What do cracks in a foundation mean?

Most foundation cracks tell you one of three things: the house settled a little and stopped, the soil under or beside it is moving, or water is pushing on the wall. The width, direction and behaviour of the crack, not its mere presence, tell you which. The guidance below comes from Fairfax County's marine clay guide, written for Northern Virginia soils.

Cracks that are usually harmless

Small settlement cracks (smaller than 1/16 inch) are not uncommon in new construction.^[1] They usually appear and stabilize during construction or within the first year.^[1] If a hairline crack has looked the same for years, you can note it and keep watching.

Cracks that point to soil or water

Timing is the first clue. Cracks that appear abruptly after five, ten, or thirty years are likely caused by soil problems.^[1]

Seasonal movement is the second. If the cracks open up during the summer months, especially during very dry years, and close up during the moist winter months, they are probably caused by shrinking and swelling of clays below the footings.^[1] That is why it helps to note where trees stand in relation to the damage.

Pattern is the third:

• Cracks through the masonry that follow mortar lines. Cracks that extend through masonry walls and follow block and brick mortar lines may indicate advanced settlement due to shrink-swell clays.^[1]
• A basement wall bowing inward. Concrete block walls that are bowing inward usually have one or more horizontal cracks about mid-height on the wall.^[1] Vertical cracks often develop near the corners, since the corners are more rigid and resist inward movement.^[1]
• A gap between wall and soil in summer. If there is a gap one-half to one inch in width between the wall and soil backfill during the summer months, then you may have high shrink-swell clays, at least near the ground surface.^[1]

Soil is not the only cause. Settlement of improperly compacted non-clay fill under a foundation, pressures exerted by water collecting next to the house, and substandard building construction can also result in cracks in walls and floor slabs and in foundation damage.^[1] Water is the cause a homeowner can most often influence; see how to keep water away from your foundation.

What to record before anyone visits

Measure and keep a record of the width and extent of the cracks and the time of the year that you made the recordings.^[1] Photographs with a ruler beside the crack, taken from the same spot, make the history clear. Note when you first saw it and whether doors or windows have started to stick.

Do not fill the crack in the meantime. Patching wall, ceiling, or floor cracks caused by Marine Clay is only a cosmetic repair and is not a solution to the problem.^[1] It also hides the evidence an engineer wants, and there is a legal side: if you patch up cracks and later sell your house without disclosing the damage, you may be sued by the purchaser.^[1]

Signs that cannot wait

Fairfax County's drainage guide is blunt: wet basements and wall or foundation cracking can have serious consequences for your home's structural integrity and your health. They should be addressed right away.^[2]

A wall moving inward can strain the lines running through it. Call the gas company immediately if you think that your natural gas pipe or gas meter is under pressure from a damaged wall, since an explosion could occur from a leaking pipe.^[1]

When to call a licensed pro

A crack that is wider than a hairline, growing, seasonal, stair-stepped, horizontal or paired with a bowing wall needs an engineer's diagnosis, not a repair estimate. In Virginia, a person shall hold a valid license prior to engaging in the practice of architecture or engineering which includes design, consultation, evaluation, or analysis and involves proposed or existing improvements to real property.^[3] Who to call and what to ask is covered in who to hire to check a cracked foundation.

Repairs such as underpinning, wall reinforcement and drainage work are contractor work, usually after the engineer's design. The Uniform Statewide Building Code requires a permit for installations or alterations involving any structural component,^[4] so ask who is pulling it. On licensing, Class C contractors take single contracts over $1,000 but less than $30,000, and Class B contractors take single contracts of $30,000 or more but less than $150,000;^[5] check the licence class against the contract before you sign.

Related reading in Virginia

If you are weighing a new foundation instead, see what pouring footings for a house involves and how much a foundation costs for a new house in Virginia. Browse excavation services or more excavation answers.

When you have a diagnosis, describe the repair once and licensed local contractors can quote it.