How to Plumb a Bathtub Drain in an Old House (Step-by-Step Guide)

Renovating a bathroom in an old house? You’re not alone—nearly 40% of U.S. homes were built before 1970, according to the U.S. Census Bureau. If you’re tackling a clawfoot tub or restoring original fixtures, knowing how to plumb a bathtub drain in an old house is essential. Unlike modern builds, older homes often feature cast iron pipes, outdated venting, or non-standard layouts that demand extra care. This guide walks you through every step with clarity, safety, and respect for your home’s historic charm.

Why Is Plumbing a Bathtub Drain in an Old House Different?

Older homes—especially those built before 1950—commonly used galvanized steel or cast iron drainpipes, which corrode over time. Additionally, many lack proper venting by today’s standards, increasing the risk of slow drainage or sewer gas backup.

“Working on pre-1960 plumbing requires understanding legacy materials and adapting modern codes without compromising structural integrity,” says Maria Lopez, a licensed master plumber with 20+ years of experience in historic home restoration.

Key challenges include:

• Non-standard pipe diameters (older drains may be 1¼” instead of today’s standard 1½”)
• Fragile or rusted connections
• Absence of P-traps (required by modern plumbing codes to prevent sewer gases)
• Floor joists that can’t be easily cut for new drain lines

Before you begin, confirm your local plumbing code allows your planned modifications. Many municipalities require permits for drain work—even in DIY projects.

Tools & Materials You’ll Need

Gather these before starting:

✅ Tools:

• Adjustable wrench
• Pipe cutter or hacksaw
• Plumber’s putty or silicone sealant
• Tape measure
• Level
• Bucket and rags
• Flashlight or headlamp

✅ Materials:

• New bathtub drain assembly (check compatibility with your tub type)
• PVC or ABS pipe (1½” diameter, unless matching existing system)
• P-trap kit (if not already present)
• Pipe primer and cement (for PVC)
• Thread seal tape (for threaded connections)
• Optional: Fernco rubber coupling (for connecting dissimilar pipes)

💡 Pro Tip: Always shut off the main water supply and verify no water remains in lines before cutting pipes.

Step-by-Step: How to Plumb a Bathtub Drain in an Old House

Step 1: Remove the Old Drain Assembly

1. Access the drain from below (via basement or crawl space) if possible. If not, work from inside the tub.
2. Use a drain removal wrench or large channel-lock pliers to unscrew the drain flange from the waste-and-overflow assembly.
3. Disconnect the overflow pipe and waste pipe carefully—old brass or galvanized fittings may snap if forced.

⚠️ Warning: Avoid excessive torque. If stuck, apply penetrating oil and wait 15 minutes before retrying.

Step 2: Inspect Existing Pipes

Check for:

• Corrosion or cracks in cast iron or galvanized pipes
• Sagging sections that could cause standing water
• Proper slope: Drain lines must pitch ¼ inch per foot toward the main stack

If pipes are severely degraded, replace them with modern PVC or ABS—but consult a pro if structural changes are needed.

📌 Note: The International Residential Code (IRC) requires all new drain installations to include a P-trap within 24 inches of the fixture. Older homes often lack this—don’t skip it!

Step 3: Install the New Drain Assembly

1. Apply plumber’s putty around the underside of the new drain flange.
2. Insert the flange into the tub’s drain hole and screw on the drain body from below.
3. Tighten until snug—do not overtighten, as this can crack porcelain or warp threads.

For clawfoot tubs, ensure the waste-and-overflow linkage aligns correctly with the lever mechanism.

Step 4: Connect to the Drain Line

1. Measure the distance from the tub drain outlet to the existing stack or new P-trap location.
2. Cut PVC/ABS pipe to length using a pipe cutter for clean edges.
3. Dry-fit all pieces first to ensure proper alignment and slope.
4. Once confirmed, apply primer and cement to PVC joints (or use rubber couplings for mixed-material transitions).

🔗 For more on pipe materials and their history, see Plumbing on Wikipedia.

Step 5: Test for Leaks

1. Fill the tub with 2–3 gallons of water.
2. Open the drain and observe all connections for 3–5 minutes.
3. Check under the tub and in the basement/crawl space for drips.

If leaks appear, disassemble, clean joints, and re-cement or reseal.

Common Mistakes to Avoid

When to Call a Professional

While many homeowners successfully complete this project, call a licensed plumber if:

• You encounter lead pipes (common in homes built before 1930)
• The main stack needs rerouting
• Local codes require inspections for drain modifications
• You smell sewer gas after installation

According to HomeAdvisor, the average cost to replumb a bathtub drain professionally ranges from $400 to $1,200—but avoiding a mistake could save thousands in water damage.

FAQ Section

Q1: Can I connect a modern PVC drain to old cast iron pipes?

Yes, but use a Fernco rubber coupling (also called a no-hub connector). These flexible adapters create a watertight seal between dissimilar materials and absorb minor movement—ideal for aging structures.

Q2: Do I need a permit to replace a bathtub drain in an old house?

It depends on your municipality. In cities like Boston or San Francisco, any alteration to the drainage system requires a permit and inspection. Check with your local building department first.

Q3: What’s the ideal height for a bathtub drain outlet?

The drain outlet should sit 8–10 inches above the finished floor to allow space for the P-trap and maintain proper slope. Measure before cutting!

Q4: Why does my old tub gurgle when draining?

This usually indicates improper venting. Without a vent, air can’t replace the water flowing out, creating suction that pulls water from the P-trap. Install an Air Admittance Valve (AAV) if running a traditional vent isn’t feasible.

Q5: Can I reuse the old overflow plate?

Only if it’s not corroded or cracked. Vintage plates often have unique threading—measure the diameter and thread pitch before buying a replacement.

Q6: How long does this project take?

For a DIYer with basic tools: 4–8 hours. Add time if replacing rotted subfloor or rerouting pipes.

Conclusion

Learning how to plumb a bathtub drain in an old house blends craftsmanship with code compliance. With careful planning, the right materials, and respect for your home’s legacy, you can achieve a functional, leak-free system that honors its history. Not only will you avoid costly repairs down the road—you’ll add authentic value to your property.

Found this guide helpful? Share it on Pinterest, Facebook, or with a fellow old-house enthusiast! And if you’ve tackled this project yourself, drop your tips in the comments below—we’d love to hear your story.