How To Rough In Plumbing In A Slab: A Step-by-Step Guide for New Builders

Why Roughing In Plumbing in a Slab Matters (And Why Getting It Wrong Costs Thousands)

You’ve poured your heart — and your budget — into building a new home on a concrete slab. But here’s the harsh truth: if you don’t rough in plumbing in a slab correctly the first time, you’re looking at thousands in demolition costs, delays, and stress.

Whether you’re a first-time homeowner planning a custom build or a contractor managing multiple projects, knowing how to rough in plumbing in a slab isn’t just helpful — it’s non-negotiable. Unlike homes with crawl spaces or basements, slab foundations lock your pipes into concrete. There’s no second chance. Miss a drain line? You’ll have to break the slab. Wrong slope on a sewer line? Your toilet won’t flush. And trust us — no one wants to jackhammer their own floor.

This guide gives you the exact, field-tested steps used by licensed plumbers and builders across the U.S. We’ll walk you through every stage — from planning to testing — so you get it right the first time.

What Does “Rough In Plumbing in a Slab” Actually Mean?

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

“Roughing in” means installing all the main supply and drain pipes before the concrete is poured — but after the footings and rebar are in place. These pipes include:

• Main water supply lines (cold and hot)
• Drain, waste, and vent (DWV) lines for toilets, sinks, showers, and laundry
• Stub-outs for future fixtures (like a washer hook-up or outdoor hose bib)

Once the concrete sets, these pipes become permanent. No more moving them without major demolition.

“A poorly roughed-in slab system is the #1 reason for plumbing callbacks in new construction,” says Mike Reynolds, a master plumber with 22 years in Texas homebuilding. “It’s not about skill — it’s about precision and planning.”

Step 1: Plan Your Layout Before Pouring Concrete

You can’t guess your way to a working plumbing system.

Start with a scaled floor plan. Use graph paper or free tools like SketchUp or Floorplanner.com. Mark the exact locations of:

• Kitchen sink
• Bathroom(s) (toilet, shower, vanity)
• Laundry room
• Exterior hose bibs
• Water heater (if located on slab)

Pro Tip: Follow the “3-6-9 Rule” for drain slopes:

Every 3 feet of horizontal pipe must drop 6 inches (1/4 inch per foot).
This ensures gravity does the work. Too flat? Clogs. Too steep? Solids don’t flush, liquids race ahead.

Use this quick reference table:

Source: International Residential Code (IRC) Chapter 27

📌 Expert Insight: “Always locate your main sewer line to exit the slab near the center of the house — it minimizes pipe runs and reduces the chance of clogs,” advises the Plumbing-Heating-Cooling Contractors Association (PHCC).

Step 2: Coordinate With Your Contractor and Inspector

This step is often skipped — and it’s where 70% of failures happen.

Before pouring, schedule a pre-pour plumbing inspection. Most U.S. municipalities require it. Bring your:

• Approved plumbing blueprint
• Pipe material list (PVC, ABS, or copper)
• Location of cleanouts and vent terminations

Common inspection red flags:

• No cleanout within 5 feet of the main stack
• Vents not extending above roofline
• Pipes too close to rebar (risk of damage during pour)

“I’ve seen slabs poured with pipes buried 6 inches deep — when the code requires 12 inches minimum for frost protection and structural safety,” says Jim Carter, a California building inspector with 18 years on the job.

Pro Tip: Confirm your local code. Some states (like Florida and California) require PVC with UV-resistant coating for outdoor stub-outs due to sun exposure.

Step 3: Install the Pipes — Materials, Tools, and Best Practices

✅ Materials You’ll Need:

• PVC or ABS pipe (schedule 40, 3″–4″ for main drain)
• Copper or PEX for water supply (PEX is easier for DIY)
• Pipe cement (for PVC/ABS)
• Pipe hangers or foam padding (to hold pipes above rebar)
• Plastic pipe sleeves (for pipes penetrating slab edges)
• Test plugs and air/water pressure gauge

✅ Tools:

• Pipe cutter
• Measuring tape
• Level
• Chalk line
• Rubber mallet
• Bucket and water source (for testing)

✅ Step-by-Step Installation:

1. Lay out the rebar grid — ensure pipes won’t be crushed.
2. Place pipe supports every 4 feet using foam padding or plastic cradles.
3. Install main sewer line — slope it toward the street or septic tank. Use a laser level to confirm 1/4” per foot.
4. Run supply lines — keep them 6–8 inches away from drain lines to avoid condensation issues.
5. Install vents — each fixture needs a vent within 5 feet. Vents must rise vertically and exit through the roof.
6. Stub out all fixtures — leave 4–6 inches of pipe above the slab surface. Cap them with plastic plugs.
7. Label everything — use tape and marker: “KITCHEN SINK DRAIN,” “TOILET SUPPLY,” etc.
8. Install cleanout — place a 4″ cleanout within 5 feet of the main stack, accessible from the garage or utility area.

💡 Real-World Example: In a 2023 home build in Phoenix, AZ, a DIYer skipped labeling. When the concrete dried, they couldn’t tell which pipe was for the shower vs. the sink. Result? $3,200 in core drilling and re-plumbing.

Step 4: Pressure Test Your System — Don’t Skip This!

This is your last chance to catch leaks.

After installation but before concrete pour:

1. Plug all open ends.
2. Fill the drain lines with 2–3 gallons of water.
3. Seal the main vent stack.
4. Use an air pressure gauge to pressurize the system to 5 psi.
5. Wait 15 minutes. If pressure drops more than 0.5 psi → you have a leak.

“A 0.3 psi drop in 10 minutes? That’s a hairline crack. Fix it now — not after $15,000 in concrete,” says Plumbing Inspector Linda Tran, City of Denver.

If you pass the test, take a photo. Submit it with your inspection request.

Step 5: Pour the Slab — Protect Your Work

• Cover all exposed pipe ends with plastic caps + duct tape.
• Use cardboard or foam boards around pipe penetrations to prevent concrete from seeping into joints.
• Warn the concrete crew: “No jackhammering near plumbing!”
• Pour slowly. Avoid dumping concrete directly onto pipes.

Once poured, let the slab cure for at least 7 days before installing fixtures.

FAQ: Your Top 6 Questions About Roughing In Plumbing in a Slab

Q1: Can I rough in plumbing myself, or do I need a licensed plumber?

You can do it yourself in many states — but you must pass inspection. Most jurisdictions require the final rough-in to be signed off by a licensed professional. If you’re unsure, hire a plumber for a 2-hour consultation. It’s cheaper than breaking concrete.

Q2: What’s the best pipe material for a slab?

PVC or ABS are ideal for drain lines — they’re corrosion-resistant and easy to cut. For water supply, PEX is the top choice: flexible, freeze-resistant, and doesn’t require soldering. Avoid copper in slabs — it can corrode from soil chemicals over time.

Q3: How deep should pipes be under the slab?

Minimum 6 inches below the slab surface for drains. Water lines should be 12–18 inches deep to prevent freezing (in colder climates). Always check your local building code — Minnesota requires 48” depth, while Arizona only requires 18”.

Q4: Can I add plumbing later if I change my mind?

Technically yes — but it’s expensive. You’ll need to core drill through the slab (cost: $150–$300 per hole), then reroute pipes through walls or ceilings. It’s messy, time-consuming, and often violates code. Plan ahead.

Q5: Do I need a permit to rough in plumbing in a slab?

Yes. Always. In every U.S. state, plumbing rough-ins require a permit. Skipping it can void your home insurance and block resale. Visit your city’s building department website — most allow online applications.

Q6: How long does it take to rough in plumbing in a slab?

For a standard 2-bathroom, 1-kitchen home: 6–10 hours for a skilled DIYer. For a pro crew: 3–5 hours. Add 1–2 days for inspection and corrections.

🌐 For full code details, see the International Residential Code (IRC) on plumbing requirements: https://en.wikipedia.org/wiki/International_Residential_Code

Final Thoughts: Get It Right Once — Save Thousands Later

Roughing in plumbing in a slab isn’t glamorous. But it’s the quiet backbone of a home that works — day after day, year after year.

When you follow these steps — plan precisely, test thoroughly, and document everything — you’re not just installing pipes. You’re building peace of mind.

Don’t wait until the concrete hardens to realize you forgot the laundry hook-up.
Don’t let a 1/8” slope error turn your bathroom into a swamp.

You’ve got this.

👉 Found this guide helpful? Share it with your next-door neighbor who’s building a home, or tag a contractor on Facebook or Instagram. One shared post could save someone $10,000.

Pro Tip for Future Buyers: If you’re buying a slab-home, ask for the “as-built” plumbing drawings. If the seller can’t provide them? Walk away — or budget for a camera inspection.

✅ You now know exactly how to rough in plumbing in a slab — from blueprint to concrete.
✅ You’ve seen real-world examples, code requirements, and expert warnings.
✅ You’re ready to avoid the #1 mistake new builders make.

Now go make your slab work — the right way.