Slab Leaks and Hydrostatic Plumbing Tests: A Houston Foundation Guide

A slab leak is a leak in the water supply or drain lines under a concrete foundation, and a plumber's leak test is how you confirm or rule one out: a meter or pressure test for supply lines, and a hydrostatic test for drains (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17). On Houston's expansive clay, leaking plumbing can create local high moisture that leads to uneven foundation movement (FPA-SC-07-0, Foundation Maintenance and Inspection Guide, 2003, checked 2026-10-17).

Signs of a slab leak

The Foundation Performance Association's test-methods paper points to these reasons to have the plumbing under a slab tested (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17):

• An unusually high water bill. The paper says a supply-line test may be warranted when unusually high water bills arrive.
• A water meter that moves when no water is in use. In the paper's supply test, all water use is stopped and automatic devices such as ice makers are shut off; if the meter hasn't changed after about 30 minutes, there's no supply leak. The paper says testing should be performed by a qualified plumber, so if the meter moves while nothing is running, ask a plumber to test the lines.
• Signs of foundation movement. The paper says supply and drain tests are often run together to confirm or rule out foundation movement caused by leaks under the slab. Our signs of foundation problems page lists what movement looks like.

Two kinds of pipe run under a slab

Most homes have two separate systems below the concrete. Supply lines carry pressurized water in. Sanitary (drain) lines carry used water out by gravity to the sewer. The Foundation Performance Association's test-methods paper treats them separately: a pressure test for supply and sprinkler lines, and hydrostatic, isolation, flow and video tests for sanitary and storm drain lines. Air conditioning condensate lines are checked visually (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17). The same paper notes that a supply leak above the slab is usually easy to spot, which is why the supply test matters mainly for pipes below it.

How a slab leak can move a foundation on clay

Houston slabs often sit on expansive clay, which swells as it takes on water and shrinks as it dries. The association's maintenance guide says leaking plumbing lines, leaking drains, dripping faucets and dripping condensate lines can create local high moisture that leads to differential movement in expansive soils, and that these conditions should be fixed as soon as possible (FPA-SC-07-0, Foundation Maintenance and Inspection Guide, 2003, checked 2026-10-17). Its owner's guide lists a leaking sewer or water line under the foundation among the kinds of water that can harm a foundation when it stays there (FPA-SC-20-0, Owner's Guide to a Better Foundation, 2016, checked 2026-10-17). The key word is uneven: one wet spot under one part of the slab is what tends to cause distress. If you're unsure which way a slab moved, our sinking foundation guide explains settlement, shrinking clay and heave.

How a hydrostatic plumbing test works

The FPA paper describes both tests step by step (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17):

• Before either test: nobody in the house uses water, and supplies to toilets, ice makers and other automatic watering devices are shut off. If the air conditioner's condensate drains into the sewer, it's turned off too.
• Supply test: the water meter is read. If it hasn't changed after an allotted time, typically 30 minutes, there's no supply leak. Another method shuts the main valve and watches a pressure gauge at a hose bib.
• Sanitary (hydrostatic) test: a plumber blocks the system's exit, normally at the cleanout, with an inflatable rubber bladder called a test ball, then fills the drain system to floor level, often watching from a toilet flange or floor drain. If the water level holds for about 30 minutes, no leak is present.
• If it fails: the plumber uses two or more test balls to isolate sections until the leak is found, and may then run a camera to locate it. A flow test can estimate how much water the leak loses in normal use; a hydrostatic test alone shows only whether the system leaks, not how much.

The paper also says some owners consider pulling and resetting a toilet destructive, and that the wax ring will likely need replacing.

Why the test comes up around foundation repair

The same paper says supply and sanitary tests are often run together to confirm or rule out foundation movement caused by leaks under the slab, and that leak testing is often done before a foundation is lifted and should always be done after lifting to make sure the lift didn't break any pipes (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17). The association's movement-evaluation guidelines list a hydrostatic leak test, with leak location and a flow test if applicable, among the items an engineer may include in an evaluation (FPA-SC-13-1, Guidelines for the Evaluation of Foundation Movement, 2015, checked 2026-10-17). Our foundation inspection guide covers that evaluation.

Who runs the test

The FPA paper says testing should be done by a qualified plumber (FPA-SC-02-0, Test Methods for Evaluating Existing Foundations, 2010, checked 2026-10-17). In Texas, the plumbing trade is regulated by the Texas State Board of Plumbing Examiners (TSBPE, checked 2026-10-17). We don't refer plumbers.

Questions to ask about a leak test

• Which lines will you test: supply, sanitary, sprinkler, or all of them?
• Where will the test ball go, and will a toilet need to come up?
• If the sanitary test fails, will you isolate the leak and run a camera?
• Will I get a written report I can hand to an engineer?
• Should the test be repeated after the foundation work?

After the plumbing: the foundation step

If a leak is found, fixing the pipe stops the water; it doesn't tell you whether the slab needs work. A professional engineer can evaluate the foundation, and our slab foundation repair page explains repair methods. Our foundation repair cost page lists published price ranges, and how long foundation repair takes covers timing. All our guides start from the home page. If you're tracking cracks, see signs of foundation problems and foundation crack repair.

Insurance: whether a policy pays for foundation work depends on that policy, so ask your insurer or agent. The Texas Department of Insurance explains policies and claims in its consumer guide (TDI home insurance guide, updated 6/1/2026, checked 2026-10-17).

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