Hide Drain Pipes Under Concrete (Sub-Slab Trenching)
A drain line can often be moved below a concrete slab, but the work involves more than cutting a trench and covering the pipe. First confirm the route, slope, slab construction, permits, and cleanout locations. Then install and test the line before backfilling and restoring concrete. Structural review is essential if the slab is post-tensioned or otherwise load-bearing.
A quiet, comfortable room is harder to enjoy when a drain pipe runs across a wall or floor. Moving it below the slab can clear the view, but the pipe must still drain well and remain serviceable. I plan these projects around two goals: a clean finished space and a drain system that can be checked and repaired.
The right method depends on the building, local code, pipe route, and slab. A plumber should assess the drainage work, and a structural professional may need to review the proposed cut. Never start by cutting concrete.
Plan the drain route before cutting
Planning means confirming where the pipe begins and ends, how it will connect, and whether gravity can carry wastewater along the route. It also means checking the slab and existing services before work starts. A neat pipe layout on paper is not proof that the slab is safe to cut.
Record the fixture, connection point, existing pipe size, and proposed route. Check invert elevations, meaning the height of the inside bottom of the pipe at key points. Those elevations show whether the line can fall continuously toward its connection without crossing a footing or other obstruction.
Identify slab thickness, reinforcement, and construction type. A post-tensioned slab contains tensioned steel tendons; cutting one can create a serious structural hazard. Locate utilities and reinforcement before any saw-cutting, and do not rely on a visual check alone. Get permits and required approvals before work begins.
How much slope does a drain pipe need?
Slope is the downward fall of a drain line along its route. Too little fall can leave wastewater and solids in the pipe; too much may also affect drainage performance. As one reference, the 2021 International Plumbing Code (IPC) sets minimum slopes by pipe size. The locally adopted code and amendments control.
Under 2021 IPC §704.1, the example minimums are:
- Pipe 2½ inches or smaller: ¼ inch per foot.
- Pipe 3 to 6 inches: ⅛ inch per foot.
- Pipe 8 inches or larger: ¹⁄₁₆ inch per foot.
For example, a 12-foot run of pipe 2½ inches or smaller would need at least 3 inches of fall under those example values. Confirm the applicable size, route length, and adopted code before setting elevations. Do not try to create the correct fall by forcing a sagging pipe into place during the concrete pour.
Check access, materials, and slab risks
A concealed drain should be out of sight, not out of reach. Before settling on a route, confirm cleanout locations, venting, pipe material, and the space needed for bedding and slab repair. Trench dimensions and pipe cover depend on the project; there is no single safe dimension for every slab.
A cleanout is an access point used to clear or inspect a drain. Keep required cleanouts reachable after the floor is finished. Do not bury one beneath concrete or a permanent finish. Check its required placement against the adopted plumbing code and the actual pipe layout.
Use a code-listed drain, waste, and vent (DWV) material approved for the application. Common examples include PVC that meets ASTM D2665, ABS that meets ASTM D2661, and cast iron that meets ASTM A888. These are examples, not blanket approvals. Confirm local acceptance, compatible fittings, and any rules for transitions between materials. Follow the pipe maker’s installation instructions.
A proposed plumbing cut may also affect the slab’s structure. Saw-cut limits, excavation depth, pipe cover, bedding, and reinforcement for the patch must be set for the specific slab. Do not cut a structural or post-tensioned slab based only on a plumbing sketch. Have a structural professional review the work, and establish tendon locations and an approved method before work on a post-tensioned slab.
Follow a controlled installation sequence
A sub-slab drain job moves through four stages: survey, excavation, installation and testing, then backfill and slab repair. Keeping these stages in order helps prevent damage and makes it easier to check the work before it is concealed. The permit, approved plans, local inspector, and manufacturer instructions govern the details.
Survey and approve. Locate existing utilities and reinforcement. Confirm slab type, pipe route, elevations, slope, venting, connection details, permits, and inspections. For a post-tensioned slab, do not cut until tendon locations and the approved approach are established.
Open and prepare. Saw-cut only within approved limits. Excavate without undermining slab edges, footings, or bearing elements. Prepare a firm, even bed for the pipe, and remove sharp debris that could damage it.
Install and test. Assemble listed pipe and fittings as directed by the manufacturer. Check alignment, continuous fall, connections, and required venting. Have the altered drainage system tested by the method required by the code official before the line is covered.
Document, backfill, and restore. Photograph the inspected pipe and connections. Place suitable backfill evenly around and over the line, then compact it without shifting or damaging the pipe. Restore the concrete using the approved structural detail. Keep every required cleanout accessible in the finished space.
What should be checked before concrete covers the line?
Before backfilling or placing concrete, verify the permit and inspection status, pipe markings, connections, slope, venting, cleanout access, and test results. These checks provide a clear record of the concealed work and a chance to correct problems while the pipe remains visible.
Do not pour over an untested line. Do not bury a cleanout. If the inspector requires a specific test or inspection point, follow that direction before closing the trench. Take photos with measurements or fixed room references so the route can be found later.
Compare route options and project scenarios
The best route is not always the shortest one. A direct path may conflict with a footing or fail to provide enough fall, while a longer route may preserve structure and meet the required slope. Compare options using measured elevations and approved plans, not appearance alone.
| Option or condition | Potential benefit | Main check before proceeding |
|---|---|---|
| Route beneath a non-structural slab area | Can move the pipe out of the finished room | Confirm utilities, slab details, fall, and access |
| Longer route to the connection | May avoid an obstruction | Recalculate total fall and confirm the connection |
| Cleanout located on the proposed route | Supports future clearing | Keep it accessible; do not cover it with concrete |
| Post-tensioned or structural slab | May require a different route or design | Structural review and approved locating method |
| Drain cannot maintain gravity fall | A below-slab route may not work as drawn | Ask a qualified plumber to assess alternatives |
Illustrative installation scenarios
These examples are planning scenarios, not reports of named or verified projects. They show how route and slab conditions can change the decision. I use this kind of comparison to test whether concealment is practical before anyone opens the floor.
- A short fixture move: The proposed line may fit beneath a slab if measured elevations allow the required fall and the route avoids structural elements. The plumber still needs to confirm venting, connection, cleanout access, and inspection steps.
- A route crossing a footing: A direct trench could undermine or cut a load-bearing element. The design needs review; the route may need to change. Do not excavate beside or beneath the footing without an approved detail.
- A post-tensioned floor: The slab cannot be treated as ordinary concrete. Locate tendons and obtain structural direction before cutting. If an approved route cannot be established, consider a different drain path rather than risking a tendon.
FAQs about drains below concrete
These answers give general guidance, not approval for a specific slab or plumbing layout. Local code, permits, the inspector, and project-specific structural details control. If any condition is unclear, pause the work and get a qualified professional to review it before concrete is cut or replaced.
Can a drain pipe be buried under a concrete slab?
Often, yes, if the route and materials comply with local plumbing and building requirements. The line must have the required slope, appropriate support and protection, and the required venting and cleanout access. Get approvals and complete required inspections before covering it.
What slope does a drain pipe need under a slab?
The required slope depends on pipe size and the adopted code. For example, the 2021 IPC lists ¼ inch per foot for pipe 2½ inches or smaller, ⅛ inch per foot for 3-to-6-inch pipe, and ¹⁄₁₆ inch per foot for pipe 8 inches or larger.
Can I cut a concrete slab myself?
Do not cut until utilities, reinforcement, slab type, and permits have been checked. Structural or post-tensioned slabs need professional review and an approved cutting method. A visual inspection alone cannot confirm that a cut is safe.
Can a cleanout be covered by concrete?
No. Required cleanouts must remain accessible for inspection and clearing. Plan their final position before cutting, and keep them reachable after the floor is repaired and finished. Confirm placement with the adopted code and the actual route.
Does every sub-slab drain need a permit?
Permit rules vary by location and by the scope of work. Check with the local building or plumbing authority before starting. Ask which permits, inspections, and tests are required for the altered drain system.
Which pipe material should I use under concrete?
Use a DWV material approved for the application and local code. PVC, ABS, and cast iron have relevant ASTM standards, but local approval, fittings, and transition rules still need checking. Follow the manufacturer’s installation instructions.
When should the pipe be tested?
Test the altered drainage system before backfilling or placing concrete, using the method required by the code official. Keep the pipe visible until required checks and inspections are complete. Do not use fresh concrete to hold a poorly aligned pipe in place.
How deep should the trench be?
There is no universal trench depth or width. It depends on pipe size, elevations, bedding, slab construction, and the approved repair detail. Have the project team set dimensions before cutting, and avoid undermining footings, slab edges, or bearing elements.
The key is to design the route before opening the slab. Confirm the code, elevations, structure, materials, access, and inspection plan; then test and document the installed line before it is concealed.
(This article was written by one of our staff writers, Michael Donovan. Visit our Meet the Team page.)
