How to Stub Up Plumbing Pipe for Second Story Toilet

Adding a bathroom on the second floor? One of the most critical—and often overlooked—steps is properly stubbing up the plumbing pipe for a second story toilet. Get it wrong, and you’ll face leaks, clogs, or even failed inspections. But don’t worry—whether you’re a seasoned DIYer or a first-time renovator, this guide walks you through the entire process with clarity, confidence, and code-compliant precision. Let’s make sure your second-story toilet drains smoothly for years to come.

Why Is Stubbing Up a Toilet Drain Pipe So Important?

When you stub up plumbing pipe for a second story toilet, you’re creating the vertical connection between the main waste line (usually in the basement or crawlspace) and the future toilet flange on the second floor. This isn’t just about running a pipe upward—it’s about slope, venting, material choice, and alignment.

According to the International Residential Code (IRC), all waste pipes must maintain a minimum slope of ¼ inch per foot to ensure proper drainage. A poorly stubbed pipe can lead to:

• Slow flushing
• Sewer gas infiltration
• Standing water in the bowl
• Costly rework during final inspection

“Rough-in errors account for nearly 30% of plumbing callbacks in residential remodels,” says Master Plumber Elena Rodriguez of the Plumbing-Heating-Cooling Contractors Association (PHCC).

What Tools and Materials Do You Need?

Before cutting into floors or walls, gather these essentials:

💡 Pro Tip: Always confirm your local building code. While most U.S. jurisdictions follow the IRC, some cities (like Chicago or Los Angeles) have stricter requirements for vertical waste stacks.

Step-by-Step Guide: How to Stub Up Plumbing Pipe for Second Story Toilet

Step 1: Locate the Main Stack

Start in the basement or crawlspace. Identify the main soil stack—the large vertical pipe (usually 3″ or 4″ diameter) that carries waste from upper floors to the sewer. Use a stud finder or consult original blueprints if available.

⚠️ Never cut blindly! Verify there are no electrical wires, HVAC ducts, or structural beams in your planned path.

Step 2: Plan the Vertical Run

From the main stack, plan a straight vertical path up through the floor joists to the second-floor bathroom location. The toilet drain should align directly above the stack whenever possible to minimize offsets.

• Use a plumb bob or laser level to mark the exact center point on the subfloor.
• Ensure the pipe runs within a plumbing chase or inside a wall cavity—not through living spaces.

Step 3: Cut Access Holes

Using a hole saw or reciprocating saw, carefully cut access holes through each floor joist. Per IRC Section R602.6, holes must be:

• At least 2 inches from the top or bottom of the joist
• No larger than ⅓ the depth of the joist

For example: In a standard 2×10 joist (actual depth: 9.25″), max hole diameter = ~3.1″.

Step 4: Install the Vertical Waste Pipe

1. Cut a length of 3″ or 4″ PVC DWV pipe (Schedule 40) long enough to reach from the main stack to 6–8 inches above the finished second-floor surface.
2. Dry-fit the pipe to ensure alignment.
3. Apply PVC primer, then cement, and join sections securely.
4. Secure the pipe every 4 feet vertically using plastic or metal pipe hangers anchored to framing.

📏 Critical Measurement: The top of the stub-out should be 12–14 inches above the subfloor. This allows room for flooring (e.g., tile adds ¾”) and proper flange installation.

Step 5: Cap and Protect the Stub-Out

Temporarily cap the pipe with a test cap or screw-on plug to prevent debris entry during construction. Label it clearly (“TOILET DRAIN – DO NOT REMOVE”).

🔗 For more on plumbing venting requirements, see Wikipedia’s overview of Drain-Waste-Vent systems.

Common Mistakes to Avoid

Even experienced contractors slip up here. Watch out for:

• Incorrect slope: Horizontal offsets must slope downward toward the stack—never flat or uphill.
• Wrong pipe size: Toilets require minimum 3″ drain lines; 4″ is better for second-story installations to reduce clog risk.
• Missing cleanouts: Install a cleanout fitting near the base of the stack for future maintenance.
• Ignoring venting: Every toilet needs a vent within 6 feet of the trap (per UPC). Without it, siphoning occurs, breaking the water seal.

PVC vs. ABS: Which Pipe Material Should You Use?

✅ Recommendation: Use PVC unless your local code specifies otherwise. It’s more widely accepted and UV-resistant if exposed during construction.

When to Call a Professional

While stubbing up a pipe seems straightforward, consider hiring a licensed plumber if:

• Your home has a slab foundation (requires core drilling)
• You’re tying into a cast iron stack (needs special adapters)
• Local codes require pressure testing before wall closure
• You’re unsure about venting or trap arm length

The average cost for professional rough-in plumbing ranges from $450–$1,200 per fixture (HomeAdvisor, 2025), but it’s worth it to avoid $5,000+ in water damage later.

FAQ Section

Q1: How high should I stub up the toilet drain pipe?

A: The top of the pipe should extend 12 to 14 inches above the subfloor. This accounts for underlayment, finished flooring (e.g., tile, hardwood), and ensures the closet flange sits flush with the final floor surface.

Q2: Can I offset the toilet drain horizontally?

A: Yes, but with limits. The horizontal trap arm (from flange to vertical stack) must not exceed 6 feet (per UPC) and must maintain a ¼” per foot slope. Avoid sharp 90° elbows—use two 45° bends instead for smoother flow.

Q3: Do I need a vent for a second-floor toilet?

A: Absolutely. Every plumbing fixture requires venting to equalize air pressure. Without it, flushing creates suction that empties the trap, allowing sewer gases into your home. The vent can tie into an existing stack or run separately through the roof.

Q4: What size pipe is required for a second-story toilet?

A: Minimum 3-inch diameter is standard, but 4-inch is recommended for upper floors to handle waste volume and reduce clogging—especially in homes with low-flow toilets that use less water per flush.

Q5: Can I stub up the pipe before framing walls?

A: Yes—and it’s often easier! Rough-in plumbing is typically done after framing but before drywall. However, if you’re certain of the toilet location, you can stub up through the subfloor early, then frame around it.

Q6: How do I test the stubbed pipe for leaks?

A: After gluing all joints, perform a water test: Cap the top, fill the pipe with water to the top, and observe for 15 minutes. No drop in water level = no leaks. Some jurisdictions require an air pressure test (5 psi for 15 minutes).

Conclusion

Stubbing up a plumbing pipe for a second-story toilet might seem like a small part of your renovation—but it’s foundational to your bathroom’s long-term performance. By following code-compliant slopes, using the right materials, and planning your venting carefully, you’ll avoid costly mistakes and ensure a smooth, odor-free flush every time.

If this guide helped you, share it with a friend tackling a bathroom upgrade—or pin it for your next project! Got questions? Drop them in the comments below. Happy plumbing! 💧🚽