Leaks & Detection
What Is a Slab Leak? Signs, Causes and Fixes
Many people think slab leaks only happen in places without basements. That is not true. A basement floor is a concrete slab, and water and drain lines often run under it. So do lines under sunrooms, garages, additions and many townhomes. Because you cannot see the pipe, a slab leak can run for weeks. This guide explains how to spot one and what fixing it involves. When you are ready, our team handles slab leak detection and repair in McLean.
What does a slab leak look like?
Often it does not look like anything at first. The clues are indirect:
- A warm or hot spot on a tile, wood or concrete floor.
- Damp carpet, lifting vinyl or cupping hardwood.
- Water seeping at a crack or joint in a basement floor.
- The sound of running water when nothing is on.
- A water meter that keeps moving with every fixture off.
- A water bill that rises with no change in use.
- Lower water pressure, or a water heater that runs more than usual.
- Musty smells or mold near baseboards.
A warm spot is a strong clue that a hot-water line is leaking. The leak heats the slab from below.
What causes slab leaks?
| Cause | How it happens |
|---|---|
| Corrosion | Older copper thins from the inside, sometimes as pinhole leaks |
| Abrasion | Pipe expands and contracts against concrete or gravel and wears through |
| Poor installation | Weak joints, kinks or damaged pipe left under the slab when built |
| High water pressure | Extra stress on pipe and joints |
| Soil movement | Clay soil swells and shrinks with moisture and shifts the slab |
Northern Virginia has a lot of clay soil, which moves as it wets and dries. That puts stress on rigid pipe under concrete.
How do I check for a slab leak at home?
- Do a meter test. Turn off every fixture and appliance. Read the meter, wait two hours and read it again. The EPA recommends this test. Our water meter leak test guide shows how.
- Shut off the water heater supply. If the meter stops moving when the cold supply to the water heater is closed, the leak is likely on the hot side.
- Walk the floor barefoot. Feel for warm spots, especially near the water heater and bathrooms.
- Listen. In a quiet house, put your ear near the floor where pipes likely run.
These checks tell you whether to call a professional. They do not show the exact spot.
How do plumbers find a slab leak?
Professionals use several tools together. Pressure tests isolate the hot and cold lines. Acoustic microphones on the floor pick up the sound of escaping water. Thermal cameras show warm patches from hot-water leaks. Moisture meters map the damp area. The goal is to mark one small spot so the repair opens as little concrete as possible.
How are slab leaks fixed?
| Option | What it involves | Good fit when |
|---|---|---|
| Spot repair | Cut a small opening in the slab, replace the failed section, patch | One leak in otherwise sound pipe |
| Reroute | Run new pipe through walls, ceilings or the basement; abandon the old line | Older pipe, repeat leaks, finished floors |
| Repipe | Replace a whole branch or all supply lines | Several leaks or failing pipe throughout |
In homes with basements, a reroute can often run overhead through the basement ceiling. That avoids breaking up a finished floor and takes the pipe out of the slab for good.
Does a slab leak damage the foundation?
It can. Water under a slab can wash away or soften the soil that supports it. Over time that can lead to settling and cracks. The sooner a leak is fixed, the less soil is affected. Water damage to flooring and walls is a separate issue that may need drying.
Is a slab leak covered by insurance?
This is general information, not insurance advice. Policies differ in how they treat the water damage, the cost to access the pipe and the pipe repair itself. Virginia's Bureau of Insurance says to give immediate notice of a possible claim and protect the property from further damage. Our guide to slab leak insurance covers the questions to ask.
Do slab leak repairs need a permit?
Fairfax County requires a residential plumbing permit to replace or repair plumbing equipment and fixtures. A plumber should tell you whether your repair needs one.
How can I lower the risk of slab leaks?
- Have water pressure checked and add or replace a pressure regulator if it is high.
- Fix small leaks quickly, before soil and flooring are affected.
- Consider rerouting old lines during a basement renovation.
- Watch the meter monthly for movement when no water is in use.
Local note: Basement slabs are common in Oakton and Vienna. Townhomes in Reston and Herndon are often slab-on-grade, so leaks show through first-floor flooring.
Related reading: Learn what to do about water coming up through the basement floor, read about pinhole leaks in copper pipes, and see what sets leak detection cost.
When should you call a slab leak professional?
Call when the meter moves with all water off, when you feel a warm spot on the floor, when water seeps through a slab, or when flooring starts to lift. Do not chip at concrete to search for a leak.
Suspect a slab leak in McLean? Call (571) 760-4451 for slab leak detection and repair from McLean Plumber Works.
Frequently asked questions
How do I know if I have a slab leak?
Look for a warm spot on the floor, damp flooring, the sound of running water with fixtures off, a moving meter, a jump in the water bill and low pressure. A meter test confirms a leak; professional detection confirms where.
What causes slab leaks?
Corrosion, abrasion where pipe rubs against concrete or gravel, poor joints, high water pressure and soil movement are the most common causes.
Can a slab leak fix itself?
No. Leaks under a slab usually get worse as water erodes soil and the hole in the pipe grows.
How long does it take to fix a slab leak?
A spot repair is often done in a day, plus time for concrete and flooring. A reroute usually takes one or two days.
Can I live in the house with a slab leak?
Usually, if you shut off the water when it is not in use and the leak is not flooding. Get it located and repaired soon to limit damage.
Sources
Related guides
Think you have a slab leak? Call to have it located.
Call McLean Plumber Works any time, day or night. A plumber picks up and talks you through next steps.