Introduction
Homeowners and renovators working with older plumbing systems often encounter mysterious components that modern plumbers rarely install. One such relic is the drum trap — a type of plumbing trap that once served a key role in drainage systems but is now largely obsolete. Whether you’re remodeling your bathroom, experiencing slow bathtub drainage, or searching for solutions to sewer gas smells, understanding drum traps can help you make the right decision about repairs or replacements.
In this article, we’ll explore everything you need to know about drum trap plumbing, including its function, differences from the modern P-trap, how to clean it, installation tips, replacement costs, and how it’s used in bathtub drains. We’ll also guide you through PVC drum traps and include visual insights based on plumbing diagrams.
What is a Drum Trap in Plumbing?
A drum trap is a type of plumbing trap that holds a reservoir of water to block sewer gases from entering living spaces. Unlike the modern P-trap, which has a curved “U” or “P” shape, a drum trap is shaped like a drum or cylinder. It connects to the plumbing drain line with inlet and outlet pipes and usually includes a threaded access cover for cleaning.
Drum traps were commonly installed in homes built before the 1950s, particularly for bathtubs, showers, and some sink fixtures. At the time, these traps were valued for their ability to retain a large amount of water, which helped prevent the water seal from being siphoned out due to poor venting — a common problem in early plumbing systems.
Drum Trap Plumbing Diagram: Understanding the Layout
In a typical drum trap plumbing diagram, you’ll see:
A vertical or horizontal drain pipe entering the side of the drum (inlet)
Another pipe exiting the opposite side (outlet)
A removable cleanout lid or cover on the top or side of the trap
The entire unit installed between the fixture (bathtub or sink) and the main drain line
This design captures water and blocks sewer gases while allowing for manual cleaning access. However, the same layout also causes sediment and debris to accumulate, leading to the primary downside of drum traps: clogging.
Drum Trap vs P-Trap: What’s the Difference?
The P-trap has largely replaced the drum trap in modern plumbing for several good reasons. Here’s a direct comparison:
1. Shape and Flow Efficiency
Drum Trap: Large, boxy, or cylindrical in shape with greater water retention but limited flow velocity.
P-Trap: Streamlined “U” or “P” shape that promotes self-scouring and minimizes clogging.
2. Cleanability
Drum Trap: Requires removal of a threaded lid that can be difficult to access or corroded shut.
P-Trap: Easily removable for cleaning or replacement, usually with slip joints.
3. Code Compliance
Drum Trap: Prohibited by most modern plumbing codes due to inefficiency and maintenance issues.
P-Trap: Fully compliant when vented properly and installed according to standard plumbing practices.
4. Clog Risk
Drum Trap: Higher due to sediment buildup and low water velocity.
P-Trap: Lower risk thanks to smooth water flow.
If you’re debating drum trap vs P-trap, the modern P-trap wins in virtually every category.
Why Are Drum Traps Obsolete Today?
Although drum traps served a purpose in early plumbing design, their flaws became more apparent over time:
They’re not self-scouring: Water flow inside a drum trap isn’t strong enough to carry away solids, so debris builds up.
They are hard to access and clean: The lid or plug may corrode or be inaccessible behind walls or flooring.
They often violate plumbing code: Drum traps are not permitted in most jurisdictions under modern building codes.
They can harbor odors: Trapped debris may decay and emit unpleasant smells.
They cause drainage issues: Slow-draining tubs and gurgling sounds often trace back to clogged or partially blocked drum traps.
Drum Trap in Bathtubs: A Common Encounter
Drum traps are most commonly found beneath bathtubs in older homes. Many homes built before the 1950s relied on drum traps to prevent sewer gases from coming up through tub drains. They were especially common in second-story bathrooms where venting options were limited.
If you’re remodeling or replacing a tub and find a drum trap underneath, it’s a strong sign the plumbing hasn’t been updated in decades. Most plumbers will recommend removing the drum trap and installing a modern P-trap in its place.
PVC Drum Traps: Are They Still Available?
Though drum traps are outdated, PVC drum traps are still available for purchase, primarily for repairs or special applications.
These newer models are usually made of Schedule 40 PVC and come with a threaded cleanout cap. Sizes often range from 1½ inches to 2 inches in diameter. You may find them in specialty plumbing supply stores or online. However, just because they’re sold doesn’t mean they’re legal to install in new construction.
Important: Always check your local plumbing code before installing a PVC drum trap. In many cities and states, they are only allowed for specific uses like laboratory sinks or solids interceptors—not for bathtubs or regular residential drains.
Drum Trap Installation: A Note of Caution
If you’re replacing or reinstalling a drum trap, consider the following:
Location & Access: Make sure the trap is installed where the cleanout lid remains accessible.
Proper Orientation: The inlet and outlet should be correctly aligned to allow water to flow smoothly.
Sealing: Use plumber’s tape or a gasket on the cleanout lid to prevent leaks or sewer odors.
Ventilation: Ensure the fixture is properly vented, as drum traps don’t solve siphoning issues on their own.
Warning: In most modern plumbing scenarios, it’s better to remove the drum trap entirely and replace it with a P-trap.
How to Clean a Drum Trap
Cleaning a drum trap is more difficult than dealing with a P-trap, but here’s how it’s done:
Step-by-Step:
Locate the trap – Usually under the bathtub or behind an access panel.
Remove the cleanout lid – This can be challenging; you may need pipe wrenches, penetrating oil, or heat if it’s rusted.
Clear debris – Use a hose, hand tool, or shop vac to remove hair, soap scum, and sediment.
Inspect the interior – Check for rust, damage, or corrosion.
Reseal and test – Apply thread tape or sealant to the lid and tighten it securely. Run water to test drainage and check for leaks.
Note: If the lid cannot be removed, or if the interior is too corroded to clean, full replacement may be the only option.
Drum Trap Replacement Cost: What to Expect
Replacing a drum trap varies in cost depending on access and complexity. Here’s a breakdown:
Basic replacement (easily accessible): $200 – $500
Moderate replacement (floor or wall removal required): $500 – $1,000
Complex replacement (structural work, crawlspace, cast iron): $1,000 – $3,000+
You may also have to pay for:
New trap parts (PVC or P-trap materials): $20 – $50
Permits or inspection fees, depending on your city
Tip: Always get 2–3 estimates from licensed plumbers before proceeding.
Should You Replace a Drum Trap?
In nearly all cases, the answer is yes — especially if:
You’re experiencing slow drain
You smell sewer gas
You’re renovating your bathroom
The trap is corroded or leaking
It’s not accessible for cleaning
Replacing a drum trap with a modern P-trap improves drainage, ensures code compliance, and reduces future maintenance headaches.
Common Issues Homeowners Face with Drum Traps
Repeated clogging under bathtubs or showers
Corroded cleanout covers that cannot be opened
Unpleasant odors from poorly sealed or dirty traps
Traps encased in concrete, making removal costly
Insurance or inspection issues in real estate transactions
Final Thoughts: Time to Update Your Drainage System
Drum traps served a purpose in the early days of plumbing, but their time has passed. Whether you’re dealing with a bathtub drain clog or tackling a full-scale remodel, identifying and replacing old drum traps can significantly improve your home’s drainage system and indoor air quality.
For most modern homes, a P-trap is the better alternative — it’s easier to maintain, less prone to clogging, and fully compliant with building codes.
If you’re unsure whether your home still has a drum trap or need guidance on plumbing upgrades