Which foundation settling cracks are normal and which are not

Almost every concrete or block foundation develops a few cracks over time. Thin vertical lines in poured concrete are usually harmless signs of the material curing and shrinking. Any crack that runs horizontally, bows inward, or keeps widening can mean the wall is moving, and a structural engineer should assess it.

Why a house settles into the ground

A house is heavy. The soil beneath the concrete footing carries the entire weight of the structure. That includes the wood framing, the roof structure, the appliances, and the people inside.

As that weight presses down, the soil naturally compresses. The house drops slightly into the earth. Builders call this initial downward movement settlement.

Many new homes settle during their first few years. Builders pour the concrete footing deep in the earth below the local frost line. This prevents frozen ground from heaving the house upward. Even with a deep footing, the soil beneath it still gives way slightly under the massive load.

Uniform settlement happens when the entire house drops at the exact same rate. The structure stays completely square. Differential settlement happens when one corner or side of the house sinks deeper than the rest.

The wood framing above must bend to accommodate the drop. The foundation wall gets pulled along with it. Concrete and masonry handle downward compression well, but they crack when bent or pulled apart.

Hairline shrinkage cracks in poured concrete

A poured concrete foundation wall begins as a thick, wet mix. Concrete needs water to remain workable as it pours into the foundation forms. Once the forms come off, that excess water slowly evaporates out into the air.

The physical volume of the wall shrinks slightly. That physical shrinkage pulls the concrete apart. The resulting hairline cracks often appear near openings or in long unbroken walls.

These fine lines run vertically or slightly diagonally down the wall. You can identify a standard shrinkage crack because it stays very narrow. If you run your hand across the wall, the concrete on either side remains perfectly flush.

The foundation repair standards from Mequon, Wisconsin say shrinkage cracks with no wall movement are not considered a structural defect. Water can seep through them into a basement, so ask a licensed pro about sealing if you see dampness.

Reading vertical, diagonal and horizontal cracks

The direction a crack travels gives clues about how a foundation is moving, though only an engineer can say why.

Vertical cracks are often the least serious. They are usually associated with shrinkage of concrete, and a crack in a flat wall that stays tight is usually not a concern. A vertical crack where one side sits out from the other deserves an engineer's look.

Diagonal cracks need a closer look. They can mean one corner of the foundation is dropping faster than the rest, or that a wall is leaning. Either way, an engineer should assess any growing diagonal crack.

Horizontal cracks are the ones to take most seriously. Soil outside the wall can push inward, especially when it is wet or frozen, and the crack is often where the wall begins to bow.

The Mequon standards say horizontal cracks are usually associated with bowing or displacement of masonry walls that are out of plumb. Call a licensed structural engineer if you see one.

Stair step cracks in block and brick walls

Block foundations and exterior brick walls crack differently than solid poured concrete. The mortar joints between masonry units are often where a crack travels.

The crack follows the joints and zigzags up and down the block wall in a stair step pattern.

The Mequon standards say step cracks in block walls can come from wall deflection, foundation settlement or shrinkage of the masonry.

A hairline step crack may just reflect shrinkage of the masonry. A step crack that grows or a wall that bulges inward should be seen by an engineer. Under the Mequon standards, a step crack wider than 1/4 inch or any sign of settlement means consulting an engineer.

Companion signs above the foundation

Foundation movement rarely stays isolated in the basement. The wood framing of the house rests on a wooden sill plate bolted to the top of the foundation wall. When the concrete drops, the framing follows. Every foundation carries a house differently, but any drop at the base pulls the upstairs floors and walls out of square.

Secondary signs appear in the living areas above.

• Interior doors stick in their frames or swing open on their own.
• Windows suddenly become hard to slide up and down in their tracks.
• Wide gaps appear between the baseboards and the finished floor.
• Floors slope noticeably or feel bouncy in one specific area.
• Exterior trim boards separate from the brickwork.

A single sticking door in August usually just means high humidity has swollen the wood. A door that jams while a diagonal crack grows in the basement directly below it points to a structural shift. That pattern can mean the foundation dropped and the framing above followed, so have an engineer take a look.

How to track a crack over time

A crack that appeared ten years ago and never moved is a scar. A crack that grows a fraction of an inch every month may be active. You need to know if a crack is spreading or if the house has finished settling.

Take a carpenter pencil and draw a small line straight across the very tip of the crack. Write the date on the concrete next to the mark. If the crack grows past your pencil mark a few months later, the foundation is still moving.

Measure the width of the crack at its widest point. Write that exact measurement down on the wall next to the date. Take a clear photograph with a metal tape measure held flat against the wall across the crack. Wait two months and measure it again.

Do this at the end of a wet spring season and the end of a dry summer. The surrounding soil expands and contracts. The crack might open and close slightly with the weather. A crack that only ever gets wider needs professional attention.

Shine a bright flashlight parallel to the wall across the crack. If the light casts a shadow on one side of the crack, one side may be sitting out from the other, and an engineer should check it.

Crack patterns that require a structural engineer

Some crack patterns leave no room for patient monitoring. Call a licensed structural engineer promptly if you see warning signs. Under the Mequon, Wisconsin standards, a crack wider than 1/4 inch or any sign of settlement means consulting an engineer for site specific engineering, and your own building department can tell you what applies where you live.

Consider hiring an engineer before a foundation repair contractor. An independent structural engineer works for you, not for a repair company. They do a full foundation inspection to diagnose the underlying problem and write a neutral repair plan. The repair company then follows that engineered plan.

Call an engineer if you spot any horizontal crack. Call them if any concrete or block wall bows inward. A plumb bob dropped from the top sill plate down to the floor will show exactly how far the wall leans out of plumb. Call one if a diagonal crack keeps growing.

The engineer will inspect the soil type, the yard grading, and the entire structure from the basement to the rafters. They can recommend what repair the house needs. They can tell you if underpinning a house is necessary to stop the footing from sinking further.

Do not patch a wide crack yourself before an engineer has assessed it. Patching can hide a symptom while the cause continues. Structural repair needs a licensed pro and, in most places, a permit.