A crack in the foundation, and how to tell whether it matters

Most cracks in a foundation are not a sign that it is failing. Concrete shrinks as it dries and nearly all of it cracks, so the question worth answering is whether anything is moving: a crack that is widening, that has one face displaced from the other, or that arrives with doors and windows that have stopped closing points to movement, and a stable hairline crack on its own usually does not.

The industry says so plainly. The National Ready Mixed Concrete Association's guidance states that all concrete has a tendency to crack and that crack free concrete is not possible, and that most early random cracks rarely affect structural integrity or service life. The ASCE Texas Section's residential foundation guidelines say the majority of cracks do not compromise structural integrity.

Where the pattern does point to movement, the measured evaluation 254 Engineering describes is on the foundation inspection page.

Read the crack before measuring it

• Fine random cracks in a slab or garage floor, with no offset. These are the shrinkage cracks the concrete guidance describes.
• Straight cracks along sawn or tooled lines. Those are contraction joints doing their job; NRMCA guidance places them at 24 to 36 times the slab thickness so the concrete cracks there rather than at random.
• A crack with one face higher or further out than the other. It has moved in more than one direction, which a plain drying crack has not done.
• A crack that has grown since it was first noticed, in length or in width.
• A crack that lines up with distress elsewhere: a door that binds, a gap at a window frame, separation in brick coursing outside.

The first two are normal. The last three are reasons to keep reading.

The signs that sit away from the crack

A foundation that moves shows it through the frame above it. The Texas Real Estate Commission's inspection standards name the typical indications in rule 535.228(a): binding, out of square and non latching doors. The ASCE Texas guidelines tell an evaluating engineer to read brick coursing, door alignment and built in surfaces alongside the measured floor.

One sticking door in humid weather is a door. Several doors along the same side of the house that began to bind in the same season is a pattern, and it means more than any single crack.

How to monitor a crack yourself

A crack's history is worth more than its size, and an owner can record that history at no cost.

• Mark both ends of the crack with a pencil line and the date.
• Photograph it square on with a ruler or a tape in frame, from a spot you can return to.
• Measure the width at two or three marked points with a crack gauge or a feeler gauge, and write the readings down.
• Note whether one side sits higher than the other, and by how much.
• Repeat at the end of a dry season and the end of a wet one. On Texas clay, a reading from a single month says little about the year.

Do not fill or paint over the crack until it has been read. Filling it removes the record an engineer would use, and a filled crack that reopens says only that something moved, not how much.

Why Texas foundations crack in seasons

Much of Texas sits on clay that changes volume with moisture. The USDA's fact sheet on Houston Black, the soil the Professional Soil Scientists Association of Texas recommended as the state soil, describes classic Vertisols that shrink and swell markedly with changes in moisture content, found on about 1.5 million acres of the Blackland Prairie, and calls their very high shrink swell potential a common limitation for building sites.

The International Residential Code treats such soils as a design condition. TDLR's technical bulletin on foundations on expansive soils summarizes the code's test, one element of which is a plasticity index of 15 or greater, and the code's requirement that foundations on those soils be designed for them.

So a slab on clay moves a little every year, and a crack can open in summer and close in spring without anything being wrong. Slab foundations, common across Texas, are covered in cracks in a concrete slab foundation.

When a crack needs an engineer

• Your own readings show it widening across seasons rather than opening and closing.
• One face is displaced from the other.
• Doors and windows along one side of the house have started to bind together.
• The crack is in a foundation that has already been repaired.
• A lender, a buyer or an insurer has asked for an engineer's opinion in writing.
• On the coast, the crack appeared after a storm. What else to check is in the hurricane damage structural checklist.

When a cracked foundation needs a repair rather than a watch, a defined scope is what lets contractors price the same job, which is what a repair specification is for.

What an engineer does with the crack

An engineer treats the crack as one piece of evidence. Under the ASCE Texas guidelines, a Level B evaluation adds relative floor elevations across the whole foundation and reports deflection and tilt against tolerances. The guidelines use the residential code's L/360 live load limit as an analogy: where the measured profile is under L/360 and no distress is visible, it is unlikely the foundation has deflected materially. They also say a tilt greater than 1 percent is usually noticeable.

The crack pattern is then read against that measured shape. A crack sitting at the low point of a measured deflection means something different from the same crack in a flat floor. ACI 224.1R, the American Concrete Institute's report on cracks, is organized the same way: causes first, then evaluation, then repair.

If your readings or the signs above point to movement, the next step is a measured evaluation rather than a repair quote. The foundation inspection page sets out what it covers, and the structural engineer guide to when you need one covers the cases where the honest answer is to keep watching.