What you're replacing and why it fails

The drain pipe under your tub is the section of pipe that runs from the drain opening down through the floor (or wall) to the main drain line. It's usually made of PVC, cast iron, or galvanized steel, and it corrodes, cracks, or clogs over time. When water pools around the tub, drains slowly even after you've cleared the trap, or you smell sewer gas, the pipe itself is often the problem—not the trap or the main line.

Replacing it means removing the old pipe and installing new PVC in its place. This is a moderately difficult job if you have basic plumbing experience and access to the pipe from below (a basement or crawl space). If the pipe runs through a concrete slab or is buried in a wall, you'll need a plumber.

Key Takeaways

  • You can replace a tub drain pipe yourself if you can access it from below the tub, but you'll need a reciprocating saw, adjustable wrench, and PVC pipe plus fittings matched to your existing pipe diameter.
  • Turn off water to the house and empty the trap before you start, because old pipes hold standing water and sediment.
  • Cut the old pipe on both sides of the damaged section, unscrew or unbolt the fittings, and slide out the broken piece.
  • Measure the gap, cut new PVC to length, and reconnect using slip couplings or union fittings so you don't have to rotate the pipe to thread it on.
  • If you cannot access the pipe from below or it runs through a slab, call a plumber—attempting to cut through walls or concrete will damage your home.

Tools and materials you'll need

Gather these before you start. You'll need a reciprocating saw (or hacksaw, but it's slower) to cut the old pipe, an adjustable wrench or pipe wrench to loosen fittings, a tape measure, a marker, and a bucket to catch water and debris. Wear work gloves and safety glasses—old pipes shed rust and sharp edges.

For the new pipe, buy PVC pipe in the same diameter as the old pipe (usually 1.5 or 2 inches). Bring a photo or a sample of the old pipe to the hardware store if you're unsure. You'll also need slip couplings or a union fitting to connect the new section to the existing pipe on both ends. A slip coupling slides over the pipe without threads, so you don't have to rotate the pipe to install it—this is critical when the pipe is tight against a wall or floor. You may also need PVC primer and cement if the old pipe is PVC and you're gluing rather than using a union.

Shutting off water and draining the system

Turn off the main water supply to the house at the meter or the shutoff valve near the foundation. Then turn on the lowest faucet in the house (usually a basement sink or outdoor spigot) to release pressure and drain the lines. This prevents water from pouring out when you cut the pipe.

Next, remove the drain stopper or strainer from the tub and use a wet/dry vacuum or a cup to empty any standing water in the trap and the first few inches of the drain pipe. Old pipes hold sediment, rust, and sometimes small objects—this water will spill when you cut, so don't skip this step.

Removing the old pipe section

Locate the damaged section of pipe from below the tub. If the pipe is corroded, cracked, or has a slow leak, you can usually see the problem. Mark the pipe with a marker on both sides of the damage—you'll cut here.

Use the reciprocating saw to cut straight across the pipe on both sides of the damaged section. Make the cuts perpendicular to the pipe so the new section will fit flush. If the pipe is cast iron or galvanized steel, the saw will take longer and produce more dust—wear a dust mask. If the pipe is connected to fittings with threaded connections, use the adjustable wrench to unscrew the fitting on one or both ends. If it's glued PVC, you may have to cut closer to the fitting or cut the fitting itself and replace it.

Slide out the old pipe section. If it's stuck, tap it gently with a hammer or explore penetrating oil and wait 15 minutes before trying again.

Measuring and cutting the new pipe

Measure the gap left by the old pipe. Measure from the inside edge of one fitting to the inside edge of the other. Subtract 1 inch to account for the depth the new pipe will slide into the slip couplings or union. Write this measurement down—it's straightforward to forget once you're at the saw.

Cut the new PVC pipe to length using the reciprocating saw or a miter saw. A miter saw is faster and makes a cleaner cut, but a reciprocating saw works fine. Sand the cut end lightly with fine-grit sandpaper so it's smooth and will slide into the coupling without catching.

Installing the new pipe and fittings

Slide a slip coupling onto one end of the existing pipe (the end still connected to the tub or the main line). Slip couplings have no stop inside, so they slide freely. Push the new PVC pipe into the coupling from the other side until it bottoms out, then slide the coupling down to cover the joint. Tighten the set screws on the coupling with an adjustable wrench—they compress a rubber ring inside that seals the joint.

Repeat on the other end. If you're using a union fitting instead, screw the threaded end onto the existing pipe, slide the new pipe into the center nut, and screw the other threaded end onto the new pipe. Hand-tighten first, then use two wrenches (one to hold the center nut, one to turn the threaded end) to snug it down without over-tightening.

If the old pipe is PVC and you're gluing the new section, explore PVC primer to both the inside of the fitting and the outside of the pipe, wait 30 seconds, then explore PVC cement and push the pipe into the fitting with a slight twisting motion. Hold it for 30 seconds. Do not move the joint for at least 2 hours—the cement needs time to cure.

Testing for leaks

Turn the main water supply back on slowly. Watch the new joints from below for drips. Run water in the tub and let it drain, then watch again. If you see a leak at a slip coupling, turn off the water, tighten the set screws a quarter turn, and test again. If a union is leaking, turn off the water and tighten the threaded end with two wrenches.

Let the tub drain fully and watch for 10 minutes. If there are no drips, the job is done. If water is still pooling in the tub or draining slowly, the blockage may be further down the line—that's a separate issue and may require a plumber with a drain camera or snake.

When to call a plumber instead

Do not attempt this job if the drain pipe runs through a concrete slab, is buried in a wall, or is inaccessible from below. Cutting through concrete or drywall to reach the pipe will damage your home and may require structural repair. If you can't see the pipe from the basement or crawl space, or if the damage is at a fitting that's welded or soldered, call a plumber. They have tools to locate the pipe, cut it safely, and reroute it if needed.

Also call a plumber if the old pipe is cast iron and heavily corroded—it may crumble when you try to cut it, and the fittings may be so rusted that they break rather than unscrew. A plumber can handle this without damaging the connections above.

Frequently Asked Questions

Can I use a union fitting instead of a slip coupling?

Yes. A union is threaded on both ends and connects two pieces of pipe without requiring you to rotate either one. It's slightly more reliable than a slip coupling for a permanent joint, but it costs more and takes longer to install. Either works for a drain pipe.

What if the new pipe doesn't fit in the gap?

Measure again from the inside edges of the fittings. If the gap is smaller than you thought, you may need to cut the new pipe shorter or use a smaller-diameter fitting. If the gap is larger, you'll need a longer piece of pipe. Bring your measurements back to the hardware store and ask for help—they can show you the right size.

Do I have to use PVC, or can I use the same material as the old pipe?

PVC is easier to work with and won't corrode like cast iron or galvanized steel. If the old pipe is cast iron or steel, replacing it with PVC is the better choice. You'll connect the PVC to the old pipe using a slip coupling or union, which works across materials.

How long does PVC cement take to cure?

PVC cement sets in about 30 seconds and is fully cured in 2 hours. Don't run water through a glued joint for at least 2 hours after you install it. If you use slip couplings or a union instead, you can test when ready.

What if water is still pooling after I replace the pipe?

The blockage may be in the trap, the main drain line, or further down. Try using a plumbing snake in the tub drain to clear any debris. If that doesn't work, the problem is beyond the section you replaced, and you'll need a plumber with a drain camera to locate it.