How to Rough-In Plumbing Under a Concrete Slab for a Bathroom

Roughing in plumbing under a concrete slab is one of the most important phases in building a bathroom. Proper layout, slope, pipe selection, venting, and pressure testing ensure that the system performs well and meets inspection standards. This guide explains each step of the rough-in process exactly as demonstrated in the transcript—simple, practical, and easy for DIY builders to follow.

1. Understanding the Bathroom Layout Before Digging

Before touching tools or digging trenches, the entire plumbing layout must be planned on paper. This includes:

• Locations of the main 3-inch drain line

• Toilet drain

• Bathroom sink drain

• Outdoor kitchen sink drain

• Garage sink drain

• Vent pipes

• Wall locations

The dotted lines in the drawings represent the pipes under the slab. Each pipe section and fitting should be measured ahead of time so material quantities are accurate and nothing is missed.

2. Establishing Wall Locations Using String Lines

The string lines act as a reference for:

• Bathroom wall positions

• Centerlines of fixtures

• Guidance for digging trenches

Steps:

1. Measure offset distances (such as 4 ft 6 in from a block wall).

2. Install tap-cons into the block or wood to tie string lines.

3. Run string lines in both directions to outline the bathroom footprint.

4. Confirm alignment and tension of the string lines.

This ensures that when the slab is poured, each stub-up pipe emerges exactly in the wall cavity.

3. Digging the Trenches

Once wall positions are marked:

• Dig trenches for the 3-inch main drain

• Dig pathways to each fixture location

• Ensure trenches are deep enough to provide proper slope(Generally ¼ inch fall per foot on drain lines.)

• Keep the base uniform to avoid pipe movement

After digging, mark fixture locations on string lines such as:

• Toilet center (e.g., 39 inches from the reference string)

• Sink drain center (e.g., 14 inches from reference)

4. Installing a Cleanout Tee for Pressure / Water Testing

A cleanout tee is temporarily installed to allow:

• Air pressure insertion

• Water filling for inspection

Process:

1. Cut the main pipe clean and square using a metal-cutting blade.

2. Deburr inside and outside edges.

3. Dry-fit the cleanout tee.

4. Mark alignment using a marker.

5. Apply purple primer and heavy-duty PVC cement.

6. Install and hold for 15–20 seconds.

A cleanout plug with a valve stem is later installed for testing.

5. Choosing the Correct Type of PVC Pipe

There are two types of PVC commonly found:

Cellular Core PVC

• Lightweight

• Not pressure-rated

• Some regions do not allow it under slabs

• Check your building code

Solid Schedule 40 PVC

• Heavier, fully solid

• Strong and rigid

• Rated at 260 PSI

• Recommended for underground slab installations

Most professionals use Schedule 40 solid PVC for all under-slab plumbing.

6. Cutting the PVC Pipe

The fastest way:

• A miter saw with a standard wood blade

It provides clean, square cuts with minimal burrs.

Measurements must include:

• Pipe insertion depth (typically 1½ inches per side for 3" fittings)

Example:If the distance between fittings is 9⅝ inches, add 3 inches insertion depth → 12⅝-

inch cut.

7. Installing the Combo Wye (Combo-Y) Fittings

A combo wye provides a smooth directional flow without creating restriction.

Important rules:

• The branch must always point in the direction of water flow.

• Never install the combo wye backwards or flat; this can cause clogs.

• Use a torpedo level to achieve the correct slope (¼ inch per foot).

Mark fittings before gluing to maintain exact orientation.

8. Running the 1½-Inch Sink Drain Lines

Sink drains use 1.5-inch Schedule 40 PVC.

Steps:

1. Dry-fit each piece.

2. Measure the distance to the wall.

3. Maintain slope using a torpedo level.

4. Add elbows when rising over a footer.

5. Glue once the alignment is perfect.

All sink pipes should rise into the wall cavity at the correct height for future traps.

9. Running Drain Lines Through Block Walls

When a drain needs to rise through masonry:

1. Determine the exit height using a level.

2. Mark the blocks.

3. Use a rotary hammer to break out the needed opening.

4. Install long-sweep 90° elbows (never short vent 90s for drains).

5. Dry-fit and mark alignment.

6. Glue fittings and insert through the block cavity.

Blocks should be filled with concrete after pipes are installed.

10. Installing the Toilet Drain

The toilet is installed using:

• 3-inch combo wye

• 22.5° elbow

• Several short connecting pieces

Important considerations:

• Always dry-fit before gluing.

• Align centerline using string line.

• Ensure distance from wall (usually 12 inches rough-in).

• Maintain slope toward the main drain.

Measurements must be precise because adjustments are difficult after gluing.

11. Installing the Vent Pipes

Vent pipes require:

• Long-sweep 90° elbows

• Proper vertical alignment through wall cavities

• Continuation above slab height

Vents allow air into the plumbing system and prevent siphoning of traps.

12. Cost Breakdown of Under-Slab Plumbing (Based on Transcript)

Approximate costs:

• 3-inch fittings: ~₹1,200 (approx. $15) each

• Schedule 40 pipe: ₹3,000–₹3,500 (approx. $35–$40) per pipe

• Total material for the project: ₹20,000 (approx. $250)

Costs will vary depending on region and store.

13. Preparing for the Water Test (Inspection Requirement)

Steps for testing:

1. Remove the cleanout cap.

2. Thread in the test balloon plug.

3. Tighten by hand only.

4. Pump to ~35 PSI using a bicycle pump (with gauge).

5. Fill pipe system with water using the lowest stub-up.

6. Let water rise to the top.

7. Allow system to sit overnight.

8. Inspector checks water level the next day.

If the water level does not drop, the system passes.

14. Backfilling the Trenches

After inspection:

1. Remove test plug and install cleanout cap.

2. Backfill around pipes using gravel, avoiding sharp rocks.

3. Do not allow gravel to enter open pipe ends.

4. Compact gravel gently around pipes for stability.

5. Grade surface flat in preparation for concrete.

Placing water lines above the slab (inside walls) makes future maintenance easier.

15. Ensuring Proper Pipe Position for the Slab Pour

Things to verify before concrete:

• Each stub-out is inside the planned wall cavity.

• All vent and sink lines extend 6-8 inches above slab level.

• Pipes are properly marked, aligned, and not leaning.

• Gravel bedding is uniform and compact.

Conclusion

Roughing in plumbing under a concrete slab requires careful planning, accurate measuring, proper pipe selection, and precise installation techniques. When done correctly, the system will pass inspection easily and provide long-term reliability for all bathroom fixtures.

This blog covers every detailed step—from layout and digging all the way to pressure testing and backfilling—so DIY builders can follow confidently and produce professional results.