Predrywall and drain pipes in the slab

[Randall Clark]

I've done hundreds of predrywall inspections and most of the time the void around drain pipes under tubs is filled with mortar. Did inspection a few weeks ago and superintendent said they intentionally leave this void in case a repair needs to be made to the pipe. In this case, there was a little bit of water in this void - ground water, someone dump water in the hole? Curious if anyone knows where, if anywhere, this could be found in residential code. Thanks for any assistance.

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[Jerry Peck]

For starters:

North Carolina State Building Code: Residential Code
- Section R506
- - Concrete Floors (On Ground)
- - - R506. General
- - - - Concrete slab-on-grade floors shalled be designed ad constructed in accordance with the provisions of this section or ACI 332. Floors hall be a minimum 3-1/2 Inches (89 mm) thick. The specified compressive strength of the concrete shall be as set for in Section R402.2.


Also read R506.2.2; R506.2.3


Section R318 Protection Against Subterranean Termites ... if they used chemical soill treatment, that space need to be retreated before being closed up.


Section P2602 Penetrations, P2606.1 Sealing of annular spaces (note, it allows 'foam', however, if 'foam' is used, then protection of the foam is required in accordance with the applicable section)


"superintendent said they intentionally leave this void in case a repair needs to be made to the pipe" = okay, sos they are installing an access door, sealed and gasketed, etc, of sufficient size for access "in case a repair needs to be made to the pipe".


The floor is part of the thermal envelope, so they need to seal that hole to maintain the floor part of the thermal envelope. North Carolina State Building Code: Energy Conservations Code, R402.4 Air leakage control, see R402.1 and R402.2

[Randall Clark]

Thank you for the references. Excellent points.

In my view, this is also a radon gas entry point and potentially, Potentially a water entry point in case of high water table/flood (or at least high humidity entry area in an enclosed space).

[Jerry Peck]

I agree on both of your points, however, I am not a 'radon person' and didn't want to step over that line. That said ... try installing a radon mitigation system with that open hole ...

Regarding water intrusion from high water, I almost included a code reference for flood, but I presumed that the elevation of the slab meets established FIRM and NC code elevations, which means that water entering there would occur at the same time it entered over the slab edge (usually at door openings). Thus that aspect should ... 'should' ... already have been addressed.