Categories: Foundations

Are Cracks In Concrete Foundations Bad?

Concrete Cracks

I often get asked about cracks in concrete foundations. Many home owners are nervous when they see cracks in concrete and wonder if they are bad or dangerous. While it’s a natural reaction to be concerned when you see something cracked the reality is that 95% of cracks in concrete are harmless and nothing to worry about.

Why Does Concrete Crack?

Concrete cracks when it is subjected to tensile stress (forces that pull apart on something vs forces that push together something). Concrete has very strong compressive strength and very poor tensile strength. That’s why concrete is reinforced with steel reinforcing called rebar. So you end up with a composite material, the concrete takes the compressive loads and the steel rebar takes the tensile forces.

There are many factors that cause concrete to crack. Some of the factors that can cause concrete cracks are:

  • Drying Shrinkage – as concrete cures and goes through it’s chemical reaction between the water and cement particles it begins to “dry” out. As with most materials that dry out they begin to shrink causing hairline cracks to appear.
  • Thermal Shrinkage – all materials shrink and expand with changes in temperature. As concrete cools down (the chemical reaction that’s created when water and cement is mixed creates heat) it shrinks. The shrinking from cooling down can also cause hairline cracks to appear.
  • Restraint – This topic is a bit harder to explain but it’s caused when concrete is confined and prevented from moving due to the structure. If the concrete wants to move due to loading, thermal or other effects and it’s restrained from doing so it will crack.
  • Settlement – If the ground around a foundation settles it can cause the concrete to crack. Settlement can cause significant damage under certain circumstances.
  • Loads – As I mentioned when concrete is subjected to loads that cause tension it will crack the concrete. Tension can be caused by many different loading conditions and it’s best dealt with by designing reinforced concrete that uses rebar.
  • Corrosion – When steel rebar corrodes inside of concrete it expands and causing the concrete around it to crack.

Which Concrete Cracks Are Bad?

Obviously this is a topic that has many answers and not easily answered in a simple way. However, what you need to know is that ALL concrete cracks. In fact, reinforced concrete must crack before the rebar can take hold and do it’s job. Most cracks are so narrow that you can’t see them unless you look very closely with good light.

So which cracks are bad? Typically if you see a concrete crack that’s less than 1/8 inch wide then I’d say you can totally ignore it. If you find a crack that’s between 1/8 inch and 1/4 inch wide it is most likely nothing to worry about. In this situation it’s important to look at the concrete on both sides of the crack and try to determine whether or not there has been any relative movement between the sides, up/down, left/right, in/out. This would be an indication of settlement and therefore a potential problem.

If you have a concrete crack that’s 1/4 inch wide or wider then I recommend you contact a local structural engineer. Cracks of that width indicate significant movement within the foundation and warrants further investigation.

Minor Concrete Crack Repairs

Minor cracks in concrete can be repaired easily by DIY home owners. Concrete cracks can be filled / sealed using a number of products. I happen to like concrete crack fillers from Sika. You can also use hydraulic cement (it expands slightly as it cures) to fill cracks. The idea behind filling the cracks is to seal them up to prevent moisture and air from moving into the concrete.

Summary

Cracks in concrete are a fact of life. ALL concrete cracks and you should understand this issue and realize that most concrete cracks are safe. If you find a wide crack and evidence of movement then contact a local structural engineer for further investigation. I hope this article has cleared up this confusing and mis-understood subject.

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

View Comments

  • Todd, great post. Your descriptions of crack sizes and things to worry about are a great reference.

    Happy New Year and good blogging in 2009!

  • I am very concerned,I am buying a home and i noticed that in my living room my concrete floor has some major cracks in them.I can feel them under my carpet.we purchased our home in 2004 so its less than 5 years old..if anyone can give me advice please do so..Thank You

    • @ EJ - It really depends on how wide the cracks are and what type of construction. Is it a slab on grade? If it's a slab on grade and there are no signs of settlement affecting the structure then you can fill the cracks the next time you change the flooring.

  • I have a big crack right up the right hand side of my concrete front stoop, from the bottom step to the top. I would say the crack is about 1/4 inch, no shifting up/down, front/back- none of the walls in the basement or the house indicate that there is any kind of foundation or structural problem as far as I can see, do you think the crack in the stairs in cause for concern?

  • I'm a young First time home buyer and after our home inspection our inspector noted the following on the house's slab on grade concrete foundation: Common minor settlement cracks were observed in the foundation. This implies that some structural movement of the building has occurred. Cracks of this type should be watched for any sign of additional movement. In the absence of any sign of ongoing movement, repair should not be necessary.

    Is this something we should be concerned about or something that should stop us from buying this place?

    thank you so much!!

    • @ 1st Time Home Buyer - Sounds like nothing serious. Without seeing it myself it's hard to say. Minor cracks are almost never a problem.

  • Hey Todd I am a 1st time home buyer and the house that I am interested in had just been built. The concrete slab foundation has a crack in it the total width of the house. You can see it from the outside and it travels from there through a bedroom through the family room under the fireplace to the other end of the house. The crack is not wide, but this is a serious concern for me because I am a 1st time home buyer. Will this be a problem down the line considering the weather (rain, coldness, etc).

    • @ Juju - Almost every concrete slab has visible cracks in it at one time or another. Without seeing your crack it's hard to say definitively if it's bad or not. However, if they crack is less than 1/4" wide and the concrete on either side of the crack is level then it's most likely nothing to worry about. If you're still concerned then I would recommend speaking with a qualified home inspector or engineer. Best of luck!!

  • Todd,

    I purchased a home that has a crack 360 degree halfway up all of the walls inside the basement. The previous homeowners had it epoxy injected but it still has small cracks apearing near the original crack (5 years later) 1/16 - 1/8 ". An engineer stated that the rebar is most likely causing the crack. He said that the home is structurally sound. We are considering finishing the basement after we have a drain tile system put it to catch a small amount of that leaks through during HEAVY rains. Is this rebar corossion a major concern? SHould it keep up from finishing the basment. The home is 50 years old.

    • @ Dave - Without seeing the home it's hard to say what the cause is. However, a horizontal crack as you describe is most likely caused by corrosion in the rebar. It's really hard to say how the rebar will act over the next 50 years. It is possible that the corrosion will continue, as that happens the crack will likely continue to grow. In a perfect world I'd suggest digging around the foundation (outside) and installing a very good water proofing system. If you eliminate the water that is causing the corrosion you may be able to stop the cycle. However, that would be extremely expensive and there is also the possibility that the crack may be very stable now.

      It really comes down to how much money are you going to spend in the basement and how many years are you looking to enjoy that space? Your situation is a bit complicated and it's really hard to say what the best approach is.

  • IN THE OLD BUILDING ON LAST FLOOR A CANTLEVER BEAM AND SLAB PORTION IS DETORORATE AND DAMAGE . REINFORCEMENT IS CORRODE AND CONCRETE BOND IS SEPARTED.FOE THIS QUESTION WATE REMIDAL MASURE TO ACTION TO OVER COME THISE CASE OR TYPES OF MANITANANCE TEQUNQIUES.

    • Esayas - Sounds like you need to hire a structural engineer to evaluate the problem. Cantilever beams and structural slabs are not something for DIY.

  • I have been repairing my basement as a result of a sump pump backup. I am replacing all 2x4s with Bluwood. When I removed the first part of the load bearing I noticed the bottom stud plate was rotted with mold underneath. I cleaned the concrete slab floor and let it air dry. Now I see hairline cracks with slight water seepage (wet areas) around some of the cracks. I supposed the buidling structure has been applying pressure to the slab for so many years that it cracked over time. (The house is 40 years old). What would you recommend to fill the hairline cracks? My plan was: Hydraulic cement the paint over the bottom plate area with Zinsser Watertite paint followed by a stud vapour barrier then the bottom stud plate.

    Thanks as always! Your site is fantastic.

    • Norm - Sounds like a fun project. Those cracks may well have been there for 40 years! Sometimes slabs get hairline cracks almost from day one. If there are no signs of significant settlement then I wouldn't worry too much about them. I would use any type of urethane concrete crack sealer that's able to deal with hairline cracks. If the cracks are really narrow it's likely that paint will be the only option.

      Good luck and have fun with that remodel.

  • I had a home inspection kept asking about the cement floor and a small crack in the garage in 1/4 slab section, told unevenness of floor in home was poor workman ship and the owner of the company came to visit after I put the inspector over the coals. he came out told me he does not see an inpressions, and to put rug over the linoleum then I wont feel anything, the small hairline crack in garage not a real concern, the impression all over the living room, the lumps and impressions and in a 9 foot span he found as much as 3/8 inch off. Telling me to put a rug over it does not sit well with me. our opinion, the home is only 5 years old.

    • Without seeing the situation its impossible for me to speculate if you've got a problem. All concrete cracks it's just a matter of if the cracks are caused by shrinkage, settlement, or a serious structural problem. Typically, if a crack is narrower than 1/4", it's seldom a structural problem. I know that probably doesn't sound reassuring to you, but hairline cracks are not something to worry about. If all else fails, you could hire an engineer to evaluate. Good luck.

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