Roughing in bathroom plumbing beneath a concrete slab can feel intimidating—especially if you’ve never done it before. One wrong measurement or misplaced pipe can lead to leaks, expensive repairs, or failed inspections. But don’t worry: with the right planning and execution, how to rough in bathroom plumbing in concrete slab becomes a manageable, even rewarding, part of your home build or remodel. This guide walks you through every critical step, backed by industry standards and real-world best practices.
Why Is Plumbing Rough-In Under Slab So Critical?
Before concrete is poured, all supply and drain lines must be precisely placed. Once the slab cures, accessing or adjusting pipes becomes nearly impossible without jackhammering—a costly and disruptive process. According to the International Residential Code (IRC), plumbing under slabs must meet strict slope, material, and venting requirements to prevent backups, corrosion, and health hazards.
💡 Pro Tip: The National Association of Home Builders (NAHB) reports that over 60% of slab foundation plumbing issues stem from improper rough-in during initial construction.
What Tools & Materials Do You Need?
Gathering the right supplies upfront saves time and prevents mid-project delays. Here’s your essential checklist:
Tools:
Pipe cutter or hacksaw
Tape measure (25-ft minimum)
Laser level or string line
PVC/CPVC glue & primer
Drill with hole saw bits (for penetrations)
Shovel or trenching tool
Materials:
Drain-waste-vent (DWV) pipes: Typically 3″ or 4″ PVC for toilets, 2″ for showers/tubs, 1.5″ for sinks
Supply lines: PEX or copper (PEX is preferred for flexibility and freeze resistance)
Pipe supports: Rebar chairs or plastic saddles to hold pipes off the ground
Cleanouts: Required by code for future maintenance
Foam pipe insulation (optional but recommended in cold climates)
📌 Note: Always check local building codes—some municipalities require specific pipe types or inspection schedules.
Step-by-Step: How to Rough In Bathroom Plumbing in Concrete Slab
Follow these 7 precise steps to ensure a code-compliant, leak-free installation.
1. Review the Bathroom Layout & Obtain Permits
Start with your architectural or plumbing plan. Confirm fixture locations (toilet, sink, shower) and verify distances from walls. Most toilets require a 12-inch rough-in from the finished wall to the center of the flange—but 10″ or 14″ options exist. ✅ Action: Submit plans to your local building department and secure a plumbing permit. Skipping this risks fines or failed inspections.
2. Mark Pipe Locations on the Subgrade
Using spray paint or chalk, mark where each pipe will run. Include:
Pressurize supply lines to 80 PSI for 2 hours (no drop = pass).
Fill DWV system with water for 15 minutes—no leaks allowed.
Schedule a rough-in inspection with your local authority. Do not pour concrete until approved!
7. Protect Pipes During Concrete Pour
Cover pipe ends with caps or tape to prevent concrete intrusion.
Alert the concrete crew about pipe locations—use stakes or flags.
Avoid heavy machinery directly over pipes to prevent crushing.
Common Mistakes to Avoid
Incorrect toilet flange height
Wax ring leaks, rocking toilet
Set flange on temporary block matching slab thickness
Insufficient drain slope
Slow drainage, clogs
Use laser level; verify ¼” per foot
No cleanouts
Impossible to snake clogs
Install cleanout within 5 ft of fixture
Skipping pressure test
Hidden leaks under slab
Always test—twice if needed
Should You DIY or Hire a Pro?
While ambitious DIYers can handle this, consider hiring a licensed plumber if:
You’re unfamiliar with local codes
The project involves multiple bathrooms
Your area has strict frost-depth requirements
According to HomeAdvisor, the average cost to rough-in bathroom plumbing under slab is $1,200–$3,500, but a mistake can cost $5,000+ in slab repairs.
FAQ Section
Q: How deep should plumbing be under a concrete slab?
A: Drain lines should be 18–24 inches deep, supply lines 6–12 inches. Depth varies by climate—consult local codes for frost line requirements.
Q: Can you move plumbing in a slab foundation later?
A: Technically yes, but it requires cutting the slab, rerouting pipes, and repouring concrete—costly and messy. That’s why accurate rough-in is crucial.
Q: What type of pipe is best for under-slab plumbing?
A: PVC for drains, PEX for supply lines. PEX resists corrosion and expands under freeze conditions, reducing burst risk.
Q: Do I need a vent for every fixture under a slab?
A: Yes—but vents can tie into a common stack. The IRC requires all fixtures to be vented within specific distances (e.g., sink within 5 ft of vent).
Q: How long does rough-in plumbing take before pouring concrete?
A: For one bathroom, 1–2 days for a skilled DIYer or pro. Always allow time for inspection scheduling.
Q: What if I forget to install a cleanout?
A: You’ll likely fail inspection. If concrete is already poured, you’ll need an above-slab cleanout or expensive slab cut—avoid this by planning ahead.
Conclusion
Mastering how to rough in bathroom plumbing in concrete slab is a game-changer for builders, remodelers, and serious DIYers. By following precise measurements, using quality materials, and adhering to code, you’ll avoid the nightmare of post-pour plumbing failures. Remember: measure twice, cut once, and always get inspected.
Found this guide helpful? Share it with a friend tackling their own bathroom build—or pin it for your next project! 💧🔧
Your future self (and your wallet) will thank you.
Leave a Reply