Pool Leak or Slab Leak? How to Tell Which One Is Losing Water

Texas summers are hard on swimming pools and hard on water bills. When the bill jumps in July, many homeowners with a pool assume the pool is to blame, and many homeowners without one start wondering about the pipes under the house. If you have both a pool and a slab foundation, the question gets harder: is the water going into the pool and out through a crack, or is it escaping from a pipe under your floor? This guide walks through the simple tests that separate the two, and why it matters to know before you call anyone.

Why the two get confused

A pool loses water in three ways: evaporation, splash-out and leaks. In a dry Texas summer, evaporation alone can take a quarter of an inch or more off the water level every day, and more on windy days. If your pool has an automatic fill valve, that lost water is replaced from your home's water supply without you noticing. The bill goes up, the pool level looks normal, and nothing in the house seems wrong.

A slab leak also shows up first on the bill. A pressurized pipe under your foundation leaks around the clock, so it adds to your usage even when nobody is home. Both problems raise the bill, both are hidden, and both get worse with time. The difference is where the water goes, and a few tests tell you that.

Step 1: Turn off the pool's auto-fill

If your pool has an automatic fill valve, it is connected to the home's water supply and can hide a pool leak by quietly topping up the pool. Find the fill line, usually at a valve near the equipment pad or a small box beside the pool deck, and close it. Some auto-fills have their own shut-off; on others you close the supply valve feeding them. Leave it off for the tests below.

This one step often changes the picture. If the water meter stops moving once the auto-fill is off, the water was going into the pool. If the meter keeps moving with the auto-fill off and every fixture in the house off, the water is going somewhere else.

Step 2: Run the meter test

With the pool auto-fill closed, turn off every faucet, the dishwasher, the washing machine, the ice maker and the sprinklers. Then read your water meter, wait one to two hours without using any water, and read it again. Most meters also have a small triangle or dial called a leak indicator that spins at even very low flow, so watch that too. Our step-by-step water meter leak test explains how to find and read the meter.

• Meter still moving: water is escaping from your home's plumbing or the line between the meter and the house. That points away from the pool and toward a hidden leak, possibly under the slab.
• Meter stopped: the home's plumbing is holding water. If the bill was high, the auto-fill and the pool are the likely cause.

Step 3: Do the bucket test on the pool

The bucket test is the standard way to tell evaporation from a pool leak. Fill a bucket with pool water and set it on the first or second step of the pool, so the bucket's water and the pool sit at the same temperature. Mark the water level inside the bucket and the pool level on the outside of the bucket. Turn off the auto-fill, run the pump on its normal schedule, and check after 24 hours.

• Both levels dropped about the same: that's evaporation. The pool is not leaking.
• The pool dropped more than the bucket: the pool is losing water beyond evaporation, which means a leak in the shell, fittings, light niche, skimmer or plumbing.

Many pool pros suggest repeating the test with the pump off. If the pool loses more with the pump running, the leak may be on the pressure side of the pool plumbing; if it loses more with the pump off, it may be on the suction side.

What the results tell you

Put the two tests together and the answer is usually clear.

The third row is less common but real. Older Texas homes with older pools can develop both problems, and fixing one won't fix the other. That's why it pays to test both before anyone starts digging.

If it's the pool

A pool leak needs a pool leak specialist. They use dye testing, pressure testing of the pool lines and sometimes underwater listening equipment to find leaks in the shell, fittings and pool plumbing. Pool leaks are a separate specialty from slab leak detection: the pipes run under your deck and yard, not under your home's foundation.

If it's your home's plumbing

When the meter keeps moving with the pool's auto-fill off and everything inside turned off, the next question is whether the leak is in the yard line or inside the house. Close the main shut-off valve where the line enters the house (often near a hose bib or in the garage) and watch the meter again. If it keeps moving, the leak is between the meter and the house. If it stops, the leak is inside the house, and a pipe under the slab is a strong possibility.

From there, professional slab leak detection narrows it down. A licensed plumber confirms the leak with a pressure test, separates the hot and cold lines, and then pinpoints the spot through the concrete with acoustic listening, thermal imaging or tracer gas. You get the location marked on the floor and a written report.

Signs that point toward a slab leak, not the pool

Some clues lean strongly one way. These suggest a leak under the house:

• A warm spot on the floor, especially on tile, which usually means a hot-water line is leaking
• The water heater running more than usual
• The sound of running water inside the house when nothing is on
• Damp carpet or flooring near an interior wall, away from any bathroom fixture
• New cracks in tile or flooring along with a rising bill

These point toward the pool:

• The pool level drops noticeably faster than the bucket test predicts
• Wet or soft spots in the yard between the pool and the equipment pad
• Cracks in the pool deck or tile line, or loose coping
• Air bubbles in the return jets, or the pump losing prime

Why the order of testing matters

It is tempting to call a pool company when you have a pool and a high bill. But if the real problem is a hot-water line under your kitchen, a week of pool testing costs time and money, and the slab leak keeps running. The reverse is also true: paying for slab leak detection when the auto-fill is feeding a cracked skimmer wastes the visit.

The two simple tests above take a day and cost nothing. They tell you which specialist to call, and when you do call, you can describe exactly what you found. That helps a slab leak detection plumber plan the visit and often shortens it. Our FAQs cover what to expect from a detection visit and what it typically costs in Texas, and our locations page lists the Texas cities we serve.

A note on water bill credits

Many Texas cities and MUDs offer partial credits for water lost to a hidden leak once it is repaired. Policies differ on what counts. Some exclude pool and irrigation leaks; others treat any hidden leak the same. Whichever leak you find, keep the invoices and any test results, and ask your utility about its rules and deadline soon after the repair. Some require the request within 20 or 30 days.

Pools are common on larger lots in Mansfield, Southlake and Grapevine, and an auto-fill can hide either kind of leak.

The bottom line

A pool with an auto-fill can hide a leak, and a slab leak can hide behind a pool. Turn off the auto-fill, run the meter test, and do the bucket test. If the meter keeps moving with everything off, the water is escaping from your home's plumbing, and that's when it's time to have the lines under your slab tested. If the meter stops and the pool drops faster than the bucket, call a pool leak specialist. Either way, you'll know where the water is going before anyone breaks concrete.