How are under-slab plumbing leaks found and repaired?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Leaks in water supply lines or sewer drains under a slab add water to the soil, and on expansive clay that water can make the ground swell under the foundation ³. Supply leaks are usually found with a water-meter check, and drain leaks with a hydrostatic (water-level) test followed by a camera inspection ¹². In Oklahoma, the repair itself is licensed plumbing work ⁴.

Why do plumbing leaks matter for foundations?

Most Oklahoma homes built on a slab have water supply pipes and sewer drains running in the soil under the concrete. When one leaks, it wets the clay in one area. Expansive clay swells as it takes on water, so a leak can push part of a slab up — called heave — while the rest stays put. Engineering research modeling a leaking water line under a slab on expansive soil found that leaks can cause differential movement and added bending stress in the foundation ³.

Leaks are also linked to problems after a foundation has been repaired. An engineering committee review lists leaking sewer or water lines as a cause of renewed movement after mudjacking, foam injection and pressed piles ⁵.

Leaks can play a part in several movement patterns:

• Leak-induced localized heave — a wet "bulb" of swollen clay lifts one area.
• Center heave — moisture builds up under the middle of the slab.
• Erosion and washout voids — running water carries soil away and leaves gaps.
• Hydrocollapse — some loose, dry soils compact suddenly when soaked.

Because adding lift to an area that is already high makes things worse, the water source is typically found and stopped before any lifting is planned.

How are supply leaks detected?

Pressurized water lines leak constantly, so the water meter is the first check.

• Two-hour meter test: Record the meter, use no water for two hours, then read it again. If the reading changes at all, a leak is likely ¹.
• Winter usage check: EPA notes that a family of four using more than 12,000 gallons a month in a cold month such as January or February probably has a serious leak ¹.
• Scale of the problem: EPA estimates that about 9% of homes have leaks wasting 50 gallons or more per day ¹.

An unexplained high water bill is often the first clue. Once a leak is confirmed, a plumber narrows down its location before opening the slab.

How are drain and sewer leaks tested?

Drain lines are not under pressure, so they do not move the meter. A hydrostatic test is used instead ²:

1. An inflatable plug seals the main sewer line through a cleanout.
2. The under-slab drain system is filled with water up to slab level.
3. The water level is watched for up to about 15 minutes. A drop suggests a leak.
4. Follow-up uses a video camera in the line and isolation testing of individual branches to pinpoint the leak.

Some facts about this test from a city building department bulletin ²:

• There is no general requirement to hydrostatically test existing homes; it is often done during a sale inspection.
• Testing is most often requested on older homes with cast-iron drain pipe (generally homes built before the early 1970s) and on homes with earlier foundation repair.
• Shallow sewer lines sit under only about 1 to 2.5 psi of water pressure in the test, which is low enough to reveal existing leaks rather than create new ones.
• Not every small drain leak releases enough water in normal use to threaten a foundation.

Engineering guidelines for underpinning list a hydrostatic leak test report for under-slab sewer and water lines among the records kept for a pier job ⁷. See slow drains or sewer backups and plumbing leaks under slab for the warning signs.

What are the repair options?

Once a leak is located, the plumber's options range from repairing a single section through the slab, to rerouting a line around the slab, to lining or replacing longer runs of pipe. The choice depends on the pipe material, its age and how many leaks the testing finds. Cost is driven mainly by the number of leaks, how much concrete must be opened or tunneled under, and the length of pipe involved.

Who may repair leaks in Oklahoma?

Oklahoma's plumbing rules state that no person or firm may perform plumbing work without first obtaining the appropriate license or registration ⁴. The Construction Industries Board licenses plumbing contractors and journeymen, including residential categories limited to one- and two-family homes and townhouses; journeymen must work for a licensed contractor ⁴.

What ongoing checks help?

A foundation maintenance guide suggests checking for dripping faucets, air-conditioner condensate drips (especially from the overflow line) and stoppages, and fixing local moisture sources as soon as possible ⁶. Leak coverage and insurance questions are covered in plumbing leaks and insurance in Oklahoma.

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• Plumbing Leaks Under Slab
• Leak-Induced Localized Heave
• Unexplained High Water Bill
• Plumbing Leaks and Insurance - Oklahoma
• Heave vs Settlement