How To Rough In Plumbing In Concrete Slab – Step-by-Step Guide for New Builds

Why Getting Plumbing Right in a Concrete Slab Matters (And Why Most People Get It Wrong)

You’re building a new home—or maybe adding a garage, shed, or basement—and you’ve hit the big decision: how to rough in plumbing in concrete slab. It’s one of those “do it right the first time” moments. Get it wrong, and you’re looking at jackhammering through 6 inches of reinforced concrete later. That’s not just expensive—it’s a nightmare.

The good news? If you plan carefully, follow code, and use the right materials, roughing in plumbing before the slab pours can save you thousands and avoid years of headaches. Whether you’re a DIYer with a plan or a contractor on a tight schedule, this guide gives you everything you need—step by step—with real-world insights and expert-backed advice.

What Does “Rough In Plumbing” Actually Mean?

Before we dive into the how, let’s clarify the what.

Rough-in plumbing means installing all the main supply lines, drain pipes, vent stacks, and stub-outs before the concrete slab is poured. Think of it as laying the skeleton of your home’s water system—everything gets buried, then covered up. Once the slab sets, you can’t move pipes without major demolition.

According to the International Residential Code (IRC), all rough-in plumbing must be inspected and approved before concrete placement. Skipping this step? That’s not just risky—it’s illegal in most U.S. jurisdictions.

“A poorly planned slab rough-in is one of the top 3 reasons for costly retrofitting in new construction,” says Mike Smith, a licensed master plumber with 22 years of experience in Texas. “Most mistakes happen because people assume ‘it’ll fit’—then it doesn’t.”

Step 1: Plan Your Layout Before You Pour

You can’t install what you haven’t designed.

Start by marking the exact locations of:

• Kitchen sink
• Bathroom fixtures (toilet, shower, vanity)
• Laundry area
• Outdoor hose bibs (if applicable)

Use graph paper or free tools like SketchUp Free or Floorplanner.com to map out pipe runs. Keep these rules in mind:

Source: International Plumbing Code (IPC) Section 704

Pro Tip: Always design your layout so pipes slope toward the main sewer line or septic tank. A 1/4-inch drop per foot is the gold standard. Too flat? Clogs. Too steep? Solids don’t flush properly.

Step 2: Choose the Right Pipes for Slab Applications

Not all pipes are created equal when buried under concrete.

Best Choices:

• PVC (Polyvinyl Chloride) – Most common for drains and vents. Resists corrosion, easy to cut, and code-compliant. Use Schedule 40.
• HDPE (High-Density Polyethylene) – Flexible, great for areas with shifting soil. Used in commercial and some residential slab applications.
• Cast Iron – Durable but heavy. Rare in new residential builds due to cost and weight.
• CPVC or Copper? Avoid for drains under slab. Copper can corrode from soil chemicals. CPVC is okay for supply lines if protected.

“For supply lines under slab, I only use PEX-A with a protective sleeve. It’s flexible, freeze-resistant, and won’t crack under pressure.” – John Rivera, Certified Plumbing Inspector, California

Critical Rule: All pipes under slab must be sleeved if they’re metal or rigid plastic. Use 2” diameter PVC or HDPE sleeves around supply lines to allow for movement and future repair access.

Step 3: Install Drain, Waste, and Vent (DWV) System First

This is where most DIYers panic. Don’t worry—it’s simpler than it looks.

Here’s How to Do It Right:

1. Lay gravel base (4–6” deep) over compacted soil. This improves drainage and prevents pipe settling.
2. Place drain lines with consistent 1/4” slope. Use a laser level or string line to check.
3. Install cleanouts every 50 feet and at every 90° bend. Use 4” PVC cleanout plugs—these are your future lifeline.
4. Stub out all fixture locations:
   • Toilet: 12” or 10” rough-in from finished wall (standard is 12”)
   • Shower: 2” drain stub-out centered under future pan
   • Sink: 1.5” drain + 1.25” vent stub
5. Vent stack must rise vertically and connect to the main stack. It must extend above the roofline later—don’t cap it now!

Pro Tip: Use no-hub couplings or rubber sleeves for connections. They handle minor shifting better than glued joints.

Step 4: Install Water Supply Lines (Hot & Cold)

Now, run your potable water lines.

• Use PEX-A tubing (cross-linked polyethylene). It’s flexible, freezes less, and doesn’t corrode.
• Avoid PVC or copper for supply lines under slab—they’re rigid and prone to failure from soil movement.
• Sleeve every PEX line in 2” diameter PVC conduit. This protects against abrasion and lets you pull new lines later if needed.
• Label everything with waterproof tape: “HOT,” “COLD,” “TOILET,” “SHOWER.” You’ll thank yourself later.

Pressure Test: Before pouring concrete, pressure-test your supply lines at 80 PSI for 30 minutes. If pressure drops >2 PSI, you have a leak. Fix it. Now.

“I’ve seen 3 homes where homeowners skipped the pressure test. All three had leaks that showed up 6 months later—under tile, under carpet. Total cost: $18,000 to repair.” – Lisa Tran, Home Inspector, Denver

Step 5: Coordinate With Other Trades & Get the Inspection

Plumbing doesn’t exist in a vacuum.

• Coordinate with your electrician: Don’t run electrical conduits too close to plumbing (min. 6” separation per NEC).
• Confirm with your foundation crew: They need to know where pipes are so they don’t crush them during pour.
• Schedule a pre-slab inspection with your local building department. Bring your plans, pipe layout, and test results.

“In 2023, 68% of failed slab inspections were due to missing cleanouts or untested lines.” – National Association of Home Builders (NAHB) Report

Once approved, you’re cleared to pour.

Step 6: Pour the Slab – And Protect Your Pipes

The concrete pour is the most nerve-wracking moment.

• Use a concrete pump if possible. It’s smoother and less likely to shift pipes.
• Place a layer of sand or foam padding (1–2”) above pipes to cushion them.
• Do NOT let heavy machinery drive over the pipes before the slab cures (72 hours minimum).
• Cover stub-outs with plastic caps or tape to prevent concrete from clogging them.

Pro Tip: Mark the location of every pipe on the ground with spray paint before pouring. Take a photo. You’ll need it when installing fixtures later.

After the Slab Cures: What Comes Next?

Once the slab has cured (typically 7–14 days), you can:

• Remove caps from stub-outs
• Install final fixtures (faucets, toilets, etc.)
• Connect to the main sewer line
• Final inspection

Note: If you’re doing a slab-on-grade home, your water heater, sump pump, and gas lines will go on top of the slab—so plan their locations accordingly.

FAQ: Your Top Questions About Roughing In Plumbing in Concrete Slabs

Q1: Can I add plumbing after the slab is poured?

Technically, yes—but it’s a last resort. You’d need to core drill through the slab (cost: $200–$500 per hole), install a trench, and patch the concrete. It’s messy, expensive, and often violates code. Always rough in before pouring.

Q2: How deep should pipes be under the slab?

Minimum 4 inches below the bottom of the slab. Most codes require 6–8” for drain lines to allow for proper slope and protection. Check your local building code—some areas (like Florida) require deeper burial due to frost or soil conditions.

Q3: Do I need a vapor barrier under the slab?

Yes. A 6-mil polyethylene vapor barrier is required by the International Building Code (IBC) to prevent moisture from rising into your home. Place it under the gravel, before pipes go in.

Q4: Can I use PVC for water supply lines under slab?

No. PVC is not approved for potable water under slab in most U.S. states due to long-term brittleness risk. Use PEX-A instead. It’s flexible, approved by NSF/ANSI 61, and rated for 100+ years of service. Learn more about plumbing material standards on Wikipedia .

Q5: How much does it cost to rough in plumbing for a 2-bath home on slab?

Typical range: $3,000–$7,000, depending on complexity. Labor is about $40–$80/hour. DIY can cut costs by 50%—but only if you’re confident and code-compliant.

Q6: What’s the biggest mistake people make?

Skipping the pressure test and cleanouts. I’ve seen homes where the toilet line clogged 3 months after move-in—and the only way to fix it was to jackhammer the bathroom floor. Don’t be that person.

Final Thoughts: Do It Right, Once and For All

Roughing in plumbing in a concrete slab isn’t just a task—it’s a foundational decision that impacts your home’s value, safety, and comfort for decades. When done right, it’s invisible. When done wrong? It’s a $10,000+ emergency.

By following this guide—planning your layout, using the right materials, testing your lines, and getting inspections—you’re not just installing pipes. You’re investing in peace of mind.

Ready to take the next step?
👉 Share this guide with your builder, contractor, or friend who’s planning a new build.
👉 Save it for your next project.
👉 Tag someone who needs to see this before they pour concrete.

Because when it comes to plumbing under slab—there’s no second chance.