What a drain trap is and why you replace it

A drain trap — the curved or S-shaped pipe under your sink — holds standing water that blocks sewer gases from coming back up through your drain. When a trap cracks, leaks, or gets so clogged you cannot clear it, replacement is faster and cheaper than repair. Most traps cost between $15 and $50 in parts, and the job takes 20 to 45 minutes if you have basic tools and no corrosion.

You will know you need a new trap if water pools under the sink when you run the faucet, if you see cracks in the plastic or corrosion on metal, or if a plunger and drain snake cannot clear a stubborn clog. A trap that has been leaking for weeks may have rotted the wood or drywall around it, so check the cabinet floor and wall before you start.

Key Takeaways

  • A drain trap is the curved pipe under your sink that holds water to block sewer gas; it needs replacement when it cracks, leaks, or cannot be unclogged.
  • Most traps are held on with slip nuts (large hand-tight nuts) that you can loosen with a pipe wrench or adjustable wrench without removing the sink itself.
  • Measure the diameter of your old trap (usually 1.25 or 1.5 inches) and the distance from the wall drain to the sink drain before you buy a new one, because trap shapes vary.
  • The job requires only a bucket, two wrenches, and a new trap kit; you do not need to shut off water or call a plumber unless the trap is corroded solid.

Tools and materials you will need

Gather these before you start: a bucket or large bowl (to catch water still in the trap), an adjustable wrench or pipe wrench, a flashlight, and the replacement trap itself. If the nuts are stuck, you may also want penetrating oil (like WD-40) and a second wrench to hold the fitting steady while you turn the nut.

For the replacement trap, measure the diameter of your current one — most are 1.25 inches (standard for bathroom sinks) or 1.5 inches (common for kitchen sinks). Bring a photo of your current setup to the hardware store, or measure the distance from the wall drain opening to the sink drain opening so the new trap will fit. Plastic traps are lighter and easier to hand-tighten; metal traps last longer but corrode over time.

How to disconnect the old trap

Place your bucket under the trap to catch any water still inside. Locate the two slip nuts — the large nuts where the trap connects to the sink drain above and the wall drain below. Turn the nut at the sink drain counterclockwise by hand first; if it does not budge after a few tries, use your wrench to loosen it a quarter turn, then finish by hand to avoid stripping the threads.

Once the top nut is loose, turn the bottom nut the same way. If the trap is metal and corroded, spray penetrating oil around the nut, wait 10 minutes, and try again. If it still will not turn, hold the fitting itself steady with one wrench while you turn the nut with the other — this prevents the whole fitting from twisting and breaking the connection inside the wall.

Pull the trap straight down and out. Water will drip, so keep the bucket underneath. If the trap is stuck, rock it gently side to side rather than yanking hard. Once it is out, look at the rubber washers inside each slip nut; if they are cracked or flattened, set them aside — you will replace them with the new trap kit.

Installing the new trap

Before you install, check that the rubber washers and metal ferrules (the rings that sit under the nuts) are in the new kit. If not, buy them separately — they cost a dollar or two and prevent leaks. Slide the top nut and ferrule onto the sink drain pipe, then slide the new trap up into place so the top opening aligns with the sink drain.

Hand-tighten the top nut first — turn it clockwise until it is snug but not tight. You should be able to turn it another quarter turn with your wrench. Do the same with the bottom nut, connecting the trap to the wall drain. Tighten both nuts a quarter turn more with your wrench, then stop. Over-tightening cracks plastic traps and strips the threads on metal ones.

Turn on the sink and let water run for 10 seconds while you watch underneath. If water drips from either nut, tighten that nut another quarter turn and check again. If water drips from the trap body itself, the trap is cracked and needs to be swapped for a new one — do not use it.

When the old trap will not come loose

Corroded metal traps sometimes seize solid. If your wrench cannot budge the nut after you have applied penetrating oil and waited, try heating the nut with a heat gun or hair dryer for 30 seconds to expand the metal slightly, then try the wrench again. If that fails, you may need to cut the trap out with a hacksaw or reciprocating saw — a job that takes 10 minutes and leaves you with a cleaner break than forcing a stuck nut.

If you cut the trap, use a file or sandpaper to smooth any sharp edges on the remaining pipe stub before you install the new trap. If the wall drain opening itself is corroded or the threads are damaged, you may need a plumber to repair the fitting — this is rare, but it is worth checking before you assume the trap is the only problem.

Choosing between plastic and metal traps

Plastic traps (usually PVC) are lighter, cheaper, and easier to hand-tighten, making them the standard choice for most homeowners. They last 20 to 30 years in normal use and do not corrode. The downside is that they can crack if you over-tighten the nuts or if something heavy falls on them under the sink.

Metal traps (usually chrome-plated brass or steel) look better and are harder to damage, but they corrode over time — especially under kitchen sinks where acidic water or salt sits. A corroded metal trap can seize, making replacement difficult. If you have hard water or live in a humid climate, a plastic trap will outlast a metal one and cost less to replace when it does fail.

What to do if water still leaks after installation

If water drips from a slip nut after you have tightened it, the washer may be seated wrong or the ferrule may be damaged. Loosen that nut, pull the trap down slightly, and check that the rubber washer is sitting flat and centered. If it is bunched or torn, replace it with a new one from a hardware store (they come in packs of five for a few dollars).

If the washer looks fine, the ferrule may be dented or the threads on the pipe may be damaged. In that case, wrap two or three layers of plumber's tape (also called PTFE tape) around the ferrule, slide it back on, and tighten the nut again. If water still leaks, the trap itself is cracked or the pipe threads are too damaged to seal — replace the trap or call a plumber to assess the wall drain fitting.

Frequently Asked Questions

Do I need to turn off the water before replacing a drain trap?

No. The trap holds standing water but is not connected to the water supply line. You can replace it with the water on. Just place a bucket underneath to catch the water that drips from the old trap when you remove it.

What is the difference between a P-trap and an S-trap?

A P-trap (the most common shape) has a horizontal section that connects to the wall drain. An S-trap curves back on itself and connects to a floor drain. Most modern homes use P-traps. Check your current trap shape before you buy a replacement, or bring a photo to the hardware store.

Can I use plumber's putty instead of rubber washers to stop a leak?

No. Plumber's putty is for sealing around sink overflows and faucet bases, not for slip nuts. Use only rubber washers and metal ferrules — they are designed to compress and seal under the nut, and they cost almost nothing.

How do I know if my trap is the right size?

Measure the diameter of the pipe where it connects to the sink drain — usually 1.25 inches for bathrooms or 1.5 inches for kitchens. Measure the distance from the wall drain opening to the sink drain opening as well. Bring both measurements to the hardware store, or take a photo of your current setup and show it to a staff member.

What should I do with the old trap after I remove it?

If it is plastic, you can throw it in the trash. If it is metal and in decent condition, some scrap metal recyclers will take it for a small amount of money. Check your local recycling center or call ahead to ask.