Foundation Cracks: Which Ones Are Cosmetic and Which Ones Need an Engineer

Walk into the basement of almost any house in Greater Boston and you will find a crack in the foundation. Buyers see one and imagine the house sliding into the ground. Sellers see the same crack and say it has always been there. Usually the seller is closer to right, but not always, and the difference is visible if you know what to look for.

The short version: thin vertical cracks in poured concrete are usually shrinkage, structurally harmless, and matter mainly as a way for water to get in. Horizontal cracks, stair-step cracks in block, cracks wider at one end than the other, and any crack where one side has moved out of line with the other are the ones to show a structural engineer before you sign the purchase and sale agreement.

Three kinds of foundation in New England houses

The age of the house usually tells you which you have.

Fieldstone or rubble, common in houses built before about the 1920s: stones laid with mortar, sometimes rough-faced on the inside.
Concrete block, common in mid-century houses: hollow blocks laid in mortar, sometimes filled and reinforced, often not.
Poured concrete, standard in post-war and modern construction.

Each one cracks in its own way, and the meaning of a crack depends on which you are looking at.

Cracks that are usually cosmetic

Poured concrete shrinks as it cures, and almost every poured foundation develops a few hairline vertical cracks, often running from the corner of a basement window or near the middle of a long wall. Signs that a crack is shrinkage:

It is vertical or nearly so.
It is narrow and about the same width along its length.
The two sides are in line: run a fingernail across it and there is no step.
Nothing upstairs suggests movement.

These matter because water follows them. A shrinkage crack that lets water in after a heavy rain is a water problem, fixed by improving gutters and grading outside or by injecting the crack from inside with epoxy or polyurethane. Water in the basement covers which of those works when. Radon can also enter through cracks, which is one reason to test; radon testing explains how.

Cracks that need an engineer's opinion

Horizontal cracks, especially in block walls about halfway up. They usually mean soil pressure is pushing the wall inward, and they are often accompanied by the wall bowing. Stand at one end and sight along it.
Stair-step cracks following the mortar joints of a block wall, which can mean either settlement or lateral pressure.
Cracks wider at the top than the bottom, or the reverse, which suggest part of the foundation is settling more than the rest.
Displacement: one side of the crack has moved in or out relative to the other.
Cracks that are growing. A pencil mark at each end, dated, shows whether a crack has lengthened. Sellers sometimes have years of evidence either way.
Signs upstairs: doors and windows that stick or no longer latch, cracks running diagonally from door corners, floors that slope noticeably.

None of these means the house is unsound. Each means you need a structural engineer's opinion before you know what you are buying.

Fieldstone foundations age in their own way

Fieldstone foundations have held up New England houses for two hundred years and can go on doing so. They are not a defect in themselves. What to look for is different:

Deteriorated or missing mortar between the stones, which needs repointing.
Loose or missing stones, or a section bulging inward.
Moisture, which a stone wall almost always lets through to some degree. Grading, gutters, a dehumidifier and sometimes a perimeter drain are the usual answers.
The sill, the timber resting on top of the foundation, which rots where water and insects have reached it. Termites and carpenter ants covers the insects.

A wall that has been parged — coated with a smooth layer of mortar inside — can hide all of this. Ask when and why it was done.

Frost, water and the soil around the house

Most foundation movement in this climate starts with water. Water-saturated soil freezes and expands, pushing on walls and lifting shallow footings, and saturated soil also presses harder on a basement wall than dry soil. Clay soils hold water and make all of it worse. A tree close to the house can dry the soil unevenly in summer.

So the first questions about a worrying crack are often outside the house. Where do the downspouts discharge? Does the ground slope toward the foundation? Has the patio settled so that it now drains toward the house?

What repairs typically involve

Repairs range from minor to very large, and prices vary so much with the house that only a contractor's quote after an engineer's diagnosis means anything.

Crack injection, for leaking shrinkage cracks.
Drainage and grading, often the most valuable repair of all.
Wall bracing for a bowing wall: steel beams, wall anchors into the yard, or carbon-fibre straps.
Underpinning or piers, for a footing that is settling, which is the expensive end.
Repointing and rebuilding for fieldstone.

Insurance generally does not cover settlement and cracking; it is treated as wear rather than sudden damage. Home insurance in Massachusetts explains what is and is not covered.

Bringing in an engineer during the purchase

If the home inspector raises a structural question, the inspector's job is done and an engineer's begins. A licensed structural engineer will evaluate the foundation, write a short letter with findings and recommendations, and give you something definite to negotiate with or walk away on. The fee is small next to the decision it informs.

Book the engineer the day the inspector raises the concern, because the inspection window is short. The home inspection guide explains how that window works and how to turn findings into a credit, and what a buyer inherits in an old house covers the rest of what comes with a fieldstone foundation.

General information as of 2026. A home inspector and a licensed structural engineer, examining the specific house, are the only reliable authorities on whether a crack matters.