Floor slab & tile crack diagnosis & repair Q&A set 2.

FAQs about how to diagnose & repair the different types, sizes, shapes, locations, & patterns of cracks found in concrete floors & slabs. Some floor slab cracks are harmless or even just "cosmetic" while others may spell trouble ahead.

This article series 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. This series also 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.

Page top photo of a supported slab on grade design is provided Carson Dunlop Associates, a Toronto home inspection & education firm.

Diagnose & Fix Cracks in Concrete Slabs

These questions & answers about the cause, diagnosis, cure or prevention of types of cracks in concrete slabs and floors were posted originally at CONCRETE SLAB CRACK EVALUATION - home - be sure to see that article.

Some of these Q&A were also posted first as comments on this page that you are viewing.

[Click to enlarge any image]

Reader question: numerous cracks appear in one day old slab

(July 27, 2012) Garry Everett said:

We have just had a 20 x 15 foundation installed (1 days old) for a patio room and is has numerous cracks already is this normal for a foundation that does not have any stress relief?

Reply:

Garry, cracks that appear rapidly and early during curing of a foundation or slab may indicate difficult pour conditions such as very hot weather, or a problem with the mix, or even settlement if the site was not properly prepared.

You should ask your contractor to take a look at the situation. Keep us posted.

Reader question: garage slab dropped 1/2 to 1"

(Aug 13, 2012) Steve Snoreck said:

Looking at purchasing an 35+plus old home. Slab in garage has a crack and has dropped about a(1/2 to 1")on one side all the way across this 2 car garage.

Also, where slab crack meets poured foundation walls, there are cracks in the foundation wall and step cracks in brick veneer. Is this a major fix and deal breaker?

Thanks for your help

Reply:

Steve if the whole slab has settled without cracking and you see a prior concrete line on the garage foundation wall that marks the slab's original height, the explanation may be poor compacting of fill before the slab was poured

Reader Question: A sizable crack has now formed. is there any recourse for this?

(Aug 28, 2012) DMN said:

We recently ( 2wks ago ) had a new concrete patio put in. A sizable crack has now formed. is there any recourse for this? The contractor basically says this happens and we are out of luck. Is he responsible?

(Sept 9, 2012) Mark M said:

We just finished a new house build and have noticed spider cracks in the garage floor running from an interior wall to a centre floor drain.

The floor was poured in winter conditions and and 6 months later these cracks are showing up. Is this a building error or part of normal settling? How can this be repaired?

(Sept 20, 2012) Cracked slab causing discolorati said:

Our kitchen floor (either vinyl or linoleum) has started staining over the past six years. The stains look like water stains.

We have a concrete slab underneath and have had some cracking in the garage. But I am wondering whether the kitchen staining could be from a crack underneath the floor. Could water stain the floor without causing any flooding or noticeable issues other than the dark color? The flooring is original, about 20 years old and the discoloring is no where else. Thanks for any response!

Kristin

Reply:

Kristin,It sounds as if you are talking about sheet vinyl flooring. It may stain from a spill from above if the surface has been damaged,or there may be moisture comingnup through the slab.

Reader Question: cracks in home under construction

(Oct 1, 2012) Carol L said:

My home is currently being built. Upon inspecting of the 'slab' there are large cracks that mimic the 2007 Daniel Friedman picture below.

y builder tells me he is going to 'fix' it. How certain can I be that 1. no water will seep through the cracks and 2. that the concrete footers will remain firm and the walls won't have settling issues.

Reader Question: cracks in slab on expansive soils and clay

(Oct 1, 2012) Carol L said:

My home is currently being built. Upon inspecting of the 'slab' there are large cracks that mimic the 2007 Daniel Friedman picture below. My builder tells me he is going to 'fix' it. How certain can I be that 1. no water will seep through the cracks and 2. that the concrete footers will remain firm and the walls won't have settling issues.

Reply:

Nancy, in the article links at the start of this article you will want to read

FROST HEAVE / EXPANSIVE SOIL CRACKS in SLABS - separate article

If your problem is a home built over expansive clay soil there are systems to address the cyclic movement that would damage a slab; without fixing the cause, just fixing the cracks would be futile.

Also you'll want to have an expert examine the building to be sure you're making the right diagnosis and that there is no related foundation damage.

Reader Question: how to know if a slab crack is caused by settlement vs stress

5/4/2014 Mark said:

Is there a way to know if a crack was caused by settlement vs. stress caused by concrete slab that broke vaper barrier for purposes of brining drainage to basement bar.?What kind of professional is able to evaluate changes caused by this change in slab integrity and give us proper solution in that rain water penetrated this hairline crack recently?thanksMark

Reply:

Mark I don't completely understand your description, but in general an experienced foundation contractor, a building inspector with experience in foundation inspections, or if necessary a structural or civil engineer IF that person has expertise and focus on foundations, could all address your question.

Typically we look at the size, pattern, location, history, and context of a crack and usually those data point to a probable cause. If you want to use our CONTACT link to send along some sharp photos of the crack and others from a more distant perspective of the wall or slab and the building and site then I might be able to comment further.

Settlement under a floor slab that causes cracking will usually also show up as out-of-level floors, a difference in floor height on either side of the crack, and possibly we will find correlations with poor site prep (inadequate soil compaction) or mis-handling of roof or surface runoff.

Watch out: check that the cracks don't extend into and continue in the foundation wallls - a potentially more serious problem.

Question:

(July 29, 2014) M fetter said:

I had a contractor pour a driveway pad 25' by 32'- 5 in. thick nov. 19 2013. contractor did'nt cover with a blanket temp was aprox 30 deg. outside and didn't cut till four day's later. slab is cracking all over the place. may I have yer input in this matter please ? thnx

Reply:

MF

I'm doubtful that anyone can diagnose concrete cracking from just your brief e-note; you're welcome to use our CONTACT link (page top or bottom to find email) to send us some photos of the cracking you observe.

Cracks might be related to the curing process, shrinkage, settlement, bad mix, etc.

From your note we still don't understand your "cutting" question. But you might want to review the control joints article

at CONTROL JOINT CRACKS in CONCRETE where we describe the timing, depth and location of these construction details.

Question: sewage flooding caued floor heaving

Aug 2, 2014) austin said:

we just experienced a major sewer flood in our basement 2ft that sat for almost 3 days when we were gone. we have a sub floor in the basement and we do know we always had a crack in the basement floor but it now looks like that crack heaved a lot under the sub floor

they are removing the sub floor right away but insurance is kinda saying that its impossible for a basemnt floor to heave from a flood is this true?

Reply:

Austin,

No, the insurance company is mistaken. It's not impossible for a basement floor slab to heave in response to flooding.

For example, expansive clay below a slab could when soaked, expand and push the slab upwards.

Or alternatiely, soaked sewage waters may have caused settlement of poorly-compacted fill below a slab, contributing to uneven settlement of the floor on either side of an existing crack.

Often a careful examination of a crack can find evidence of its age and can identify recent movement by noticing raw or fresh surfaces on either side of a crack in the slab, wall or other masonry material. That detail can help with insurance analysis too.

Question: how to build a smaller structure atop a larger slab area

(Aug 11, 2014) Adam said:

Hi,I have an opportunity to buy a property in Northwestern Wa that has a 36x60 slab already installed. I believe it was intended for a modular home that never quite happened. I would like to be able to build a smaller structure on part of the slab, say 24 x 36. Is there a legitimate way to do this?

Reply:

Probably yes, Adam but you need to do some exploring.

Typically the slab would have been poured with a perimeter foundation intended to support the weight of the building on its perimeter or edges. There may have been other points of support inside the slab as well. Without knowing the slab design and ingredients I'd be careful about placing loads just anywhere.

And careful attention to sealing details will be needed at the smaller building's perimeter to prevent water from entering the structure.

You might, for example, need to build a smaller building permiter footing of solid concrete blocks set atop the existing slab.

Check with your engineer about the slab thickness, hardness, reinforcement, versus the loads that will be imposed by the new structure. Most likley you'll be fine.

(Aug 11, 2014) Adam said:

Thanks for the response. As a followup question, how would drainage be handled? If the building ends on top of a slab (i.e. a 24x36 structure on a 60x36 slab), how do you stop rainwater from migrating under the smaller-than-the-slab building?

Reply:

Solid question:

If you are not going to cover the protruding slab with exterior roofs, say as in a porch roof, the use of a sealant at the sills is about all you've got. If I were doing such a job I'd build a new masonry sill at least 1 solid concrete block high and set my wood framed wall sills atop that to permit a water tight seal at the slab and to avoid putting wood onto a sometimes wet surface.

Reader comment:

10/10/2014 Eng.Hasan Al-Bahkali said:

very important subjectEng.Hasan Al-Bahkali

Question: add insulation as part of concete crack repair?

(July 4, 2015) Philip said:If structures already in place make it undesirable to expose the foundation wall (for crack repair) along all of its length is it reasonable to install exterior insulation (as in FPSF style) on only a part of the foundation wall?

(similar with improved drainage, etc..)The section with problems is where the foundation is lower (a garage joined to the house but with entrance at lower level)

Reply:

I'm not sure, Philip. A lot depends on the extent, severity and cause of the cracking, the crack pattern, shape, location, and thus the implication for the stability of the foundation wall. I mean to say if there is a collapse risk clearly we need a professional assessment and foundation repair.

If the crack is not structurally significant (engineers may disagree since some call any crack a "failure") and if the only concern is water leakage, sure, steps outside to get water away from the foundation are appropriate.

I'd add that an outdoor repair to get water away from the foundation is a better approach than an interior band-aid anyway, but the cost is usually more as well. Start by making sure roof and surface runoff are directed away from the foundation.

Question: how to repair leaky hairlilne cracks in the basement floor

(July 22, 2015) chris west said:we have several hairline cracks in our concrete basement floor which are leaking. the cracks are hairline, and not wide enough to force any products down in the crack.any suggestions on how to resolve it?

Reply:

Darn.

Hydrostatic pressure is usually more powerful than we'd imagine, so I agree that not only can you probably not successfully force some sealer into a hairline crack but it will probably be ugly.

Options are:

1. Best: working outside, for example checking surface and roof runoff, remove or redirect the source of water that's passing under the slab

2. Second best: cut a sump pump hole through the floor, install a sump pump, and over time the pump will lower the water table under the floor, relieving the water pressure and stopping leaks up through the floor

3. Hire a repair contractor to cut open the cracks using an inverted Vee shaped cut, making the crack opening large enough to insert an epoxy sealer that can be applied when the contact surfaces are clean and dry.

At InspectApedia.com see WET BASEMENT PREVENTION

Use our CONTACT link at page bottom to send me some photos and I can comment further.

...

Continue reading at CONCRETE SLAB CRACK EVALUATION - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see CONCRETE SLAB CRACK FAQs-3 - more-recent Q&A about diagnosing & fixing cracks in slabs

Or see these

Concrete Crack Diagnosis & Repair Articles

• CONCRETE CORROSION DUE TO HYDROGEN SULFIDE
• CONCRETE DECK PIER PROCEDURE
• CONCRETE REINFORCEMENT, FIBER
• CONCRETE SHRINKAGE CRACKS
• CONCRETE SHRINKAGE CRACK REPAI
• CONCRETE SLAB CRACK EVALUATION
• CONCRETE SLAB CRACK REPAIR
• FOUNDATION DAMAGE by MATERIAL or INCLUSIONS

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