P-traps are S-shaped pipe components that keep toxic sewer gases from escaping into your residence. They are required on all plumbing fixtures, including sinks, tubs and showers. Toilets have built-in traps. Sink P-traps are usually accessible after installation, and are designed to be easily taken apart. Tub and shower traps usually aren't accessible, however, so they are designed to be permanent. A bathtub trap begins at the end of the tub drain assembly, where it is glued onto a fitting screwed to the drain outlet. The other end is glued to a length of horizontal pipe. It ties into the waste stack that empties into the sewer.
Wrap plumbing tape around the threaded outlet of the bathtub drain and screw on a 1 1/2-inch PVC female threaded adapter. Tighten it with adjustable pliers. This fitting has a slip connection on the other end. Cut a 4-inch nipple from PVC pipe with a hacksaw, deburr the edges with a file and fit it inside. Don't glue any pipes until you have completed the trap assembly.
Slip the trap onto the nipple and swivel the other end to face the waste stack.
Measure the distance between the end of the trap and the waste stack with a tape measure and cut a piece of 1 1/2-inch PVC pipe to that length. Fit one end into the trap.
Place a 1-foot level on a straight board exactly 4 feet long and raise one end of the board by 1 inch. Note the position of the bubble with respect to the center lines. Set the level on the pipe and lower the end that intersects the waste line until the bubble is in the same position. This ensures the pipe has the 1/4 inch-per-foot slope required for safety. Mark the point where the top of the pipe intersects the stack.
Cut into the waste stack on the mark and install a sanitary tee with a 1 1/2-inch inlet that faces the P-trap. If the waste line is cast iron, use a fitting with a threaded inlet and screw in a PVC male adapter.
Fit the horizontal waste line to the tee inlet and the outlet of the trap. You may have to cut off a small section of the pipe to get it to fit.
Make marks on the pipes and fittings with a felt marker so you can line them up in the same positions after you disassemble them. Take the assembly apart and begin gluing the pieces together with PVC cement, starting at the sanitary tee and working toward the tub. Glue the nipple connected to the trap inlet to the fitting in the tub drain last.
1 1/2-inch PVC female adapter
1 1/2-inch PVC pipe
PVC bathtub P-trap
Things You Will Need
If you don't have a clear path between the P-trap and the waste stack, you may need to bend the horizontal waste line with fittings. If you do, be sure the assembly maintains the required 1/4-inch-per-foot slope at every point.
Don't use a conventional sink P-trap for a bathtub. If it comes loose, you may not know it until water has caused serious damage to the framing around the tub.