outside of the pipes and the inside of the fittings you are connecting. Immediately follow with a layer of PVC cement. Push the pipe firmly into the fitting, giving it a quarter-turn twist to spread the cement. Hold the joint together for at least 30 seconds to allow the solvent weld to set.

Before you can test the drain, you need to install the faucet and water supply. Install the new tub/shower valve body, securing it to the wall framing according to the manufacturer's specific depth instructions. Connect the hot and cold water supply lines to the valve, using Teflon tape or pipe dope on threaded connections. Install the tub spout, ensuring a watertight seal against the tub wall. Now, turn the main water supply back on slowly. Open the hot and cold faucets to bleed any air out of the system. With the water running, use a flashlight to carefully inspect all the supply line connections at the valve for any drips. Next, test the drain. Close the tub drain with its stopper and fill the tub with several inches of water. Inspect the connection between the drain shoe and the tub from your access panel or below. There should be no leaks. Finally, open the drain and let all the water rush out. As it drains, thoroughly inspect the P-trap and all the waste line connections you made. If any leak is found, shut off the water immediately, drain the system, and remake the leaking connection.