Photograph of a basement floor slab crack How to Evaluate Cracks in Poured Concrete Slabs & Floors

  • CONCRETE SLAB CRACK EVALUATION - home CONTENTS: Types of cracks that occur in concrete slabs, shrinkage cracks, settlement cracks, frost heaves. Causes and problems of floor slab cracking; How to repair cracked concrete floors. Slab on grade construction or "patio home" construction cracks; Diagnosing cracks in ceramic tile over concrete slab floors
  • POST a QUESTION or READ FAQs about cracked concrete slabs and floors
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Floor slab & tile crack diagnosis & repair:

This article describes the types of cracks that occur in poured concrete slabs or floors and explains the risks associated with each, thus assisting in deciding what types of repair may be needed. Cracks in concrete floors or slabs occur in poured concrete slabs may be found both in basement and in slab on grade or "patio home" construction and have a variety of causes and cures that we discuss here.

This article series describes how to recognize and diagnose various types of foundation failure or damage, such as foundation cracks, masonry foundation crack patterns, and moving, leaning, bulging, or bowing building foundation walls.

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Three Types of Cracks in Concrete Slabs & Floors

Types of foundation cracks, crack patterns, differences in the meaning of cracks in different foundation materials, site conditions, building history, and other evidence of building movement and damage are described to assist in recognizing foundation defects and to help the inspector separate cosmetic or low-risk conditions from those likely to be important and potentially costly to repair.

Each type of basement slab, floor slab, or slab on grade crack is discussed and described with photographs below.

Cracks come to the job along with the concrete, riding in the same truck! At a Journal of Light Construction conference (Boston 1985) a lecturer informed us that "Every concrete truck that comes to your job to pour a slab has at least four cracks in it. It's up to you to either provide control joints, or not. If you leave out control joints the cracks will occur in a messier pattern at natural stress points in the slab."

Three Types of Poured Concrete Slab Floors - Different Implications of Cracks & Movement

Floating slab on grade schematic (C) Carson Dunlop AssociatesBefore we describe crack, movement, and damage patterns and diagnosis in concrete slabs, we need to introduce three different types of poured concrete floor slabs.

We use the three Carson Dunlop Associates Sketches shown here to comment on the occurrence, causes, and significance of cracks and movement in poured concrete slab construction.

We define types of foundations and their common vulnerabilities also

[Click to enlarge any image]

Floating concrete slab characteristics:

In the sketch at above left, the floor slab (left side of the foundation wall) is simply "floating" sitting atop gravel and soil inside the foundation wall.

This is an idealized sketch.

The author's first construction job (construction at the Fleet antiaircraft missile training center, Dam Neck, VA) consisted of raking roughly level loose-fill dirt inside of building foundations.

Over several summers of this labor we never once saw anyone using a soil compactor and rarely did we see gravel poured inside of the foundation walls before the slab was poured.

The bad news about typical floating slab construction (where the soil is not compacted) is that anything that causes the soil to settle risks slab cracking and settlement. Flooding, leaks, or simply poor handling of roof and surface runoff can send water under a building where it causes loose soil to settle.

The good news about cracks in floating slab construction is that the damage is to the floor, not to the structure that is supporting the building. Only if you see a floor slab crack that continues up in the foundation wall where the crack meets the wall would the structure be obviously involved.

More good news: if there is significant soil settlement under a floating slab, the slab is likely to break and follow the settling soil downwards; a sudden precipitous collapse of a floating slab is less likely than the next case we describe.

Supported poured concrete floor slab characteristics

Schematic of a supported slab on grade (C) Carson Dunlop Associates

As you can see from the Carson Dunlop Associates sketch above, the supported slab is a lot like the floating slab - it claims to have gravel and claims to have compacted soil below the slab.

But the edges of the floor slab rest on a lip built into the poured concrete footing which also supports the building walls.

The good news about a slab with this design is that a little soil settlement below the slab will not cause the floor to tip nor crack provided it has been adequately reinforced.

The bad news about a supported slab design is that if there were significant soil settlement below the slab and if it lacked proper reinforcement at the time of construction, it might collapse.

Where may this occur: if you inspect a garage built on what was originally a sloping hill, you can expect that the interior of the garage foundation was filled with lots of backfill soil. If your builder was the same fellow who hired the author (as we described above), all of this fill was left un-compacted. At the low end of the garage where the most fill was added, significant soil settlement can occur. If the slab was also not reinforced and if a lot of soil settlement occurs under this floor, it could collapse suddenly, say when your car is parked there.

Slab on Grade - Monolithic Concrete Slab Construction Characteristics

Monolithic slab on grade construction diagram (C) Carson Dunlop Associates

As you see in the Carson Dunlop Associates sketch above (used with permission), a monolithic slab is poured at the same time as the building footing that is going to support the building's walls.

If structural cracks appear in a monolithic slab they might trace to footing settlement which might be a structural concern, depending on the amount of settlement, its origin, and type of building construction.

Notice that the sketch shows insulation on the exterior of the slab - unless special methods are used, it can be difficult otherwise to insulate this floor from the surrounding soils, an important factor in cold climates and where heating costs are increasing rapidly.

Where exterior foundation insulation is carried up above grade and right under the building exterior siding, there may be a risk of wood destroying insect attack on the wood-framed wall.

The articles listed below explain how we recognize and diagnose signs of cracking, damage, movement in these different concrete slab construction methods.

How to Diagnose & Repair Concrete Floor Slab Cracking

Reader Question: How do I diagnose and repair cracks in ceramic tile covered slab in a nearly-new home

We bought a house two years ago, after it sat empty as a model home for a few years, so now it’s about five years old. I believe it is a slab on grade foundation type--common in this area.

We live in Austin, Texas and the drought has been really bad here, and recently we have discovered a few hairline cracks in a few different tiles. These tiles are on the first story and so are on top of the concrete. One crack is about 4 feet long and runs straight through about 4 different tiles, not following the grout lines. (Theses cracks are hairline, and are barely noticeable.)

There is a lot of limestone and rock in the area, and so we never thought we’d ever have a problem here, unlike Houston, where foundation problems are everywhere. I kinda freaked out and started putting a level to everything in the house, and all the door frames are perfectly level and the countertops are very, very close to being perfectly level.

There a few small hairline cracks throughout the house in the sheetrock, but these were there when we moved in and haven’t gotten worse, and seemed pretty normal to me. There is even a third story, which I know you’d see the most movement, (if there was sinking involved) but everything looks good up there(maybe a very slight slope to one side, like a ¼ inch over 10 feet)

Well, mentioning the drought, a few neighbors have said they have a few cracks too, and they had a foundation company come out, and the company told them it was because of the drought, and told them to soak the foundation.

I went out and bought soak hoses and are going to set that up, and I also bought a nice laser level and checked the slope of the foundation. The foundation slopes about 1 inch over about 20 feet towards the left side of the house, which happens to be the downhill side. But with almost everything else level in the house, I’m starting to think it was already like that, and I just never noticed it before.

So this leads to my main question—how much slope in a foundation is generally acceptable to build on? Because I know brand new foundations won’t be perfectly level, but about how much can a new foundation be off-1 inch, 2 inches, 6 inches?

Thanks, C.M.

Reply: diagnose the crack type, impact on foundation, probable cause, and choose extent of repairs

Thanks for the interesting tile and slab crack question. A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a foundation, slab, or floor cracking problem because someone with experience might see clues that escape even a smart, careful, thorough homeowner performing a DIY investigation. That said, here are some things to consider:

Decide if the floor and concrete cracks are structural or not

I agree that the cracks you describe are more likely due to slab settlement or movement and that considering Austin has been in a drought, soil settlement (rather than rising due to suddenly newly-wet expansive clay soils) is probably at work.

I also agree that other crack sources such as concrete slab shrinkage are probably not at fault. I think that slab shrinkage cracks appear early in building life, are not structural, and are less likely to suddenly telegraph through a ceramic tile floor. (To be accurate, severe shrinkage cracks in a slab could show up in a tiled floor if the floor were not properly installed to prevent those cracks from telegraphing through the tile, but that just doesn't sound like your case.)

It's useful to distinguish between a crack that only affects the floor slab (usually not structural, the building is not threatened) and a crack that includes the foundations and footings (structural, the building might be threatened depending on extent of movement).

Evaluate the extent of actual foundation movement vs. slab cracking

While building codes expect footings to be poured level, I'm not surprised to read that a foundation and footing slope one inch over 20 feet in new construction. A key diagnostic step will be to convince yourself that this out of slope condition is as-built or that it is the result of settlement.

Examine the sloping foundation walls wherever masonry foundation materials and surfaces are visible. A one-inch change in slope, if it happened after construction, would often be expected to produce vertical or stair-stepped cracks in the foundation wall. (While it's theoretically possible for an entire foundation or even building to settle or tip without cracking (I've found a few), you should not find upstairs floors, windows, doors, all dead level if the building had shifted one inch.)

Also take a look at the location and pattern of floor cracks, If a crack line is more or less straight, and if it runs towards the foundation walls or more or less at right angle to the foundation wall, then if the foundation wall and footing had settled you'd expect to see wall cracks in the same area.

Evaluate the extent of floor slab settlement cracks

If those clues of actual footing settlement and foundation movement are absent, you are more likely seeing settlement in the concrete slab itself. Often floor slabs are not poured on compacted fill (they should be). The result can be future slab settlement and cracking, exacerbated by changes in site conditions (more water, less water, freezing, drying) that may affect the soil below the slab.

If the floor slab is poured with its perimeter sitting atop foundation footings (and presuming the footings are intact) the slab "hangs" on the footings, and perhaps also is "supported" by piers that may have been poured under the floor slab to support Lally columns that march down the center of a basement to support a main girder.

In that construction settlement of the slab may produce cracks as the floor bends and dirt below it settles. Cracks tend to be away from and sometimes roughly parallel to the foundation walls, or to appear as islands around the Lally columns.

If the floor slab was poured atop of dirt that covered the footings, or inside of the footings, the entire slab may settle or tip even at its perimeters.

In any case, the combination of pouring a slab on soft fill and changes in soil moisture invite soil settlement and slab cracking.

In fact my first job in construction was raking a huge dirt pile out to "level" inside of an already poured and built footing and masonry block foundation wall. Once the dirt was roughly "level" via my hand rake, the builder went ahead and poured his floor slab. "Compacted fill" was not in our vocabulary.

See SETTLEMENT CRACKS in SLABS for details about cracking floor slabs due to settlement.

See SETTLEMENT IN FOUNDATIONS for a more broad explanation of foundation settlement diagnosis, evaluation, and repair.

Repairing Slab Cracks and Cracked Floor Tiles

We discuss several slab crack repair alternatives at CONCRETE SLAB CRACK REPAIR. Unfortunately there is no magic band-aid that will make the cracks disappear, especially where ceramic floor tiles are installed. If radon and water entry are not an issue in your area you might live with the cosmetic defect for a while.

Repair of the cracked floor will require removal of the cracked ceramic floor tiles, including enough mastic and crud removal that you can bed replacement tiles smoothly in place. Before replacing the tiles that were removed you might want to also install mesh tape over the floor slab cracks to reduce the chances that those cracks telegraph again through the new tiles.


Watch out: do not lay ceramic tiles across control joints as movement there is likely to cause cracking in the filed finish-floor .

Question: settlement at steps & cracking in stamped concrete patio

Crack indicating settlement in concrete steps (C) CL Hairline cracking in stamped concrete patio (C) CL

[Click to enlarge any image]

2016/09/06 Anonymous said:

I had a stamped patio installed about four months ago. Recently I noticed that the steps appear to have settled some. They used to be tight against the house, but there is now about a 3/16" gap. I am wondering if this is anything to be concerned about. I have also noticed some hairline cracks in various places on the patio less then 1/32" wide. Also, any specific recommendations on how best to seal the gap against the house and the cracks? Thanks for any help.
[See the picture for which I've sent you a link. ] - C.L.



Settlement for new steps or patios is most-often due to inadequate site preparation such as building on poorly-compacted soil, though also roof spillage or surface runoff also cause settlement trouble.

I have posted your photos of both the concrete step settlement and the cracking in the stamped concrete patio along with your question here so that other readers can comment.

It looks as if the steps are settling to the right (in the photo) away from the building wall, perhaps from footing settlement, inadequate footings below the steps, or construction, as I speculated, on poorly compacted fill. Unfortunately we cannot predict how much more settlement is going to occur without knowing more about the construction procedure and site preparation. For the amount of movement shown in your concrete stair photo, expensive repairs are not yet justified but they might become necessary if settlement continues.

If settlement continues you may be able to use a slab-jacking or helical pier repair as an alternative to reconstruction.

I would use a flexible concrete-colored sealant to keep water out of the joint between concrete steps and the wood-sided house wall. This is particularly important when a builder simply pours concrete on the ground and against a wood-framed, wood-sided structure as that design invites rot and termite or carpenter ant attack. Keeping the area dry reduces that risk.

As long as the crack in the stamped concrete patio is just hairline in width I would not try to seal that crack as the repair will be ugly and probably won't address the underlying cause anyway. See SETTLEMENT CRACKS in SLABS.

More about stamped concrete is at STAMPED CONCRETE CLEANING

Concrete Crack Diagnosis & Repair Articles


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