The Short Answer
Interior cracks tell you that something in the structure has moved. They do not tell you how much, in what direction, or whether it is still happening.
That distinction is the whole substance of this guide. A crack is an observation. Foundation movement is a conclusion, and the route between them runs through measurement rather than through interpretation of the crack itself. A foundation evaluation is where that measurement happens.
Why Interior Walls Crack
Drywall is a rigid panel system fastened to a timber frame, with joints taped and finished to appear continuous. It has very little tolerance for movement. Almost anything that shifts the frame produces a visible line somewhere, which is why interior cracking is so common and so frequently misread.
The usual contributors:
Seasonal humidity. Timber framing takes up and releases moisture through the year. In San Antonio the swing between a humid spring and a dry August is substantial, and the framing moves with it. Fine cracks that open in dry months and close in humid ones are the signature.
Curing and settling of new construction. New homes crack. Framing dries out, fasteners set, and joints move during the first couple of years. It is ordinary and it is not a foundation matter.
Thermal movement. Ceilings under a hot attic expand and contract more than the walls below them, which produces cracking at the wall-to-ceiling junction.
Actual structural movement. When the foundation moves unevenly, the frame above it is bent. The drywall cracks where the stress concentrates, which is normally at an opening corner.
Patterns and What They Suggest
| Pattern | Common association | What it does not establish |
|---|---|---|
| Hairline along a taped joint | Seasonal or finishing movement | That movement is absent |
| Fine cracking at wall-to-ceiling junction | Thermal cycling, truss uplift | Foundation involvement |
| Diagonal crack from a door or window corner | Stress concentration at an opening | That the foundation has moved |
| Diagonal crack with displacement | Relative movement between sections | The magnitude or direction of it |
| Crack reopening after repair | Ongoing movement | Where the movement originates |
| Multiple diagonal cracks in one area | A shape in the movement pattern | Whether it is active now |
The right-hand column is the one people skip. Every pattern has an association, and no pattern has a proof.
The Two Things That Actually Matter
Change over time. A crack that has been identical for three years is behaving differently from one that has widened since spring. This is measurable by anyone with a pencil and a phone camera, and it costs nothing.
The floor profile behind the wall. Elevation readings across the plan show whether the structure under that wall has actually moved and by how much. A diagonal crack above a flat profile is a framing question. The same crack above a measured deflection is a foundation question.
Everything else, including crack width, is context rather than conclusion.
On crack-width rules
You will find charts online assigning severity bands to crack widths. We do not publish one. Width varies with the finish, the framing, the age of the building, and how the crack formed. A narrow crack with displacement can matter more than a wider one without it. Applying a universal number to your wall is how people end up either alarmed about nothing or reassured about something real.
How to Document Interior Cracks Properly
- Mark both ends of the crack with a light pencil line and write the date beside it. Painter’s tape works if you prefer not to mark the paint.
- Photograph the crack with a ruler held flat against the surface so the width is readable in the image.
- Take a second photograph showing the wider context so the location is clear later.
- Note whether the two sides of the crack sit flush or whether one is proud of the other.
- Repeat after a wet season and after a dry one.
If your marks are still at the ends of the crack a year later, it is dormant. If the crack has run past them, it is active and worth measuring properly.
Contractor Evaluation Versus Engineering Assessment
If you do move to an assessment, it is worth knowing what you are getting. A contractor evaluation, including ours, documents observed conditions and measurements and proposes a repair scope where one is warranted. A Texas licensed professional engineer provides an independent structural opinion and may apply a seal, which is required for certain permits and for consequential structural conclusions.
They serve different purposes and neither replaces the other. We will tell you which one your situation calls for.
When to Request an Evaluation
Request one when a crack has visibly widened between dated checks, when displacement appears, when the same crack keeps returning after repair, when cracks are appearing in several rooms rather than one, or when the cracking is accompanied by doors that have progressively worsened or floors that have become uneven.
If none of those apply, documenting through a season is genuinely the better step, and our guide on when a crack signals a larger foundation problem sets out the escalation criteria in more detail.