Under Slab Plumbing

Under Slab Plumbing

pvc elbowIn most pole buildings, any water and waste plumbing lines are placed beneath the poured concrete slab on grade. In some cases (especially post frame houses), elevated wood floors are used to create an under floor crawl space, making these lines accessible in the future, without the need for costly jackhammer or concrete cutting work for repairs.

Even in the case of a simple garage, or shop, is can be prudent to at least rough in plumbing for a future bathroom. Once the concrete slab has been poured, things literally become “cast in stone” and placing lines after the fact can be an expensive task, at best.

What to do with the dirt

The entire area of any concrete slab or perimeter foundation should be cleared of rock and debris. If natural soil on site is rocky, it should be replaced with clean, compactable fill. Rocks can easily damages lines underneath a concrete slab, causing leaks.

Slant the Waste

Waste lines, beneath a concrete slab, must have the proper downward slope; they cannot be placed perfectly flat, or running uphill. Lines need to be sized appropriately for the number of bathrooms which will be using the lines. An experienced plumbing contractor will know the requirements for each. In the event under slab waste lines will be lower than adjacent sewer lines or septic tanks, a waste pump will be needed.

Water Lines require TLC

All under slab water lines should be carefully protected from any damage which can occur during trench backfill. Pressure test both water lines and waste lines both before trenches are backfilled and again after the grade has been prepared to pour the slab. This is one of those check twice scenarios – as a damaged pipe, beneath a concrete slab, is not a pleasant, nor inexpensive, experience.


Make sure to confirm with the Building Department when plumbing inspections will be required. The inspector is going to want to verify correct sized lines have been used, and (most importantly) will be checking the pressure gauges the plumber has set on under slab waste and water lines. As long as the pressure gauges are show pressure is being held, the lines are undamaged and all should good forward smoothly.


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