Signs of a slab leak

What is a slab leak?

A slab leak means a water supply pipe running inside or beneath a concrete floor is losing water. Because the pipe is always under pressure, it can leak day and night, even when no one is using water. The term is sometimes used loosely for drain problems under concrete too; those typically become apparent during or after use and need a different investigation. This guide covers supply-side leaks.

Why do some buildings have pipes below concrete?

Some builders lay water lines in or under the slab before the floor is poured. Others run pipes through a crawlspace, a basement or the walls. Brisbane buildings vary in age and style, so slab plumbing can’t be assumed for any home or unit. Check your own building: is the ground floor concrete, or is there open space below? Do supply pipes reach fixtures from the floor or the wall? Was plumbing rerouted overhead in a past remodel?

Could a warm floor be a hot-water slab leak?

A spot that feels warm underfoot may sit over a leaking hot-water line. Treat it as a clue only: it applies to hot lines only (a cold-line leak gives off no warmth), and afternoon sun, radiant floor heat, ducts and appliances also warm floors. A water heater that keeps firing with no hot water in use strengthens the clue. Recheck the spot in the morning and at night — warmth that follows the sun is probably sunlight.

Can sounds under flooring indicate leaking water?

Water forced out of a pressurized pipe can make a soft hiss or rushing sound, easiest to hear late at night with every fixture off. Rule out these sources first: a toilet refilling, an ice maker or water heater cycling, or plumbing in an attached unit. A sound that stays in one floor area while everything is off is worth noting.

Moisture or flooring changes

• Hardwood or laminate that cups, swells or lifts at the edges
• Carpet pad that feels damp in one area
• Grout that darkens or stays wet
• Tiles that sound hollow or work loose
• A musty smell in one room that airing out doesn’t fix

Some changes build slowly; others can spread quickly. Photograph affected finishes promptly if it’s safe. Do not delay action on active water loss just to gather photos.

Pressure drops and usage anomalies

A large supply leak can weaken pressure at several fixtures at once. A small one may show up only as higher water use on the bill. A building-wide pressure drop suggests loss upstream of the fixtures. An increase in metered units indicates more water was recorded, but a higher dollar total can also reflect charges or rate changes. Irrigation, running toilets and buried service-line leaks produce the same patterns, so neither clue points to the slab by itself.

Other causes of similar floor symptoms

How to check what you’re seeing safely

1. Log it. Write down the symptom, the room and the date.
2. Touch-test the floor with the back of your hand at two different times of day.
3. Listen in that spot at night with all water use stopped.
4. Photograph flooring from the same angle each time.
5. Watch the water heater for firing when no hot water is in use.
6. Check the meter (see below).

Slab problem or fixture problem?

• Is the damp area next to a toilet, tub, shower, dishwasher or washer? Inspect that fixture’s seal, hoses and base first.
• Does it get wet only after the fixture is used? That can point to the fixture or its drain, but some supply connections also leak mainly during use.
• Is it wet even after hours without use? A supply leak stays on the list.
• Does meter movement stop when that fixture’s shutoff is closed? Then the fixture is the likely source.

What the water meter can add

The meter is the best early check for any hidden supply leak. Stop all water use, including irrigation and appliances, and look for movement on the meter’s leak indicator. Movement means water is escaping somewhere past the meter — it doesn’t tell you whether that’s under the slab, in a wall or in the yard. Our water meter leak check guide explains the full steps. Open the meter box only if it’s easy and safe to reach, and leave City equipment alone.

What to avoid before a professional inspection

• Don’t break concrete or tile on a hunch. You may open the wrong spot.
• Don’t carry on as normal if the meter is moving fast.
• Don’t act on one clue. A warm spot alone isn’t enough.
• Don’t hide damage under rugs or new flooring. You’ll lose the trend.
• Don’t pour sealant products into the plumbing to try to fix an unseen leak.

When a slab concern is urgent

What professional confirmation usually involves

A professional investigation first confirms that water is being lost. It then isolates which part of the plumbing is leaking and narrows the location with methods suited to the floor and pipe type. The result is usually a likely zone rather than an exact guaranteed spot, and sometimes a small opening is needed to confirm. Slab leak detection explains that service.

Finding the leak versus fixing it

Finding the leak and fixing it are two separate decisions. Detection tells you whether there’s a leak and roughly where. Repair options, such as a spot repair, rerouting the line or wider replacement, can only be compared properly after that. See repair, reroute or replace a water pipe? and slab leak repair.

Slab leak warning signs: FAQs