What Is the Minimum Burial Depth When Running Sanitary Lines Under a Building Slab?

Recently on the ASPE Connect Open Forum, Andrew Saggese, a Mechanical Engineer with Emtec Consulting Engineers in Ronkonkoma, New York, said that he gets conflicting information from plumbing contractors about the minimum pipe cover when calculating inverts under building slabs for sanitary PVC. To make matters worse, the codes are not exactly clear on this issue. Both the 2015 International Plumbing Code (IPC) Section 306.3: Backfilling and the 2012 Uniform Plumbing Code (UPC) UPC Section 314.4: Excavations call for the minimum backfill above the top of the piping to be 12 inches. However, the commentary for the 2018 IPC has the following statement concerning minimum ground cover for piping:

“It is not the intent of this section to require that all piping be buried with at least 12 inches (305 mm) of cover. Sections 305.4 and 305.4.1 indicate the required burial depths for some types of piping systems. Piping in trenches under a slab-on-ground building need not be buried any more than what is necessary to provide for a full thickness of the concrete slab above.”

Andrew wondered how other ASPE members typically calculate their inverts inside buildings. As usual, our experts had a lot to say.

Get Familiar with the ASTM Standard

Our experts strongly recommend reading and understanding ASTM D2321, which is the standard for installing thermoplastic piping below grade but provides information applicable to many materials. Most manufacturers reference this standard as it provides detailed information for designers and installers regarding burial depth as well as details for minimum trench width.

If the trenches will have traffic such as construction equipment crossing over or along them, the standard requires the minimum cover over the pipe to be 3 feet. If a hydraulic compactor will be used, the minimum cover is required to be 4 feet before the hydraulic compactor can be used. Thermoplastic piping materials obtain their strength from the compaction of the fill around the pipe; however, the standard leaves it up to the design professional to specify if compaction testing is required.

Make Sure the Contractor Follows the Approved Documents

While the contractor may want to run the line as shallow as possible to save excavation and backfill costs, you need to state the depth requirement clearly in the specifications and ensure that they are followed to avoid potential failure and liability.

Other Advice

If you have a nominal 6-inch concrete slab on top of 6 inches of crushed stone, you should start the invert 1 foot, 6 inches below the slab. This allows the plumbing contractor enough depth to bury the pipe in the rough grade and out of the way of nominal traffic to spread the crushed stone.

Review the structural drawings for variations in the concrete slab thickness at the edges and at columns and grade beams. Don’t forget to look at the depth of the grade beams under which the pipes pass. Provide 4 to 6 inches of clearance to the top of any column footings or grade beam footings that are encountered. The structural engineer should be consulted if the depth of the structure needs to be adjusted.

Finally, avoid direct contact between the concrete slab and the piping materials, as the pipe will move due to expansion and contraction at a different rate and frequency than the concrete slab.

Can You Help?

The following recent discussion posts need input. Click on one of the following if you can offer some expert advice.

  1. If you’ve specified or have knowledge of 0.8-gpf water closets on a project, what has been the owner’s experiencing regarding the drainline performance?
  2. Does anyone have a code reference that states the required distance of the natural gas vent from the IPR from an ignition source?
  3. Should we provide a separate slab to cover underground water tanks at the basement level?

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