Where does the drain go in a bathtub rough-in?

Is there a standard bathtub drain location?

No single standard exists. Kohler's spec sheet for a 60 in alcove tub states that all dimensions are nominal and tells installers to measure the actual product for rough-in details. The drain side, left or right, is part of the model number.

A secondary source, Engineer Fix, explains the convention. The rough-in dimension is measured from the centerline of the drain opening to the finished wall or the stud wall, and in new construction it is taken from the stud face. A drain centered in a 30 in wide tub therefore sits about 15 in from the back wall stud. Treat that as an illustration, since the maker's sheet governs.

How is an alcove tub drain located and measured?

Face the tub apron. A left-drain tub has the drain at your left end. Build the stud pocket from the maker's rough-in drawing, then locate the drain from the end wall and back wall studs. The finished wall adds thickness, so allow for backer board and tile in the tub's fit.

• Wyndham's alcove tub sheet: build a plumb stud pocket to the specification sheet, rough in the plumbing, then lift the tub into position with the drain aligned to the pipe.
• ASB's alcove manual: confirm the drain end is where you need it before starting, and provide an access panel on the back side of the plumbing wall.
• ASB's drawing shows a drain cutout of 8 in by 10 in maximum, centered on the drain.
• Kohler's K-1184-LA sheet: an integral flange helps keep water from seeping behind the wall and simplifies alcove installation.

Keep the subfloor level and the tub supported on its feet or base, not its rim, as Wyndham's guide notes.

Where does the drain go on a freestanding tub?

Wherever that tub's drain opening falls. Waterworks has you determine the tub position, then mark the floor at the center point of the drain opening and drill the hole. Kohler's freestanding guide instead sets a standard PVC or brass drain pipe at a model-specific height above the finished floor and lowers the tub over it, with the pipe centered in the drain hole.

How does a solid stone tub change the rough-in?

Stone is carved, so the drain and any overflow follow that piece. Stone Forest notes that hand carving causes small size variances and that exact measurements should be taken from the tub once it is on site. Aquatica's stone tub procedure advises cutting the floor opening only once the tub is on hand, after checking that no pipes, joists, ducts or wires are in the way.

Ask for the dimensioned drawing, give it to your plumber, and confirm the final drain position against the tub itself before cutting. Pietra Fonte pieces are made to order and every quote gives exact dimensions and weight. Aquatica also advises that the floor under its stone tub support a minimum of 1,400 lb. Weight and floor load are in the weight guide, and sizes in bathtub sizes.

What size trap does a bathtub need?

A 1-1/2 in trap at minimum under the IRC. JASpector's summary of IRC 2021 Table P3201.7 gives a bathtub a 1-1/2 in minimum trap, and says the drain must be trapped, vented and sized under the drainage chapters. Waterworks specifies 1-1/2 in DWV piping, and Aquatica lists a 1-1/2 in tailpiece, male adapter, pipe and trap.

A bathtub also needs an approved waste and overflow. The overflow is built into some tubs and absent on others, so the drain kit must match. Local amendments can change these rules, so a licensed plumber should confirm.

Does a bathtub drain need an access panel?

Often, yes. Kohler tells installers to keep all plumbing connections around a freestanding bath easily accessible for maintenance. Waterworks recommends access from below the finished floor, and notes that where access is limited, as with slabs, slip joint gaskets replace trap adapters. JASpector's summary says many jurisdictions require an access opening for concealed slip-joint connections.

Aquatica also recommends access from the floor below. Without it, the procedure raises the tub 3-1/2 in on lumber blocks to make the connection, tests for leaks, then lowers it. Decide on access before walls and ceilings close.

How do you check the rough-in before the tub arrives?

1. Get the maker's rough-in sheet or dimensioned drawing for the exact model, drain side and size.
2. For an alcove, frame the pocket to the sheet and measure the drain from the stud face, allowing for backer board and tile.
3. For a freestanding tub, wait until the tub is on site, set it, and mark the drain center on the floor.
4. Check below the floor for joists, pipes, ducts and wires before cutting.
5. Match the drain kit to the tub, with or without an overflow, and set the trap size from code.
6. Leave the connections accessible, and get the plumbing inspected before closing walls or ceilings.