Slab Leaks and Wet Basement Floors

The common wet basement floor causes are condensation on the cool concrete in humid weather, groundwater pushing up through cracks and the joint where the floor meets the wall, a floor drain or sump that is not keeping up, and a plumbing leak in or under the slab. A leaking appliance is the other possibility. Where the water sits and whether it follows the weather will usually tell you which one you have.

Wet basement floor causes, from most to least common

• Condensation. Warm, humid summer air meets a cool slab and leaves a film of moisture, often over a wide area and worst in corners with little air movement.
• Groundwater seepage. After heavy rain or snowmelt, the water table rises and water comes up through floor cracks and along the perimeter joint.
• Sump pump problems. A failed pump, a stuck float or a power outage lets the pit overflow.
• Floor drain backup. A blocked drain or sewer line sends water up through the lowest opening in the house.
• Appliance and fixture leaks. Water heaters, washing machines, softeners and laundry tubs all sit on the basement floor.
• Slab leaks. A pipe buried in or under the concrete has failed.

The plastic sheet test

This simple test separates moisture from the air and moisture from below. Dry a section of bare floor. Tape a piece of clear plastic sheet, about half a metre square, tightly to the concrete on all four edges. Leave it for a day or two.

If droplets form on top of the plastic or the floor around it is damp while the concrete under it stays dry, the source is condensation from room air. If the underside of the plastic fogs or the concrete beneath it darkens, moisture is coming up through the slab.

Condensation or groundwater?

Condensation shows on hot, sticky days, particularly when basement windows are open and letting humid air in. The floor feels clammy more than wet, cold water pipes drip, and it clears when a dehumidifier runs and the windows stay shut. Southwestern Ontario summers are humid enough that many basements need a dehumidifier from June through September.

Groundwater follows rain and thaw, sometimes a day behind. It appears in lines, along cracks and at the edge of the floor where the slab meets the wall, and it may leave white mineral deposits when it dries. Much of the London area has clay-heavy soil that holds water around foundations. This water is under pressure from below, so sealing the top of the slab with paint does not hold it back. Drainage does: working weeping tile, a sump pump with a backup, extended downspouts and soil graded away from the house.

Signs of a slab leak

True slab leaks are less common than the causes above, but they do the most hidden damage. In many Ontario basements the water supply lines run overhead in the joists, so the pipes under the floor are mostly drains. Supply lines do run under the slab in some houses, including many slab-on-grade homes and additions with no basement, and in floors with in-floor hot water heating.

• A damp or dark spot that does not change with the weather and never fully dries.
• A warm patch on the floor, which points to a hot water or heating line.
• The sound of water running or hissing when everything is turned off.
• A water meter that keeps moving with every fixture closed.
• For a drain leak, dampness that appears after heavy water use, sometimes with a sewer smell, and no movement on the meter.

A plumber confirms a supply leak with a pressure test and locates it with listening equipment. A broken drain under the floor is found with a camera run through the pipe. The repair means either opening the concrete at the leak or rerouting the line overhead and abandoning the buried section.

Check the simple things too

Before assuming the worst, trace the water back to its highest point. Look at the base of the water heater and its relief valve pipe, the washing machine hoses and drain standpipe, the softener and its drain hose, and the laundry tub trap. Lift the sump pit lid and confirm the pump runs when the float is raised. Pour a bucket of water down the floor drain and see that it goes away.

Why finished floors make it worse

Bare concrete gets wet and dries again with little harm. Finished floors do not. Carpet and underpad hold water against the slab. Laminate swells at the edges. Vinyl plank is waterproof on top, which means it traps moisture underneath where it cannot evaporate. This is where the musty smell comes from, and where mold gets established.

If a finished floor has been wet, a moisture meter can check the slab, the subfloor and the base of the walls without lifting everything, and show what needs to come up. Our moisture inspections run $249 to $549.

Start with the plastic sheet test and a note of the weather each time the floor is wet. Two days and a roll of tape will tell you whether to buy a dehumidifier, look at drainage or call a plumber.

Common questions