How to Replace a Section of Drain Pipe

Some drain pipe damage is beyond patching — a cracked ABS trap arm, a split PVC drain line, or a section chewed by rodents needs to come out entirely. Learning how to replace a section of drain pipe is one of the most empowering DIY plumbing skills: the materials cost under $30, the job takes an hour or two, and the technique is the same whether you're fixing a two-foot section under a sink or a ten-foot run in a crawlspace.

When Replacement Beats Repair

Patch the pipe when the damage is a pinhole or hairline crack in an otherwise sound line (see our guide on fixing a leaky pipe). Replace the section when:

• The pipe is cracked or split lengthwise — patches don't hold on split plastic.
• A section is crushed, deformed or bellied (sagging and holding water).
• The pipe is chewed by rodents — the damage is usually wider than it looks.
• A previous repair has failed — a joint that's been patched twice needs replacing.
• You're rerouting a short section for a remodel or fixture move.

This guide covers ABS and PVC drain pipe (DWV), the black and white plastic pipe in virtually all modern homes. If your damaged line is cast iron or galvanized steel, the cutting and joining methods differ — and that's usually a job for a pro.

Tools and Materials

Tools:

• Hacksaw with fine-tooth blade, plastic pipe cutter, or reciprocating saw
• Measuring tape and marker
• Deburring tool, utility knife or sandpaper (120-grit)
• Rags and a bucket
• Torpedo level (for checking slope)
• Safety glasses and gloves

Materials:

• Replacement pipe — same material and diameter as the existing (ABS for ABS, PVC for PVC; check the printed markings)
• Two couplings — repair couplings (which slide fully onto the pipe) make the job much easier in tight spaces than standard couplings
• Solvent cement + primer — ABS cement for ABS (no primer needed), PVC primer + cement for PVC
• Pipe straps or hangers for support

Material warning: never join ABS to PVC with the wrong cement. Standard ABS cement won't bond PVC and vice versa — use a listed transition cement or a mechanical (no-hub) coupling. Our guide on mixing ABS and PVC has the details.

Step 1: Plan Your Cuts

Measure the damaged area and mark your cuts at least 2 inches into solid, undamaged pipe on each side. For cracks, cut past the visible ends — hairline cracks extend further than they appear, so give yourself margin. If the damage is near a fitting, it's often easier to cut back to the fitting and replace from there.

Measure the gap between your cut marks. Your replacement piece will be this length minus the socket depth of the couplings — but don't cut the new pipe yet. Dry-fit first (Step 3), then mark and cut.

Check what's around the pipe before cutting: electrical wires, other pipes, ductwork. In walls and ceilings, cut a generous access opening — working blind guarantees a bad joint.

Step 2: Cut Out the Damaged Section

1. Make sure no water will flow through the line while you work — don't run fixtures upstream.
2. Support the pipe on both sides of your cuts if the run is long. An unsupported ten-foot run will drop when cut, stressing the remaining joints.
3. Cut square. A square cut seats fully into the coupling socket; an angled cut leaves a gap that becomes a leak path. Use a miter box with a hacksaw if freehanding worries you, or a rotary pipe cutter for the cleanest cut.
4. Remove the damaged piece and deburr all cut edges — inside and out. Burrs inside the pipe catch debris and start future clogs; burrs outside prevent full socket insertion.
5. Clean the pipe ends with a dry rag. Solvent cement needs clean, dry surfaces to weld properly.

Step 3: Dry-Fit the New Section

Dry-fitting — assembling everything without cement — is the step that separates good results from crooked, stressed joints. Solvent cement sets in seconds; there's no adjusting after.

1. Slide a repair coupling fully onto each existing pipe end (repair/slip couplings slide completely onto one pipe, then slide back to bridge the gap — that's what makes them work in tight spots where you can't flex the pipe).
2. Cut your replacement piece to fit between the couplings, accounting for how far pipe inserts into each socket (mark insertion depth on the pipe with a marker — typically 3/4 to 1 inch).
3. Assemble everything dry and check: does the new section sit naturally without forcing? Is the slope right (see below)? Are the couplings positioned where they'll end up?
4. Mark alignment with a marker line across each joint — when you cement, you'll reassemble to these marks.
5. Disassemble, keeping parts in order.

Step 4: Solvent-Weld the Joints

Work one joint at a time — cement waits for no one. For the full technique with photos-level detail, see our how to glue ABS pipe guide.

1. For PVC: apply purple primer to the outside of the pipe end and the inside of the coupling socket. For ABS: skip primer (ABS cement is self-priming).
2. Apply cement generously to the pipe end first, then a lighter coat inside the socket. Work fast.
3. Push the pipe into the socket with a quarter-turn twist, aligning to your marks. Hold for 30 seconds — the joint will try to push back out.
4. Wipe excess cement from the joint exterior.
5. Repeat for each joint, then let the assembly cure: 15 minutes before handling, 2 hours before running water through it (longer in cold weather).

Safety note: solvent cement fumes are strong — ventilate the area (open windows, run a fan). The cement is flammable: no open flames, no smoking, and keep it away from water heater pilot lights.

Getting the Slope Right

Drain pipe must slope downhill toward the sewer or septic — but not too much. The code requirement is:

• 1/4 inch per foot for pipe 2-1/2 inches and smaller
• 1/8 inch per foot is allowed for 3-inch and larger pipe

Too little slope and solids settle out, causing clogs. Too much slope and water outruns the solids, leaving them behind — also causing clogs. Use a torpedo level on top of the pipe: a 1/4-inch-per-foot slope reads as roughly level with the bubble just touching the line, or measure directly (a 4-foot section should drop 1 inch end to end).

If your replacement section won't hold slope on its own, that's what hangers are for — which brings us to support.

Supporting the Pipe

Horizontal drain pipe needs support every 4 feet (and within a foot of every joint, ideally). Unsupported pipe sags over time, creating bellies that trap solids and start the clog cycle. Use plastic pipe straps or J-hooks sized for your pipe — never wire, which cuts into plastic over time.

Vertical pipe needs support at each floor level. When you open a wall or ceiling for this repair, add strapping to any nearby unsupported runs you find — future you says thanks.

Common Mistakes

• Skipping the dry-fit. The number one cause of crooked, leaking DIY joints.
• Wrong cement for the material — or worse, mixing ABS and PVC with regular cement. Match cement to pipe, always.
• Not deburring. Internal burrs catch hair and debris; the "mystery recurring clog" six months later is often a burr from this repair.
• Forgetting slope. A replacement section installed dead level (or back-sloped) will clog repeatedly.
• Running water too soon. Give solvent welds the full cure time. A joint tested at 20 minutes that holds may still fail at 20 days.
• Cutting without checking what's behind. Wires, other pipes and ducts share wall cavities. Look before you cut.

When to Call a Professional

• The damaged section is inside a finished wall or ceiling and you're not comfortable opening it up (or the repair requires rerouting).
• The pipe is cast iron, galvanized or lead — different tools, different skills.
• Damage is extensive — multiple bad sections usually mean the whole run is failing.
• The line is the main building drain or sewer lateral — bigger pipe, bigger consequences, often permit and inspection required.
• You find mold, major water damage or structural issues when you open things up.

A plumber typically charges $200–500 to replace an accessible section of drain pipe, more if walls need opening and patching. DIY materials run $20–30 — but only DIY it if you can do the solvent welding properly. A bad joint inside a wall is far more expensive than the service call you skipped.