Even if it seems like a simple task, roughing in the plumbing for a bathtub drain is a job regulated by plumbing codes that vary widely by locality and a project best left to a qualified plumber. It is not, however, difficult to understand the basics of the job and what it entails.
Locating the Drain Line
Installing a tub drain in a bathroom with an existing finished floor will require you to cut a hole in the floor to accommodate the drain and trap. The position of the tub drain will vary, depending on the size and design of the tub, and you'll need to cut an access hole in the floor at the drain end of the tub to allow for positioning and installing the drain line. This hole should be 9 inches wide and extend at least 12 inches from the wall. If you're installing a claw foot tub, the drain line will be exposed, so the hole in the floor will need to be more precise. Drop a plumb line from the center of the roll at the edge of the tub; the point where the line meets the floor is the position of the drain rough-in hole.
You'll rough in the tub drain with 1 1/2-inch pipe with male threads that extend 1/2-inch above the level of the floor. The drain will also require a P-trap and must be vented; specific requirements for the trap, drain line and vent will vary by local building codes. Rough-in requirements for the tub's supply lines will depend on the design of the tub. Tub/shower combinations will typically have supply lines installed in the wall that the tub backs up against while a claw foot tub may have the supply lines emerging from the floor near the tub drain.
Plumbing codes will have a requirement for the slope of the drain line as it moves away from the tub. A typical drain line slopes down a minimum of 1/4-inch per foot of linear distance traveled.
Drain lines must be vented to prevent potentially dangerous sewer gases from escaping into the building. A bathtub drain can utilize a wet vent, which is a system that functions as both a vent and a drain for multiple fixtures. In this case, the tub's drain line would connect to a vent stack to which the drain lines of other fixtures are also connected. Plumbing code requirements for wet vents are complex and vary by location. An individual vent is often used when a fixture is too far away from an existing wet vent to be easily connected to it; in this case, the fixture's drain connects to a vent stack that only serves that fixture.