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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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."
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
at FOUNDATION CONSTRUCTION TYPES.
[Click to enlarge any image]
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.
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.
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.
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 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:
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).
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.
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.
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 .
[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
Continue reading at CONTROL JOINT CRACKS in CONCRETE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Apr 21, 2016) Danielle said:
My slab foundation has a crack in an outside corner of the house. It's path is from one wall to the other wall, about 11 inches long. I discovered this crack when the carpet was removed. I see no water damage, but there is a huge shrub that was planted way too close to the foundation which means the roots may have caused this problem? I have not seen any bugs either. Radon gas is probably not a problem since this house is a sieve (not really great construction). Can I leave this crack alone and carpet over it or should it be inspected?
Danielle I would not carpet over an open crack; I'd seal it first.
(June 29, 2016) Carol said:
New home construction - pulled up cracked tiles - large crack in concrete flooring with inspector markings of "blk" in two areas around crack. What does "blk" stand for?
"blk" is not a standard building inspection abbreviation that I've seen. Sorry I don't know. Perhaps you could ask your local building inspector and let me know what she or he says.
Don't just put down more tile without a better evaluation of the cause of the cracking and its impact.
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Please see the main text version of this article at Shrinkage Cracks in Slabs
We discuss the recognition and significance of concrete shrinkage cracks in detail at Shrinkage Cracks in Slabs. Just below you can read a summary of this topic.
Shrinkage cracks in poured concrete are easily recognizable and can be distinguished from other types of cracks that occur later in the life of a foundation wall or floor slab.
The photograph of cracks above were taken of shrinkage cracks in a concrete slab floor in a home built in 2006. The cracks in this case ranged in width (measured across the crack) from "hairline" (less than 1/16") to about 3/32" in the basement floor slab of this particular home. They may appear larger.
What is unique about shrinkage cracks in concrete is that they usually appear to be discontinuous, as shown in this photo. The crack will meander along in the concrete, taper to a stop, and then continue beginning in a parallel line to the first crack, meandering again through the concrete. This is characteristic of concrete (or mud) shrinking while giving up its moisture.
You can see the shrinkage of even a perfect concrete floor slab with no visible cracks in the field of its surface if the floor was poured inside of an existing foundation. Look for the gap between the edges of the slab and the foundation wall? Look also for the stains or concrete debris on the wall at the slab level? These confirm that at the time the slab was poured it was touching the wall.
Depending on the mix and pour conditions as well as the site work preparation significant concrete shrinkage cracks can appear in a basement slab. On occasion we may also see vertical dislocation in a floor slab crack, that is, one side has settled or tipped away from the other side of the cracked concrete.
Our photographs above show shrinkage cracking and ensuing minor settlement cracks occurring in the same poured concrete basement floor. We suspected that site drainage defects or possibly nearby site blasting for additional construction contributed to the 1/16" to 1/8" vertical dislocation across some of the cracks in this floor where the cracks were about 3/16" wide.
Happily the building owner at the site where we made the photos above did not report water entry through these openings, though it certainly might be expected as the home ages and its footing drains stop working, particularly if surface runoff and roof runoff are not kept away from the building.
The photo at left shows some concrete shrinkage cracks that are larger than usual. Note that these cracks begin at building foundation wall inside projection corners - a condition that probably creates stress points as the slab cures.
If you click on and enlarge the photo you can see clear evidence that the cracks are discontinuous, multiple, and very roughly parallel in this area - good evidence that they were caused during the concrete curing process and not by a later event which "broke" the slab.
When we see combined slab shrinkage and slab settlement condition we suspect that the concrete pour not only allowed for excessive shrinkage, or perhaps shrinkage without control joints, but also the pour was made on top of poorly prepared soils. For example if a slab is poured on poorly-compacted soil, after shrinkage cracking occurs, we may see uneven settlement among sections of the cracked slab.
Settlement cracking following shrinkage cracking may also occur. In both of these cases we anticipate that significant horizontal dislocation in a poured concrete slab should be small unless steel reinforcement was omitted or was improperly installed. Since some contractors use a fiber-reinforced cement and may omit steel reinforcement in floor slabs, this condition may occur.
We discuss the recognition and significance of control joints in poured concrete slabs in detail at Cracks at Control Joints in Concrete. Just below you can read a summary of this topic.
The first photograph shows an expansion joint in a basement floor slab. Notice that we do not see other cracks in this slab. Shrinkage cracks that occur at control joints such as shown in the second, close up photo here, are occurring where they are supposed-to, although the width of this particular crack was surprisingly large. These cracks are not normally a defect in the slab but may be a source of water or radon entry into the building and may need to be sealed. Use a flexible sealant.
We discuss the recognition and significance of settlement cracks in poured concrete slabs in detail at Settlement Cracks in Slabs. Just below you can read a summary of this topic.
Settlement cracks in a conventional concrete floor slab which has been poured inside a separate foundation wall (and often resting at its edges on the building's foundation wall footings) are usually not connected to the foundation wall and are not supporting any structure [except possibly Lally columns, discussed next].
These cracks may not be a structural concern, but there are cases where a serious hazard can be present, such as garage floor cracking when the floor was poured over soft, loose, inadequately-compacted fill and where the floor slab was not pinned to the garage foundation walls. Soil settlement under a garage floor, perhaps aggravated by groundwater which can increase soil settlement, can lead to first hollowing-out of space below the floor and second, sudden collapse of the floor structure.
We have used a heavy chain, dragging it across the garage floor and listening to changes in the sound it produces, to find areas of significant soil voids below the floor. The pitch of the chain noise drops significantly when passing over a void below the concrete.]
Settlement cracks in a concrete floor around a supporting Lally column might be indicative of a serious problem such as building settlement if the columns are settling. Independent footings may have been provided supporting Lally columns in the building interior and those may be settling independently of the floor slab which may have been poured around and even over them (See photo above). But beware, where slab thickness and local building codes allow, supporting columns may bear directly on a poured floor slab without their own (deeper) pier or footing. In that case floor slab cracking and settling can cause column movement and may be a structural concern.
Settlement cracks in a monolithic slab or floating slab floor may be more serious, depending on their extent since in this case the edges and other portions of the slab are, unlike the cases above) expected to support the upper portions of the building structure.
A monolithic concrete slab is one which includes the building footing as part of the slab, created in a single continuous pour of concrete.
A floating concrete slab is one which is poured at a (generally) uniform thickness on the ground without a separate footing. [Beware, in areas of wet soils, expansive clays, freezing climates, or unstable soils, floating slabs may be exposed to extra stresses and may tip or crack. Proper site work and drainage are important as is proper engineering design of such structures.
We discuss the recognition and significance of frost heaving damage and cracks in poured concrete slabs in detail at Frost Heave/Expansive Soil Cracks in Slabs. Just below you can read a summary of this topic.
Frost heaves or expansive soils damage to building floor slabs can range from minor to extensive in buildings depending on soil and weather conditions, site preparation, and slab construction details, as we elaborate here.
The photograph above shows a rather straight crack across a garage slab near the garage entry door. What is happening here and why is this particular crack straight if it's a frost or soil heave crack?
In freezing climates building foundations include a footing which extends below the frost line. This is true for both the occupied space as well as garages. When a concrete slab is poured either abutting the top of such a foundation, or poured extending over the edges of such a foundation, there is risk of cracking across the concrete at the interior edge of the buried footing.
The combination of water under a garage floor (watch out for driveways and sites that slope towards the garage or home) and freezing can cause the portion of the slab which rests directly on the soil to move up and down during freeze/thaw cycles. Since a garage is often colder near the garage entry door than in other areas, there is extra risk of these cracks occurring there, but they can occur anywhere. When there is freezing and heaving of a slab, particularly one which omitted reinforcing steel, or did not extend the steel over the footing, these garage floor slab cracks may appear during freeze/thaw cycles.
Similar floor slab damage might occur in areas of expansive clay soils if the proper moisture level is not maintained.
Basement floor heave patterns - frozen floor drains: Basement floors can be frost heaved in other patterns in buildings which are unheated or which lose heat. We have found basement floor slabs broken and heaved above buried drain lines which ran below the basement floor of a home which remained unheated during freezing weather.
A clogged drain sitting full of liquid combined with prolonged freezing weather was the culprit in most of these cases. The heaved concrete was raised following exactly the path of the frozen (and burst) buried, clogged drain line. This problem can be epidemic in older homes which were constructed using a downspout drain line extending below the basement slab.
Basement and garage floor random heave and crack patterns:
Cracked and heaved concrete or settled concrete can occur in more random patterns in any concrete floor where there has been frost heaving, soil contraction/expansion, or simple soil settlement, as shown in this photograph.
Garage or basement floor sloped or semi-uniform settlement may also produce a tipped floor even if the concrete is not cracked, or the floor may settle uniformly. This condition occurs if the concrete was reinforced by steel or fiber cement, but was poured inside of a separate concrete or masonry block foundation. We see this condition more often in garages in which the slab was reinforced but poured on poorly-compacted soil. The problem may be worst if in addition to poor compaction, water runs under the slab, causing additional or more rapid soil settlement.
My first construction job (for pay) was to rake level the backfill soil that the contractor had dumped inside of the newly-completed garage foundation in a series of homes. No compaction of any kind was performed. When a lot of fill, several feet or more in depth, was required to bring the slab to the desired height, there was a good chance that the slab would settle or tip in the future.
Garage slabs which were poured inside of the foundation walls but which were pinned to the foundation sides (typically using re-bar set into holes punched into the masonry block foundation), the slab was resistant to settlement or movement even if there was modest soil settlement below.
In a garage where the slab has settled you can often spot the original level of the slab and thus can measure the amount of settlement. Look for a concrete line above the level of the top of the slab and found along the masonry block or poured concrete foundation wall. we have seen this line ranging from a fraction of an inch to six to eight inches above the current level of the slab!
Readers should also see CONCRETE SLAB CRACK REPAIR and may also want to see SINKING BUILDINGS where we include case histories of both building settlement and slab cracking, heaving, settling: diagnosis and repair. Also see How to Diagnose & Evaluate Foundation Cracks since those pages also assist in distinguishing among types of cracking in concrete foundations (vertical supporting walls and footings).