Picture the morning of the pour: the forms are set, the vapor barrier is down, the plumber's white pipes stick up out of the gravel, and the concrete truck is twenty minutes out. That is the last moment a toilet can move six inches for the price of a short length of PVC. After the pour, the same change usually means saw-cutting and jackhammering the slab, re-trenching, re-inspecting and patching. In 2025 and 2026, owners and builders commonly report a few thousand dollars for one relocated fixture, sometimes much more once flooring and cabinets are involved.
Slab-on-grade is the default foundation across much of the South and Southwest, and it is common on one-story house plans everywhere. Plumbing in a basement or crawlspace can be reworked later. Under a slab, the underground rough-in is close to permanent. Use the stages below as a walk-through checklist, and confirm the specific code requirements with your building department or plumber, because local amendments to the IRC and the plumbing code vary.
Stage 1: Freeze the decisions that set drain locations
- Fixture schedule. List every fixture that drains through the slab: toilets, showers, tubs, sinks, the laundry standpipe, the kitchen island, the wet bar, and any floor drain.
- Tub type. Alcove, drop-in and freestanding tubs all need their drains in different places. A freestanding tub needs the exact model's spec sheet, because the drain position varies between manufacturers.
- Shower design. Decide on a center drain or a linear drain, and on a curb or a curbless entry. A curbless shower usually needs a recessed area in the slab, which the concrete crew has to form.
- Kitchen island. Lock in the island's size, the sink position and the dishwasher side. An island sink also needs a venting method your inspector accepts.
- Floor outlets. Mark any electrical floor boxes for the island, a sofa floating in the great room, or a dining table. These are conduits the electrician sets alongside the plumbing.
Most changes to stock plans happen right here, and the drawings rarely keep up. Suppose you widen the island in a plan like this 4-bedroom Modern Farmhouse with 2742 sq ft, or swap a tub for a walk-in shower. In that case, mark the change in writing on the plumbing sheet and give a copy to the builder, the plumber and the drafter. Changes made by phone tend to get lost.
Stage 2: Layout and underground rough-in
- Reference lines. Have the plumber measure from the form boards or from string lines tied to batter boards, not from the gravel or from guesses about where walls will go.
- Finished-wall offsets. The standard toilet rough-in is 12 inches to the center of the flange from the finished wall. Drywall, tile and backer board add to the stud line, so the plumber has to add their thickness too.
- Slope. Check that the drain lines have the slope the code requires. A common figure is 1/4 inch per foot for smaller pipe, with less allowed for larger pipe in many jurisdictions.
- Cleanouts. Confirm that the cleanouts on the building drain will end up somewhere reachable, not under a kitchen cabinet.
- Tub boxes. An alcove tub needs a blockout around its drain so the trap can be connected later. Check that the box is there, sized correctly and well anchored.
Homes with many bathrooms multiply the risk. A layout like this contemporary 4-bedroom with 5.5 baths and 3730 sq ft, or this 3-bedroom Craftsman with 5.5 baths, can have more than a dozen slab penetrations. Number every stub-up on a printed floor plan so nothing gets overlooked.
Stage 3: Pressure test and inspection
- Test method. Under-slab drain, waste and vent piping is usually tested in one of two ways. A water test fills the system to a set height above the highest fitting. An air test holds a set pressure for a set time. Ask which one your jurisdiction requires.
- Test duration. The test should still be holding when the inspector arrives. A gauge that has dropped, or a water level that has fallen in a stub, is a leak that is still cheap to find.
- Water lines. If supply lines run under the slab, often PEX in some regions, they are tested too. Ask whether the plan keeps joints out of the slab.
- Inspection card. Do not let anyone schedule the pour until the underground plumbing inspection has been signed off.
- Photos. Photograph every stub-up with a tape measure stretched to a form board in the frame. These photos are your only map once the concrete is down.
A passed inspection only means the pipe is sound and installed to code. It does not mean the shower drain is in the middle of your shower. The inspector checks the plumbing work. Checking that it matches your plan is the owner's or builder's job.
Stage 4: Sleeves and conduits for the future
- Utility sleeves. Set PVC sleeves through footings and the slab edge for the water service, the gas line, and any irrigation or low-voltage lines that have to cross the foundation.
- Outdoor kitchen or patio. If you might add a sink, gas grill or fire feature on the patio, sleeve the route now.
- Future bathroom. Plan for a future bath in a bonus room or a garage conversion. A capped drain stub with documented measurements costs little now and saves cutting the slab later.
- Island and floor power. Install conduit to every planned floor box. Add a spare conduit run if the furniture layout is still uncertain.
- Regional items. Ask whether your area expects a radon vent stub or a perimeter sleeve for termite treatment.
Sleeves are among the cheapest items in the whole build. Open, single-level layouts such as this 1706 sq ft Modern Farmhouse ranch or this 3-bedroom Acadian-style home with 1879 sq ft leave few walls to hide future lines in, so a sleeve under the slab is often the only easy route later. Cap and label every sleeve, and record where it is on your photo plan.
Stage 5: The owner's tape-and-string walk before the pour
- Wall lines. Stretch string between the form boards along the main interior walls of the kitchen and the bathrooms. Snapping chalk on the vapor barrier works too.
- Two dimensions per stub. Measure each drain from two perpendicular references and compare the numbers with the plumbing plan, not with memory.
- Kitchen island. Mark all four corners of the island with tape or spray paint. Then check that the sink drain lands where the sink base will go and clears the dishwasher.
- Showers and tubs. Confirm the center drain or the linear drain position, the slab recess, and that no drain sits under a planned bench or a glass wall.
- Floor boxes. Find every floor outlet and sit an imaginary sofa or table around it to check the position.
Plan the walk for the day the inspection passes, with the plumber on site if possible. A good builder welcomes it, because finding a mistake now costs the builder far less than finding it later. Bring the plan sheet, a 25-foot tape, string, painter's tape and your phone. For larger homes, such as this 4-bedroom Modern Craftsman with 3237 sq ft, set aside a full hour. If your design includes a center kitchen or a broad open floor plan, give the island and the floor outlets extra attention.
The most common mistakes, and how to catch them
- A shifted island. The island grew or moved on paper, but the drain did not move with it. Catch it by taping the island footprint on the vapor barrier.
- Wrong tub drain. The drain was set for an alcove tub, but a freestanding tub was ordered, or the tub's drain is on the other end. Catch it by bringing the tub spec sheet to the site.
- Forgotten floor outlets. The floor boxes for the island or the living room never made it to the electrician. Catch it by counting the conduits against your list.
- Toilet against the wall. The plumber measured 12 inches from the stud line, so the finished wall leaves too little room. Catch it by checking the wall-finish thickness.
- A shower drain under the bench. The drain sits under a planned bench or a glass panel. Catch it by drawing the shower outline on site.
- No sleeves. Nobody set sleeves for gas, water or the patio. Catch it by walking the perimeter with your sleeve list.
- No photos. Without measured photos, nobody knows what is under the slab. Take them before a single yard of concrete is placed.
