You’ve bought an older home, renovated the kitchen, upgraded the bathroom—but now you realize: there’s no plumbing in the concrete slab. Maybe you want to add a sink, a shower, or even a laundry room where none existed before. You’re not alone. Thousands of homeowners across the U.S. face this exact challenge every year. The good news? You can put plumbing in an existing concrete slab—but it’s not as simple as drilling a hole and calling it a day. Do it wrong, and you risk water damage, structural issues, or code violations. Do it right, and you unlock new value, comfort, and functionality in your home. In this guide, we’ll walk you through how to put plumbing in existing concrete slab—safely, legally, and without breaking the bank.
Why Can’t You Just Drill Holes Through the Slab?
Before we jump into the “how,” let’s clear up a dangerous myth: you cannot simply drill holes through a concrete slab and run pipes through them.
Concrete slabs aren’t just floors—they’re structural foundations. Drilling randomly can:
Compromise load-bearing capacity
Hit rebar or post-tension cables (which can snap violently)
Create pathways for moisture, leading to mold or foundation cracks
According to the American Society of Civil Engineers, over 60% of slab failures in retrofit projects stem from unauthorized drilling or improper pipe routing. That’s why professional plumbers and structural engineers always start with a slab scan.
💡 Pro Tip: Always hire a certified concrete scanning service (cost: $150–$300) before any cutting. They use ground-penetrating radar (GPR) to map rebar, conduits, and post-tension cables beneath your slab. This single step can save you $10,000+ in repairs.
Step 1: Plan Your Plumbing Layout (And Get Permits)
Question: Can I add plumbing without a permit?
Answer: No—and here’s why.
Most U.S. municipalities require permits for any plumbing work involving concrete slabs. Why? Because improper installation can lead to:
Sewage backups
Water contamination
Structural collapse
What you need:
A detailed plumbing layout (showing drain, vent, and supply lines)
Approval from your local building department
A licensed plumber to sign off (required in 48 states)
📌 Real Case: In 2023, a homeowner in Phoenix tried to install a bathroom without a permit. When a pipe leaked months later, their insurance denied the claim—costing them $22,000 in repairs. Always get the permit.
Tools & Materials Needed:
Tape measure & chalk line
Concrete saw (rental: $80–$120/day)
Jackhammer (for larger cuts)
PVC or PEX piping (schedule 40 PVC recommended for drains)
Pipe clamps, couplings, and sealant
Dust mask, safety goggles, ear protection
Step 2: Cut the Slab — The Right Way
Question: How deep do I cut into the concrete to install pipes?
You need to cut a trench—not just a hole. The trench must be deep enough to:
Allow proper slope for drains (minimum 1/4 inch per foot)
Accommodate pipe diameter + insulation + backfill
Standard Trench Dimensions:
2″ Drain
8–10 inches
6–8 inches
3″ Drain
10–12 inches
8–10 inches
Water Supply
6–8 inches
5–6 inches
Step-by-Step Cutting Process:
Mark the path with chalk—follow your approved layout.
Use a concrete saw with a diamond blade. Cut 2 inches deep first to create a groove.
Switch to a jackhammer to break out the center. Work slowly—avoid hitting rebar.
Clean debris with a shop vacuum and broom.
Inspect the exposed slab—if you see rebar, stop. You’ll need to reroute the trench or consult a structural engineer.
⚠️ Warning: Never cut within 12 inches of the slab’s edge. That’s where the foundation is thinnest and most vulnerable.
Step 3: Install the Pipes — Drain, Vent, and Supply
Now that you’ve opened the path, it’s time to install the plumbing system. Here’s how to do it right.
A. Drain Line Installation
Use schedule 40 PVC for drains (cheaper and more durable than ABS in slab applications).
Slope the pipe at 1/4 inch per foot. Use a laser level to check.
Install a cleanout plug every 20 feet (required by IPC code).
Wrap the pipe in foam insulation to reduce condensation and noise.
B. Vent Pipe
Every drain needs a vent to prevent suction and gurgling. You have two options:
Through Roof
Code-compliant, best performance
Requires cutting roof, higher cost
Air Admittance Valve (AAV)
No roof penetration, easier install
Not allowed in all states (check local code)
🔍 Expert Insight: “AAVs are great for retrofits—but in humid climates like Florida or Louisiana, they can fail faster due to moisture buildup.” — Plumbing Today Magazine, 2024
If you’re unsure, go with a traditional roof vent. It’s the gold standard.
C. Water Supply Lines
Use PEX-A tubing (most flexible, freeze-resistant).
Run supply lines above the drain to avoid cross-contamination.
Use pipe clamps every 3–4 feet to secure them to the slab edge.
Pressure test the system at 80 psi for 30 minutes before pouring concrete.
Fill with high-strength patching compound (e.g., Sakrete High Strength Concrete Mix).
Trowel smooth and cure for 48 hours.
✅ Pro Tip: Add a waterproof membrane (like RedGard) over the patched area if it’s near a shower or sink. Prevents moisture seepage into the slab.
Step 5: Final Inspection & Testing
Before you tile or lay flooring:
Pressure test water lines (80 psi for 30 min — no drop allowed).
Smoke test drain lines (use a smoke machine to detect leaks).
Schedule a city inspector—they’ll check slope, venting, materials, and permits.
Take photos of the finished work for your records (and future home sale).
💬 Homeowner Story: “I thought I saved $3,000 by skipping the inspection. Two years later, my basement flooded because the drain had no slope. The inspector said I had to tear out the entire floor. Lesson learned.” — Mark T., Austin, TX
Pros vs. Cons: Installing Plumbing in a Concrete Slab
✅ Adds value to older homes
❌ High upfront cost ($2,000–$8,000)
✅ Creates functional spaces (bathrooms, kitchens)
❌ Time-intensive (3–7 days)
✅ Permanent, durable solution
❌ Risk of slab damage if done wrong
✅ No need to raise floor height
❌ Requires permits and inspections
✅ Long-term savings vs. moving
❌ Hard to access later for repairs
💡 Bottom Line: If you’re planning to stay in your home 5+ years, this is a smart investment. If you’re flipping, consider a raised platform instead.
Frequently Asked Questions (FAQ)
Q1: Can I install plumbing under a concrete slab without cutting it? No. You cannot run pipes under a slab without access. The only way is to cut a trench. Some homeowners try “slab-on-grade” systems with raised floors, but those aren’t true plumbing under slab—they’re above it.
Q2: How much does it cost to put plumbing in an existing concrete slab? Costs vary by region and complexity. On average:
DIY (materials only): $800–$1,500
Professional installation: $3,000–$8,000
Permits + inspections: $200–$500
Concrete scanning: $150–$300
Q3: How long does it take to install plumbing in a concrete slab? Typically 3–7 days, depending on:
Trench size
Weather (concrete needs 7 days to cure)
Permit wait times (some cities take 1–2 weeks)
Q4: Can I use PEX for drain lines? No. PEX is only for water supply. Use PVC or ABS for drains. PEX can’t handle gravity flow or solids and will clog or collapse.
Q5: What if I hit a post-tension cable while cutting? STOP IMMEDIATELY. Do not touch it. Post-tension cables are under extreme tension. If severed, they can whip back with lethal force. Call a structural engineer and your local utility locator (call 811).
Q6: Will this affect my home’s foundation? Only if done improperly. Cutting the correct trench depth, avoiding rebar, and properly curing the patch will not harm your foundation. But cutting too deep or too wide? That’s a recipe for cracks and settling.
Conclusion: You Can Do This—But Do It Right
Installing plumbing in an existing concrete slab isn’t a weekend DIY project for beginners—but with the right plan, tools, and respect for the process, it’s absolutely doable. Whether you’re adding a half-bath, a laundry room, or upgrading your kitchen, you’re not just adding fixtures—you’re adding value, comfort, and functionality to your home.
Remember: ✔️ Scan the slab first ✔️ Get permits ✔️ Slope drains properly ✔️ Test before you cover ✔️ Hire a pro if you’re unsure
And if you found this guide helpful, share it with a friend who’s staring at their concrete floor wondering, “Can I even do this?” 📲 Tap the share button below—your next-door neighbor might be one click away from saving thousands.
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