Foundation crack leaking water: causes and repairs

Why foundations crack

Concrete shrinks as it cures, and it is strong in compression but weak in tension. Almost every poured foundation develops some cracks. Others come from:

• Settlement of the soil under part of the footing
• Soil pressure, particularly from wet or expansive clay, which pushes walls inwards
• Frost, when wet soil freezes and expands against the wall or lifts shallow footings
• Tree roots and changes in soil moisture during drought
• Openings. Cracks often start at the corners of basement windows and at beam pockets, where stress concentrates.
• Cold joints, where one pour of concrete met another

A crack is a path. Whether water uses it depends on how much water is in the soil outside.

Reading a crack

Vertical or slightly diagonal hairline cracks in poured concrete, roughly uniform in width, are usually shrinkage cracks. They are the commonest kind and are seldom a structural concern. They often leak.

Diagonal cracks that widen towards one end can indicate settlement.

Horizontal cracks in a poured wall, or along a mortar joint in a block wall, suggest pressure from the soil outside. These deserve attention, especially if the wall is bowing.

Stair-step cracks in block or brick follow the mortar joints. Minor ones are common. Wide or displaced ones point to movement.

Cracks where one side has shifted in or out relative to the other, or any crack that is getting wider.

Floor cracks. Basement slabs are not structural in most houses and commonly crack. Water coming up through them shows pressure under the floor.

Have a structural engineer look at:

• Cracks wider than about six millimetres
• Horizontal cracks and bowing walls
• Cracks with displacement
• Cracks that are growing. Mark the ends and date them, or fix a crack gauge across, to find out.
• Doors and windows upstairs that have started to stick, or new cracks in drywall above

Why it leaks

After rain or during a thaw, the soil against the foundation fills with water. If the drainage at the footing, called the weeping tile, cannot carry it away fast enough, pressure builds and water comes through any opening: cracks, tie-rod holes, the joint between wall and floor, and pipe penetrations.

So a leaking crack has two parts: the opening, and the water behind it. Good repairs deal with both.

First aid during a leak

• Photograph the leak while it is happening. It shows exactly where water enters.
• Move belongings and lift anything off the floor
• Catch and remove the water with a wet vacuum, towels or a pump
• Check that the sump pump is running
• Outside, look for the cause: a downspout emptying at that wall, a blocked eavestrough, ponding water or a full window well
• Start drying as soon as the water stops

Hydraulic cement pressed into a crack can slow an active leak for a time. It is rigid and usually lets go as the wall moves with the seasons.

Repair from the inside: injection

For poured concrete walls, the standard interior repair is injection.

• Polyurethane foam is injected through ports along the crack. It reacts with moisture, expands to fill the crack through the full thickness of the wall, and stays flexible. It is the usual choice for stopping water.
• Epoxy bonds the two sides together and restores strength. It is used where structural repair is wanted and the crack is dry and stable.

Injection is done from inside, in a few hours, without digging. The wall has to be accessible, which may mean removing drywall and insulation. It does not work on block walls, which are hollow.

Repair from the outside

The soil is excavated down to the footing along the affected wall. The crack is sealed, a waterproofing membrane is applied, a drainage layer is added, and the weeping tile is checked and repaired or replaced before backfilling.

This is the most thorough repair, because it keeps water out of the wall altogether. It is also the most disruptive and expensive, and decks, driveways and neighbouring properties can make access difficult.

Managing the water instead

Where a wall has many cracks, or is block, or the outside cannot be reached, an interior drainage system collects water at the base of the wall and leads it to a sump pump. The wall still gets wet, but the basement stays dry. It needs a reliable pump and a backup.

Fix the drainage outside

Often the cheapest improvement, and worth doing whatever else you choose:

• Extend downspouts at least a couple of metres from the house
• Clean eavestroughs
• Regrade so that the ground slopes away from the foundation
• Keep garden beds and edging from damming water against the wall
• Clear and cover window wells
• Do not pile snow against the house

Drying the basement

Water from a crack usually gets behind finished walls before anyone sees it.

• Remove baseboards and open the wall at the leak
• Take out wet insulation and any drywall that is swollen or moldy
• Lift wet carpet and remove the underpad
• Run a dehumidifier and air movers until the framing and concrete read dry
• Leave the wall open until the crack has been repaired and tested by the next heavy rain if possible

Ground water has passed through soil and is not clean. Carpet and porous materials that stayed wet are usually replaced.

Before finishing a basement

Have known cracks sealed first. Use materials that tolerate moisture, such as rigid foam insulation against the concrete, and keep wood framing and drywall off the floor. A leak that appears behind a finished wall is much more expensive than one found on bare concrete.

Insurance

Seepage of ground water through foundation walls and floors is excluded by most home policies, and the repair of the crack is treated as maintenance. Some insurers offer ground water coverage in a broader water endorsement. Sewer backup and overland water are separate coverages again. Ask your broker exactly which kinds of water your policy covers.

More: basement flood cleanup.