Under-Slab Sewer Plumbing: What to Check Before Concrete

01Set fixtures from finished dimensions

Start with the final floor plan, not a rough sketch of the room. Toilet rough-in, shower drain location, wall thickness and cabinet clearances all depend on the finished surfaces and the selected fixtures. Account for tile, backer board and the actual flange or drain assembly.

A change from a tub to a shower, or moving a toilet a few feet, can change branch sizing, venting and elevation requirements. Resolve those changes on the approved plan before rough-in rather than trying to make a fitting compensate during the pour.

02Coordinate structure, bedding and pipe protection

Drainage needs consistent bedding and a uniform approved slope. A fitting suspended over an uneven trench can settle or crack once concrete and soil loads arrive. Keep debris and concrete out of open piping with appropriate temporary protection.

Do not run pipes through structural footings or cut reinforcement simply because the route is shorter. The structural design and inspector determine acceptable penetrations and protection. Sleeves or isolation at a penetration address movement and contact; they do not grant permission to alter the structure.

03Photograph and inspect while everything is visible

Before concealment, ask the contractor to document the drain route, branch connections, vent takeoffs and cleanout access. Include reference measurements to fixed walls so the pictures remain useful after the slab hides the piping.

The required rough plumbing test and inspection must happen before the pour. For plastic drains, do not substitute a compressed-air test for an approved water test. Test plugs must be selected and restrained by trained personnel; a plug can release suddenly under load.

• Fixture positions checked against finished walls
• Grade and bedding verified along the entire route
• Structural penetrations approved
• Required test and inspection accepted before concealment

04Avoid the hidden-work mistakes

Common problems include a toilet flange planned at the wrong finished-floor height, an inaccessible cleanout, a branch fitting installed against flow and an unvented fixture. A successful leak test does not automatically clear those layout defects.

Schedule a final pre-pour walk-through after other trades have worked in the area. A passing test from the previous week will not reveal a pipe displaced by later trench work. Keep permits, inspection records and the pipe-location photos with the house records.