Underground Plumbing for a Slab-on-Grade
- Staff Desk
- 2 hours ago
- 6 min read
If you’re building on a slab and you’re at the “underground” stage for plumbing, this is one of those weeks where small decisions pay dividends for decades. Below is a simple, field-tested walkthrough inspired by a real jobsite: bringing pressurized water and drains into the home, pressure-testing the drain system, and prepping the pad so you can place rebar and pour concrete with confidence.
Whether you’re an owner-builder or a GC keeping the schedule tight, use this as a checklist and sanity check.
The Two Systems You Must Rough-In Now
You’re bringing in:
Pressurized water service
Best practice today: PEX-A/PEX-B in a sleeve (a continuous conduit from outside into the mechanical area).
Why it matters: If the water line ever fails, you can pull a new line through the sleeve without jackhammering your slab.
Sanitary drainage/vent (DWV)
Includes your main building sewer connection, toilet drops, shower traps, lav drains, kitchen, utility, and future stubs.
Must be installed at the correct slope, with trap locations and vent paths accounted for.
Pro insight: A generation ago folks ran copper directly under slabs. If a rock nicked the pipe or corrosion started, you were tearing up floors. Sleeved PEX is cheap insurance and the standard for modern builds.
Bringing the Sewer Under the Footing (The Right Way)
You’ll often see plumbers tunnel under the perimeter footing to bring in the main sewer. With the right mini-excavator and a narrow bucket, this is a 10–15 minute dig.
Why under (not through) the footing?
Keeps the structural element intact.
Minimizes cold joints or dowel detailing headaches.
Faster on most sites with the right equipment.
Must-dos
Maintain uniform slope (typically 2% for 3–4 in runs unless your code allows less).
Bed the pipe in clean sand or fine gravel—no voids, no point loads.
Protect the pipe with warning tape above the trench before backfill.
Tip: If your foundation contractor “boxed out” a sleeve through the footing, use it. If not, tunneling beneath is often cleaner and avoids core drilling.
Sleeved PEX: How to Do It So It’s Actually Serviceable
A sleeved water service only helps if you can pull a replacement line later:
Continuous sleeve from the exterior to the mechanical space—no hidden couplings under the slab.
Generous radius at turns; avoid tight bends that will snag the pipe.
Cap and tape both ends during the slab phase to keep debris out.
Label the sleeve at both ends. Future-you will thank you.
Field note: Wrap the PEX end and sleeve mouth with bright tape during construction so nobody kicks mud, pea gravel, or form oil into the sleeve.
Layout First: Strings and Batter Boards Aren’t Optional
Underground rough-in is only as good as your layout. Your plumber needs:
Form boards set and strings pulled for the building perimeter.
Centerlines for walls that host fixtures—especially bathrooms and kitchens.
Final fixture locations to measure from.
This is how the crew locates:
Toilet flanges (no trap in the slab; the toilet has its own).
Shower drains (with embedded P-traps).
Lavs, kitchen sink, laundry, island vents, future stubs.
If strings aren’t up, you’re guessing. Guessing underground means moving drains later, which is not fun and never cheap.
Shower vs. Toilet: Traps Explained
Toilet: No separate P-trap in the slab. The toilet’s built-in trap seals sewer gas.
Shower: Trap in the slab (directly under the drain). If you’re building a zero-threshold walk-in with a recessed pan, set the trap now at the correct elevation for your drop.
QA tip: Before backfill, put a straightedge over the shower recess and measure to your trap inlet. Confirm your slope to the drain and compatibility with the chosen drain body.
The Drain Stack Water Test (Your Inspector Will Do This)
Before you bury anything, your inspector will want proof the DWV system is tight.
How it’s done:
Cap/plug all DWV openings.
Install a test tee or riser to create a head of water (often 10–12 feet).
Fill with water and hold for 24 hours (local requirements vary).
Inspector taps the stack, checks all glued/jointed areas for weeping.
Why it fails:
Rushed primer & cement technique (not fully seated, rushed cure).
Dirty or wet fittings.
Glue-starved joints.
Micro-cracks at transitions or discrete couplings.
Remember: This isn’t pressurized—just static head. If it leaks now, it’ll leak forever. Fix it before backfill.
Backfill, Compaction, and Pad Prep (Don’t Skip the Boring Stuff)
Once you pass inspection:
Backfill with care
Sand or fine granular fill at and around pipes to protect them.
No cobbles, debris, or sharp rock contacting pipe.
Compact in lifts
Follow geotech or local standard (e.g., 6–8 in lifts).
Plate compactor or jumping jack depending on soil type.
Achieve target density—don’t “eyeball it.”
Final pad grade
Bring to sub-slab elevation with engineered fill.
Laser level and mark high/low spots.
Prep for vapor barrier, termite treatments (where applicable), rebar or post-tension, and insulation (if spec’d).
Slab curls, random cracks, and hollow spots are often compaction problems dressed up as “concrete issues.” Get the dirt work right.
Quick Code-Savvy Checklist (Common Requirements)
Always verify with your AHJ (Authority Having Jurisdiction), but this will keep you honest:
Slope: 1/4 in per foot (2%) for small diameter drains unless your code allows less.
Trap arms & venting: Within allowable distances, proper sizing, and vent takeoffs above the trap weir height.
Cleanouts: At base of stacks, at changes of direction beyond the limit, at intervals per code, and accessible.
Pipe bedding: Sand/fine granular, compacted, no point loads.
Sleeves/penetrations: Sized with annular space sealed appropriately where required.
Water test: Full head for required duration, documented if your inspector wants photos/notes.
Inspections: Underground plumbing signed off before vapor barrier and steel.
Sleeved water service: Continuous, labeled, with gentle bends and capped.
Owner-Builder Tips That Save Time (and Rework)
Decide on fixtures early. Rough-in heights and centerlines for wall-hung toilets, linear drains, freestanding tubs, or curbless showers are specific.
Photograph everything before backfill. Take wide shots and close-ups with a tape measure in frame. Label by room.
Mark the slab after inspection—spray paint fixture centerlines and stub locations on the subgrade. It’s a great double-check before vapor barrier goes down.
Future-proof: Add capped stubs for a water softener, hose bibbs, an exterior shower, RO system, pot filler, or detached ADU line—cheap now, priceless later.
Protect sleeves and stubs during the pour. Use bright covers and flag tape so nothing gets buried in concrete or cut by a finisher.
Sequence at a Glance
Forms set and strings pulled (perimeter & interior layout).
Trench and install main sewer (often under the footing).
Run DWV branches to all fixtures; set traps (showers).
Pull sleeved PEX water service; cap both ends.
Cap DWV, fill riser—water test (hold per code).
Inspector verifies—pass.
Pipe bedding, backfill and compact in lifts.
Final subgrade, vapor barrier, termite treatment (if required), insulation (if spec’d), rebar/PT.
Slab inspection(s) as required.
Pour concrete.
Common Mistakes (and How to Avoid Them)
No layout strings: Leads to misaligned toilets or showers. Pull strings; measure twice.
Trap in the toilet line: Don’t do it—the toilet is already trapped.
Out-of-tolerance slope: Bellies (too flat) or noisy drains (too steep). Check with a digital level.
Debris in sleeves: Cap and tape sleeves from day one.
Skipping compaction around pipes: Settling equals cracked finishes and stressed piping.
Pouring before inspection: If your AHJ wants to see it, don’t bury it.
Quick FAQ
Do I really need a sleeve for the water service?
If it passes under the slab, yes. A sleeve lets you replace the line without demolition and protects it from abrasion.
How high should I water-test the DWV?
Typical is 10–12 feet of head above the highest fitting under test, held for 15–24 hours (local rules vary).
Can I run the sewer through the footing instead of under it?
You can if the footing was boxed out or you’re detailing a proper sleeve/penetration. Many crews prefer to tunnel below to avoid structural detailing and patching.
Where’s the trap on a toilet rough-in?
Inside the toilet. Your rough-in is just a 3–4 in drop to the branch with the correct flange height and orientation.
What’s the right slope for a shower trap arm?
Follow code (commonly 1/4 in per foot), and mind maximum trap-to-vent distances based on pipe size.
Final Word: Pass the Test, Protect the Future
Underground plumbing is one of those phases where careful layout, clean workmanship, and patient testing prevent the ugliest kind of callbacks—the kind hidden under concrete. Use sleeved PEX for water, set traps and slopes by the book, hold your water test, photograph everything, and compact like your slab depends on it—because it does.
Get this right and the next phases (vapor barrier, steel, and the pour) go smoothly. Get it wrong and you’ll spend time and money on do-overs. Aim for “boringly correct” here—and your future self will never think about it again.