If you have pulled up an old bathroom floor and found a squat metal canister near the tub drain, you have probably met a drum trap. They were common in houses built before the middle of the last century and they still turn up during remodels. They do work, after a fashion, but they come with problems a modern P-trap avoids. Here is how the two compare and what replacing a drum trap involves.
What a drum trap is
A drum trap is a short cylinder, usually lead or cast iron, around 3 to 4 inches across. The inlet pipe enters low on one side and the outlet leaves higher up, so a pool of water sits in the drum and blocks sewer gas. Many have a screw cap on top, sometimes set flush with the bathroom floor so it could be opened for cleaning.
They were popular because they held a large volume of water and were thought to resist siphoning. In practice the same feature that made them hard to siphon also made them collect debris.
Side by side
A standard P-trap is a simple U-bend with a trap arm running to the vent. If you are curious about other shapes, our P-trap alternatives roundup covers bottle traps and more.
| Feature | Drum trap | P-trap |
|---|---|---|
| Self-cleaning | No, solids settle in the drum | Yes, flow scours the bend |
| Code status | Not allowed for new work in most codes | Standard |
| Typical materials | Lead, cast iron | PVC, ABS, chrome brass |
| Access | Top cap, often seized | Slip nuts or cleanout |
| Typical replacement cost | Often a few hundred dollars with a plumber, more on a slab | Parts usually under $40 |
Why drum traps cause problems
- Debris settles. Water slows in the wide drum, so hair and soap scum drop out and build up.
- Slow tub drains. The usual complaint, and plunging rarely fixes it for long.
- Seized caps. Brass caps in lead bodies can be impossible to open without damaging the drum.
- Cracking. Old lead can split, causing slow leaks into the ceiling below.
Replacing a drum trap step by step
- Get access from below (basement, crawl space or ceiling) or through the floor.
- Shut off water to the bathroom so no one uses the tub.
- Cut out the drum and enough pipe either side to reach sound material.
- Fit a 1 1/2 inch P-trap, or 2 inch where local code asks for it on the tub, using no-hub couplings to join to the existing pipe.
- Make sure the new trap arm runs to a vent within the allowed length. See trap arm length rules.
- Fill the tub, release it and watch every joint for leaks.
Safety notes on lead
Lead drum traps and pipe are a health concern only if you create dust or fumes. Wear gloves and eye protection, cut rather than grind, avoid sanding, and wash your hands before eating. Scrap lead can usually go to a metal recycler. If the trap is buried in a slab, budget for concrete work and consider a pro.
How to tell if you have a drum trap
Look at the floor near the drain end of the tub for a round brass or chrome cap, often 3 to 4 inches across, sometimes painted over or hidden under vinyl. From a basement or crawl space you may see a short fat cylinder of grey metal between the tub drain and the waste pipe. Lead looks dull grey, is soft enough to scratch with a key and has wiped, bulbous joints rather than threaded fittings. Cast iron drums are darker and heavier with threaded or caulked joints. If neither is visible, a slow tub that a plunger helps only briefly, in a house built before roughly 1960, is a strong hint.
Some drum traps were also used under kitchen and lavatory sinks in very old homes, so if you are opening walls anywhere in an older property it is worth checking every fixture drain while you have access. Replacing them all at once is usually cheaper than calling a plumber back for each one.
Typical costs to budget for
Costs vary widely with access. A drum trap reachable from an unfinished basement is often a straightforward job for a plumber, typically a few hundred dollars including parts. If the ceiling below has to be opened and patched, add drywall and paint. On a slab, breaking and patching concrete can push the total well past a thousand dollars. Get at least two quotes and ask whether the price includes connecting to a proper vent, since many old tub drains were never vented to modern standards.
Can you keep a drum trap?
Existing drum traps are usually allowed to stay until you remodel, but most plumbers recommend replacing one whenever the floor or ceiling is open anyway. If you keep it, open and clean the cap now and then, and pour water in if the bathroom goes unused so the seal does not dry out. For other old-house drain quirks, read J-trap vs P-trap.
Frequently asked questions
Are drum traps illegal?
They are not allowed in new installations under most codes, but existing ones are generally permitted until a remodel.
Can I clean a drum trap?
Yes, by removing the top cap and pulling out debris, though old caps often seize.
Why does my tub with a drum trap drain slowly?
Debris settles in the drum. Cleaning helps briefly; replacement fixes it.
Is a drum trap made of lead dangerous?
Not in normal use. Take care to avoid dust and fumes when removing it.