How to Run a Plumbing Vent for Shower and Tub

If your shower gurgles or your tub drains slowly, the culprit might be a missing or improperly installed plumbing vent. Without proper venting, your drainage system can’t function efficiently—and worse, it may violate local building codes. In this guide, you’ll learn exactly how to run a plumbing vent for shower and tub, whether you’re a weekend DIYer or a seasoned contractor. We’ll walk you through everything: from why vents matter to step-by-step installation that meets modern plumbing standards.

Why Do Showers and Tubs Need a Plumbing Vent?

Every plumbing fixture in your home—including showers and bathtubs—requires a vent to maintain proper air pressure in the drainage system. Without it, negative pressure can form as water flows down the drain, causing:

• Slow drainage
• Gurgling sounds
• Sewer gas odors (due to trap siphoning)
• Potential health hazards

According to the International Residential Code (IRC), all fixtures must be vented within a specific distance from the trap (typically 5 feet for 1.5″ pipes, 6 feet for 2″ pipes). Ignoring this rule isn’t just risky—it’s often illegal.

💡 Did You Know? A study by the Plumbing-Heating-Cooling Contractors Association (PHCC) found that over 30% of drainage issues in new builds stem from improper or missing venting.

What Happens If You Don’t Vent a Shower or Tub?

Imagine drinking a thick milkshake through a straw with your finger covering the top—you’d struggle to get liquid out. That’s essentially what happens in an unvented drain.

Without a vent:

• Water flow creates a vacuum behind it.
• This vacuum pulls water out of the P-trap (the U-shaped bend under your tub).
• Once the trap is empty, sewer gases enter your bathroom—posing odor and health risks.

The U.S. Environmental Protection Agency (EPA) warns that prolonged exposure to sewer gases like hydrogen sulfide can cause headaches, nausea, and respiratory irritation—even at low concentrations.

Types of Plumbing Vents for Showers & Tubs

Not all vents are created equal. Here are the most common options:

⚠️ Note: While AAVs (like Studor vents) are permitted under the IRC and many local codes, some municipalities still ban them. Always check with your local building department first.

For more on venting principles, see Plumbing Vent Systems on Wikipedia.

Step-by-Step: How to Run a Plumbing Vent for Shower and Tub

Follow these steps carefully. Safety first—turn off water supply and wear eye protection.

Step 1: Confirm Local Code Requirements

• Most U.S. jurisdictions follow the IRC P3108 or UPC 704 venting rules.
• Maximum distance from trap to vent:
  • 2-inch drain line: up to 6 feet
  • 1.5-inch drain line: max 5 feet

Step 2: Choose Your Vent Type

• If walls are open (new build): use a conventional vent tied into the main stack.
• If remodeling: consider an AAV (if allowed) installed 6 inches above the fixture’s flood rim.

Step 3: Install the Drain & Trap

• Use 2-inch PVC or ABS pipe for tubs/showers (required by code for fixtures with high flow).
• Install a P-trap directly under the drain outlet. Ensure it’s level and sealed with plumber’s putty or silicone.

Step 4: Run the Vent Pipe

• From the top of the trap arm, install a sanitary tee pointing upward.
• Connect 1.5-inch or 2-inch vent pipe (match drain size or go one size smaller).
• Slope the vent upward at 1/4 inch per foot toward the main vent stack or AAV location.

Step 5: Tie Into Main Stack or Install AAV

• Option A (Main Stack): Connect vent to the home’s main vent stack in the attic or wall. Extend through roof with proper flashing.
• Option B (AAV): Mount the valve vertically in an accessible cabinet or wall chase. Never bury it in a wall without an access panel.

Step 6: Test the System

• Fill the tub halfway and drain it quickly.
• Listen for gurgling.
• Flush nearby toilets—if they bubble, your vent is too small or too far.
• Perform a smoke test (professional method) or water test (DIY): pour 2 gallons of water rapidly—drain should clear in under 15 seconds with no noise.

Common Mistakes to Avoid

• ❌ Venting after the trap – The vent must connect before the trap weir.
• ❌ Using undersized pipe – Shower/tub drains need minimum 2-inch drain lines.
• ❌ Horizontal dry vents below flood level – Vents must rise vertically until at least 6″ above the highest fixture.
• ❌ Ignoring cleanouts – Add a cleanout plug near the trap for future maintenance.

FAQ Section

Q1: Can a shower and tub share the same vent?

Yes, if they’re close together (within 5–6 feet of the same trap arm) and the vent is properly sized. This is called a common vent or wet vent, often used when a shower and tub share a wall.

Q2: How far can a tub be from its vent?

Per IRC, a 2-inch trap arm (standard for tubs) can extend up to 6 feet from the P-trap to the vent connection. Measure horizontally—not along pipe bends.

Q3: Do I need a separate vent for a shower if there’s a sink nearby?

Possibly. If the sink is within 3 feet and uses a wet vent configuration, it can vent both fixtures—but only if the shared pipe is 2 inches or larger. Otherwise, each needs its own vent path.

Q4: Can I use an Air Admittance Valve (AAV) for my shower?

Yes, if your local code allows it. The AAV must be installed at least 4 inches above the horizontal branch drain and 6 inches above the fixture’s flood level (e.g., top of tub). Never install in a sealed cabinet without airflow.

Q5: What size vent pipe do I need for a shower or tub?

Typically 1.5-inch is sufficient for a single fixture. However, if wet-venting with other fixtures, you may need 2-inch. Always match or exceed the drain pipe size upstream of the vent connection.

Q6: Why does my tub gurgle when the toilet flushes?

This usually means shared drainage without proper venting. The toilet’s rapid flush creates suction that pulls air through the tub’s trap. Solution: add a dedicated vent or verify existing vent isn’t blocked.

Conclusion

Knowing how to run a plumbing vent for shower and tub isn’t just about avoiding slow drains—it’s about protecting your home’s air quality, complying with safety codes, and preventing costly repairs down the road. Whether you choose a traditional roof vent or a modern AAV (where permitted), correct installation ensures smooth, silent, and safe operation.

✅ You’ve now got the knowledge to vent like a pro.
If this guide saved you a service call or helped you pass inspection, share it with a friend or on social media—your DIY community will thank you!

Got questions? Drop them in the comments below—we’re here to help you build smarter, safer bathrooms.