How To Replace a Bathtub Drum Trap

If you’re having repeated problems with a clogged or slow-draining bathtub, the issue may be an old drum trap. Upgrading to a PVC P-trap can reduce clogging and make the drain easier to clean. The project can be simple if your plumbing is already accessible, but you may need the help of a licensed plumber if it’s not.

In the video above, This Old House plumbing and heating expert Richard Trethewey demonstrates the step-by-step process of replacing a bathtub drum trap. We’ll outline when you can take the project on yourself and walk you through the steps to complete it, as well as when it’s best to hire a pro.

Understanding Bathtub Traps

Bathtub traps prevent sewer gasses from entering your living space while allowing water to flow freely from the tub to the main drain line.

Drum Traps vs. P-Traps

Both drum traps and P-traps perform the same function, but they’re shaped differently. Drum traps, once common in older homes, are cylindrical containers that act as a physical barrier to collect debris and block gas. However, they have several drawbacks:

• Prone to clogging due to their shape
• Difficult to clean and maintain
• Often made of materials that corrode over time

P-traps, on the other hand, only collect water, which creates its own seal against the backup of sewer gasses. These offer several advantages:

• Self-scouring design that reduces clog risk
• Easier to clean and maintain
• Typically made of durable, corrosion-resistant PVC

Drum traps are no longer up to code in many areas, and P-traps are the modern standard for bathtub plumbing. 

Benefits of Upgrading to a PVC P-Trap

Replacing your old drum trap with a PVC P-trap can significantly improve your bathtub’s drainage, resulting in the following benefits:

• Improved water flow and drainage
• Reduced risk of clogs and blockages
• Easier maintenance and cleaning
• Compliance with modern plumbing codes
• Increased durability and longevity of your bathtub plumbing system

Tools and Materials Needed to Replace a Drum Trap

Before starting the project, gather the following tools and materials:

• Reciprocating saw with metal-cutting blade
• Two pipe wrenches
• PVC pipe cutter or hacksaw
• Pliers
• Tape measure
• Permanent marker
• Pocket knife
• PVC P-trap
• PVC pipes and fittings
• PVC primer and cement
• Pipe dope
• Single-strand wicking
• Bucket

Preparing the Bathtub Work Area

The biggest problem you’re likely to encounter with this project is accessing the drum trap in the first place. In the example in the video, Trethewey had to cut through the ceiling below the bathtub to access the necessary plumbing. You may have to cut into your finished wall, floor, or ceiling, which greatly increases the cost and complexity of the project. Consider the time and expense of replacing these materials before cutting into them.

Once the plumbing is exposed, you can begin preparing the area. Before beginning the drum trap replacement, do the following:

• Clear the area where you’ll be working to provide ample space.
• Lay down towels or a tarp to protect the floor from water and debris.
• Gather all necessary tools and materials within reach.
• Turn off the water supply to the bathtub to prevent any accidents.

Steps To Replace a Bathtub Drum Trap

Note that the features or dimensions of your home’s plumbing may require slightly different fittings than shown in the video. However, these are the steps Trethewey followed to replace the old drum trap with a new PVC P-trap:

1. Remove the old drum trap using a reciprocating saw with a metal-cutting blade. Be cautious when lowering the trap, as it will be full of water. Have a bucket on hand to collect the water.
2. Loosen the old threaded adapter attached to the existing drain line. Use two large pipe wrenches—one to hold the pipe steady and the other to turn the adapter.
3. Once loose, remove the adapter with pliers.
4. Apply pipe dope to the threads of a PVC male adapter. Wrap the threads with single-strand wicking for extra protection against leaks.
5. Insert the male adapter into the existing iron fitting and tighten it with pliers.
6. Repeat the pipe dope and wicking process on the existing iron male fitting, then install a threaded PVC female adapter.
7. Dry fit (without glue) the PVC P-trap and pipes to ensure proper alignment.
8. Measure and cut the final piece of PVC pipe using a PVC cutter or hacksaw.
9. Use a pocket knife to remove any rough burrs from the cut end of the PVC pipe.
10. Dry fit all parts one last time, then use a permanent marker to draw alignment marks on the pipes and fittings.
11. Disassemble the PVC components and clean them with PVC primer.
12. Apply PVC cement and permanently assemble the pipes and fittings, aligning the marks drawn earlier.

Tips for a Successful Trap Installation

To ensure a leak-free and long-lasting installation, keep these tips in mind:

• Work carefully in cramped spaces to avoid damaging surrounding structures.
• Double-check all measurements before cutting pipes.
• Ensure all connections are tight and properly aligned.
• Allow sufficient drying time for the PVC cement before testing the new trap.

Common Challenges and Solutions for Replacing Drum Traps

This project requires precision work within a tight space. Here are some solutions to problems you may encounter. 

• Tight spaces: Use flexible PVC pipes or adjustable fittings to navigate cramped areas.
• Corroded or stuck fittings: Apply penetrating oil and allow it to sit before attempting to loosen stubborn connections.
• Misaligned pipes: Use offset fittings or flexible couplings to accommodate slight misalignments.

Maintenance Tips for Your New P-Trap

To keep your new P-trap functioning properly, follow these best practices:

• Regularly clean the drain to prevent hair and debris buildup.
• Avoid using harsh chemical drain cleaners that can damage PVC pipes.
• Periodically check for leaks and tighten connections if necessary.
• Schedule periodic plumbing inspections to catch and address any potential issues early on.

When To Call a Professional

Changes to your home’s plumbing system should be completed by a licensed plumber to maintain the integrity of your plumbing and the resale value of your home. In fact, local building codes may require it. However, many DIYers still take on these jobs themselves. While we recommend completing this job under the supervision of a licensed plumber, you must ask for help under the following circumstances:

• If you encounter extensive corrosion or damage to surrounding pipes
• When dealing with lead pipes, which require special handling
• If local building codes require licensed plumbers for this type of work
• When you’re unsure about your ability to complete the project safely

Our Conclusion

Replacing a bathtub drum trap with a modern PVC P-trap is a far more straightforward job when the necessary plumbing is already exposed. If your home or bathroom is already undergoing renovation, this is the perfect time to check your plumbing and see whether your bathtub may benefit from a new P-trap. This upgrade not only enhances the efficiency of your plumbing system but also ensures compliance with modern plumbing codes, providing peace of mind for homeowners.