Underground Plumbing Before the Slab: A Commercial Coordination and Inspection Checklist

It is written for general contractors, developers and site superintendents running ground-up work in the St. Louis metro, where underground is usually the first trade inspection on the job.

Why is underground plumbing the most schedule-sensitive part of the job?

Everything above the slab can be adjusted. Below the slab, the pipe is fixed the moment the concrete goes in. A cleanout in the wrong spot, a floor drain that misses a depression, or a riser that lands in a wall that moved on the latest addendum all have to be fixed by cutting and re-pouring. Underground also sits between the site contractor and the concrete crew, so a missed inspection holds up two other trades.

What should be coordinated before underground starts?

• Current drawings — confirm everyone is working from the latest architectural, plumbing and structural sheets, including any addenda that moved walls or fixtures
• Building pad and grade — the pad should be at subgrade, compacted, and approved by the testing agency before trenching
• Points of connection — invert elevations and locations where the building sewer, storm and water service meet the site utilities
• Footings and grade beams — where pipe crosses or runs parallel to foundations, and whether sleeves are required
• Slab details — depressions, trench drains, floor sinks, slope-to-drain areas and slab thickness changes
• Other trades below slab — electrical conduit, radon or vapor mitigation, and any in-slab systems that need the same trench space
• Equipment — locations of grease interceptors, sand/oil separators, sump pits and lift stations, and who furnishes each

A short pre-installation meeting with the site contractor, concrete sub, electrician and plumber settles most of this in an hour.

How should the underground be laid out?

Layout should come from the coordinated drawings, not from scaling a sheet in the field. On commercial work we lay out fixture and riser locations with survey equipment from the building control lines so the pipe lands where the walls will be. Confirm finished floor elevation and slab thickness before setting stub heights, and mark every riser with what it serves so the crew pouring concrete knows what not to disturb.

Items that commonly get missed

• Floor drains and floor sinks in mechanical rooms, kitchens and restrooms shown on architectural or equipment drawings but not the plumbing plan
• Trap primers and their supply lines
• Cleanouts required at changes of direction and at the building exit
• Sleeves through grade beams and footings
• Condensate and indirect waste receptors for equipment that has not been selected yet

What testing and inspection happens before backfill?

The plumbing permit has to be in hand before underground starts, and the work has to be tested and inspected before it is covered. Permit and inspection requirements vary by jurisdiction. The City of St. Louis lists sewer laterals, soil stacks, vent pipes and water supply lines among the work that requires plumbing permits and inspections. St. Charles County requires inspections to be requested by 5 p.m. the business day before the inspection date. O'Fallon schedules inspections through its online portal and says portal inspections are typically completed the next day. Confirm the current process with the jurisdiction on your job.

The adopted plumbing code sets the test method for drain, waste and vent piping and for water piping. In practice, that means the underground is filled or pressurized and held for the required time with the joints exposed, so the inspector can see that nothing leaks.

• Pull the permit and confirm the inspection request process before mobilizing
• Install and test the underground in sections if the building is large or phased
• Keep joints exposed and the test on the system when the inspector arrives
• Have the approved drawings and the permit card on site
• Record pipe locations and depths before backfill for the as-builts
• Get the approval in writing or confirmed in the jurisdiction's portal before backfill

What happens after the underground passes inspection?

Backfill should use the bedding and backfill material specified, placed and compacted in lifts so the pipe holds grade and the slab does not settle over the trench. Protect risers and stubs with caps and marking so they survive vapor barrier, rebar and concrete placement. A plumber should check stub locations against the layout one more time right before the pour.

Photograph the underground before backfill and keep those photos with the as-builts. When a future tenant or owner needs to add a fixture or chase a problem, those photos are worth far more than the time it took to take them.

How do you protect the pour date?

• Hold the pre-installation meeting before the pad is turned over
• Get equipment selections for interceptors, separators and sumps settled early
• Schedule the underground inspection when the work is ready, not on the day of the pour
• Build in time for a re-inspection if something needs correcting
• Keep the plumber on site through the pour to protect stubs

Legacy Plumbing installs underground sanitary, storm and water service and site utilities on commercial new construction across the St. Louis metro, and coordinates testing and inspection with the jurisdiction. Send your drawings for a bid — commercial bids are returned within one business day — or call (636) 978-0120.