Bathtub Drain Plumbing Diagram: Parts, Sizes and Rough-In

A bathtub drain plumbing diagram runs from the overflow faceplate and drain shoe into one waste-and-overflow tee, down a tailpiece to a 1-1/2 inch P-trap, then along a trap arm no longer than 6 feet to a vent.

What does a bathtub drain plumbing diagram show?

A bathtub drain plumbing diagram shows two paths that meet below the tub. Water leaves through the drain flange in the tub floor and the overflow opening in the end wall, the two pipes join at a waste-and-overflow (W&O) tee, and a tailpiece drops into a 1-1/2 inch P-trap. The trap arm then carries water to a vented drain line inside the wall or floor.

The supply side sits in the same end wall, where a mixing valve feeds the spout below it and a shower riser above it.

Clearing a clog rarely goes past the stopper and trap. Our guide shows how to unclog a bathtub drain through the stopper and overflow without opening a wall.

What are the parts of a bathtub drain, and how does each one fail?

A bathtub drain has seven parts between the tub and the drain line: overflow faceplate, overflow elbow and tube, drain flange, drain shoe, W&O tee, tailpiece and P-trap, then the trap arm and vent. Each fails in a recognizable way, which tells you where to look before cutting drywall.

Lift-and-turn, toe-touch and trip-lever stoppers all thread into or ride inside the same drain assembly, and our guide to bathtub drain stopper types covers how each one seals.

What size is bathtub drain plumbing under the code?

The 2021 International Plumbing Code rates a bathtub at 2 drainage fixture units and sets its minimum trap size at 1-1/2 inches in Table 709.1. That row covers tubs with or without an overhead shower or whirlpool attachment, and the table’s note says a showerhead over the tub does not raise the fixture unit value.

The code also says a trap must drain the fixture rapidly and match the fixture outlet, which is why tub W&O kits are built for a 1-1/2 inch line. Table 709.1 sets a minimum, so many plumbers run the drain at 2 inches past the trap.

Homes built under the International Residential Code follow its own trap and vent chapters (Chapter 32 for traps, Chapter 31 for vents). We did not check the IRC table text, so confirm residential numbers against your adopted edition.

How far can the bathtub trap be from the vent?

For a 1-1/2 inch trap arm, IPC 2021 Table 909.1 allows 6 feet of developed length from the trap weir to the vent fitting, at a slope of 1/4 inch per foot. Section 909.1 states that “each fixture trap shall have a protecting vent” placed so the slope and length stay within that table.

Developed length is measured along the pipe centerline through every fitting, not straight across the floor plan. When a corner tub sits far from the stack, the fix is a vent closer to the trap, not a steeper pipe.

A trap arm that is too long or too steep can siphon the trap dry, and our explainer on how a P-trap holds its water seal covers why that lets sewer gas into the room.

What are the rough-in dimensions for bathtub plumbing?

Bathtub rough-in starts from the tub, not a fixed number. Vintage Tub & Bath’s alcove tub installation guide says to set the tub in place with the overflow connection installed, drop a plumb line from the overflow centerline to the floor, and mark that point as the drain rough-in. Valve and spout heights come from the valve maker’s drawing.

Chart sources: House Made Clear (heights), IPC 2021 Tables 909.1 and 709.1 (trap arm, trap), Signature Hardware (cutout).

Drain end and floor cutout

Signature Hardware’s alcove tub instructions call for a 6 by 12 inch floor opening for a 1-1/2 inch drain and W&O kit, because the drain and overflow hang below the bottom of the tub. Hunker describes an opening about 12 inches long and 3 to 6 inches wide, cut from about 2 inches behind the drain mark to the edge of the subfloor at the faucet wall.

1. Dry-fit the tub with the W&O kit and drop a plumb line from the overflow centerline.
2. Mark the drain centerline on the subfloor.
3. Check for joists, then cut from about 2 inches past the mark to the wall plate.
4. Set the trap so the tailpiece drops straight into it.

Hunker notes most alcove tubs are 60 inches long and 30 or 32 inches wide, advises framing the alcove about 3/4 inch larger than the measured tub, and reminds buyers to match a right-hand or left-hand drain tub to the existing drain.

Valve, spout and shower riser

House Made Clear gives about 28 to 32 inches above the tub floor as the planning range for a tub and shower valve, and about 80 inches above the finished floor for a fixed shower head. Spout height guidance varies between secondary sources, so set it from the valve maker’s piping diagram. House Made Clear notes that some valve makers also limit the pipe type and length between valve and spout. Our tub spout replacement guide covers slip-on versus threaded spouts on that stub-out.

Which waste-and-overflow kit should you buy for a bathtub?

Pick a W&O kit by three choices: body material (PVC, ABS or brass), stopper style, and whether the kit matches the drain pipe you already have. Plastic kits solvent-weld or slip-joint onto PVC or ABS drains, and brass kits come in thinner and heavier tubing, where a lower gauge number means a thicker wall.

Check that the kit’s overflow tube covers the drop from your tub’s overflow hole to its drain, and trim it to fit.

How do you replace a bathtub waste-and-overflow?

Replacing a bathtub W&O takes access to the drain end from behind or below, a drain key, and a new kit sized to your 1-1/2 inch line. You remove the old flange from inside the tub, free the old assembly at the trap, and set the new one with fresh gaskets.

1. Open the access panel, or cut one behind the drain end or in the ceiling below.
2. Remove the stopper and overflow faceplate from inside the tub.
3. Unscrew the drain flange from the drain shoe with a drain key or a flange tool.
4. Loosen the slip nut at the trap, or cut the tailpiece above a glued trap.
5. Pull the old assembly out and scrape old putty off the drain hole.
6. Set the new shoe and overflow elbow with their gaskets against the outside of the tub, then thread in the flange with the sealant the kit calls for.
7. Connect the tee, cut the tailpiece to length and join it to the trap.
8. Run the leak test below before closing the panel.

The IPC calls for access to concealed slip-joint connections. If your tub has no panel, adding one now saves the next repair from a drywall patch. A tub on a slab with no access means working from inside the tub with a drain flange tool, or calling a plumber before you break tile.

How do you leak test a bathtub drain?

Test a bathtub drain in two stages: a high fill that loads the overflow gasket, then a full drain that loads the shoe, tee, tailpiece and trap. Many W&O leaks show in only one of those states, so a tub that seems fine during a shower can still stain the ceiling during a bath.

1. Close the stopper and fill the tub until water reaches the bottom of the overflow opening.
2. From the access panel or the ceiling below, check the overflow elbow and its gasket.
3. Open the stopper and watch the drain shoe, tee, tailpiece and trap while the full tub empties.
4. Wrap dry paper towel around each slip joint to catch small drips.
5. Tighten slip nuts by hand plus a small turn, then repeat the test.

A gurgle as the tub empties points to the trap arm and vent, not the W&O.