How Should I Replace Dual Drum Traps with a Single P-Trap?
If you live in an older home, you may have encountered a "drum trap"?a cylindrical plumbing fixture once common in bathtubs and floor drains. While they were designed to hold a large water seal, they are notorious for collecting hair, grease, and sediment, leading to stubborn clogs and difficult cleanouts. Modern building codes now require P-traps for these applications. Replacing dual drum traps with a single, efficient P-trap is a high-impact home improvement project that will modernize your plumbing system.
Why You Should Replace Your Old Drum Traps
Unlike modern P-traps, which use a self-scouring design to move waste through the pipe, drum traps allow heavy solids to settle at the bottom. Over decades, this buildup restricts flow. Furthermore, the metal covers on drum traps often rust shut, making them impossible to clear without cutting the pipe. A PVC P-trap provides a simpler, smoother path for water and is much easier to maintain.
Tools and Materials Needed
- PVC P-Trap Kit: Usually 1.5-inch or 2-inch depending on your branch line.
- PVC Primer and Cement: For solvent welding the new joints.
- Reciprocating Saw or Hacksaw: To remove the old lead or galvanized iron pipes.
- Shielded Transition Couplings (Fernco): To connect new PVC to old cast iron or copper lines.
- Spirit Level: To ensure proper "pitch" or slope.
Step-by-Step Conversion Process
1. Identify the Existing Layout
Dual drum traps often suggest that two fixtures (like a tub and a nearby floor drain) were plumbed into separate cylinders. Before cutting, identify where the waste line exits toward the main stack. You will need to consolidate these lines into a single, properly vented waste arm.
2. Remove the Drum Traps
Since most drum traps are made of heavy lead or cast iron, use a reciprocating saw with a metal-cutting blade. Cut the pipe as close to the trap as possible while leaving enough "meat" on the existing pipe to attach a transition coupling later. Warning: Be careful of stagnant water inside the drum; keep a bucket and rags nearby.
3. Account for Venting (Crucial Step)
A P-trap requires a vent to prevent siphoning. Drum traps were often "unvented" because their volume was thought to resist siphoning. When you switch to a P-trap, you must ensure the trap arm connects to a vent stack within the distance required by code (usually 5 to 8 feet for 2-inch pipe). If no vent exists, you may need to install an Air Admittance Valve (AAV).
4. Install the New P-Trap
- Attach a shielded rubber coupling to the existing waste line.
- Dry-fit your PVC pipe from the tub/shower drain assembly to the new P-trap.
- Ensure the "trap weir" (the exit point of the trap) is lower than the fixture drain but higher than the connection to the main stack.
- Check for a 1/4 inch per foot slope on the horizontal run.
5. Solvent Weld the Joints
Once the dry-fit is perfect, apply primer and glue to the PVC components. Hold the joints firmly for 30 seconds to ensure a permanent bond. Re-tighten the stainless steel bands on your transition couplings using a nut driver.
Common Pitfalls to Avoid
- Deep Seals: Do not make the "U" portion of the P-trap too deep. A trap seal should be between 2 and 4 inches. Too deep, and the trap won't self-scour.
- Double Trapping: Never install a P-trap if there is already another trap further down the line. This creates an air lock that prevents drainage.
- Improper Materials: Avoid using "flexible" accordion-style drain pipes. These are not code-compliant and will trap hair just as badly as the old drum trap did.
Conclusion: Better Flow and Peace of Mind
Replacing dual drum traps with a single P-trap is a significant upgrade for any vintage home. Not only does it bring your bathroom up to modern plumbing codes, but it also virtually eliminates the foul odors and slow drains associated with old-style plumbing. If your pipes are buried in a concrete slab, this project may require a jackhammer; if they are accessible via a crawlspace or basement, it is a manageable weekend DIY project.
