Owner Builder: Underslab Plumbing

Underslab plumbing is critical to the building process. It includes any plumbing related lines that are installed underneath the concrete slab. That may be just waste lines or both waste lines and water lines.

Clean Trenches

The foundation area and trenches should be clear of rocks and debris. If the soil that is present on the building site is rocky, excavated material should be removed from the site and replaced with clean fill. Rocks can damage lines resulting in leaks underneath the slab.

Important Aspects of Underslab Plumbing

Waste Lines

Waste lines must have the correct slope within the foundation area. They should also be sized according to the number of bathrooms that will be in the building. A house that has 6 bathrooms, of course, will require larger waste lines than a house that has 2 bathrooms. The pluming contractor should know exactly what is required, both in the size of lines and the amount of fall.

Sometimes underslab plumbing wastes lines run lower than city sewer lines. If that is the case, a waste pump will be needed.

Water Lines

Water lines can be run under the slab or in the attic. If underslab, water lines should be protected from damage that can occur during backfill. Testing of water lines and waste lines should take place before concrete is placed. Test once before backfill and again once back fill and underslab prep is complete. If lines have been damaged during backfill, it is important to identify it prior to pouring concrete.

Underslab Plumbing Videos




Underslab Plumbing Inspection

There are some key issues that the plumbing inspector will be looking for. He or she will want to see properly sized waste lines and required flow. Shading, the protecting of lines with material, will be checked. Also, and most importantly, the inspector will be checking the pressure gauge your plumber has set on the waste and water lines. If the pressure gauge holds, it demonstrates that lines are undamaged.


If the excavator has replaced any rocky soil with clean fill, backfilling will be uneventful and no problem. If there are rocks in the foundation area, it is likely they will end up in the backfill and can potential damage waste lines and water lines. The resulting underslab leaks are a nightmare to deal with after concrete is poured. Be certain that all rocky material is removed from the foundation area immediately following trench excavation.