Concrete Slab Cracks – Don’t Go Crackers
People believe that cracks in concrete slabs, such as basement floors and driveways, are construction defects. It is wisely written that there are two types of concrete slabs: those that are already cracked and those that going to crack. Armed with this bit of wisdom, you can develop realistic expectations about slab cracks. The following discussion deals primarily with new home construction; however, if you use good judgment, this information applies to slabs found in most homes and light commercial buildings.
Experts will tell you that slabs may not crack if you follow proper procedures. Proper procedures include: preparing and compacting the supporting material under the slab, using the correct concrete mix, delivering the mix promptly to the job site, prompt and professional laying and finishing at the job site, and curing at the correct temperature and moisture. If even one of these procedures is not followed to near perfection, slab cracks can result.
It is reasonable to spend the time and money required to lay concrete by the book in commercial construction (with large budgets). In residential construction, one must balance the cost required to follow proper procedures against the benefit of having crack-free slabs. Because most slab cracks are cosmetic, the cost often exceeds the benefit; thus, it’s common and reasonable to see minor cracks in residential slabs.
One usually asks three questions when evaluating cracks in residential concrete slabs:
- where is the crack?
- how big is the crack, both vertically and horizontally?
- how active is the crack?
Answers to these questions will help determine how to deal with the crack.
Crack location is important because different locations have different intended uses. Cracks in areas intended for vinyl, tile, or wood floor coverings might adversely affect the cosmetic and functional performance of these materials. Vinyl can reveal very small cracks. Cracks with vertical displacement can cause cracks in tile and grout and squeaks in wood floors. Cracks that run through stem walls and footings can be more serious than other cracks because cracks in structural components may adversely affect structural integrity.
Size matters when evaluating concrete cracks. The National Association of Home Builders (NAHB) standard for cracks in interior slabs requires repairing cracks exceeding 3/16 inch wide, or high (vertical displacement). State regulators may have different standards. Smaller cracks may be patched, but this is usually not a good solution because the patch may be more noticeable than the original crack and because the patching material may crack or become dislodged. It is often best to leave hairline cracks alone.
Cracks with vertical displacement in driveways, walkways, and patios can create a trip hazard. Generally accepted rules for these cracks in older construction are less clear. Displacement of 1/4 inch or more can present a trip hazard, but some inspectors will not call out a crack under ½ inch displacement, particularly in older homes. Extra care should be taken if children or those with reduced mobility regularly use concrete slabs with greater than ¼ inch vertical displacement.
Evidence of continuing crack activity is the most difficult question to answer and can have the greatest impact on how best to deal with a crack. Evaluating continuing activity is a matter of judgment and experience. For example, cracks where prior repairs have been attempted can be a sign of continuing activity. It can also be a sign of a poor repair job or the normal aging of the repair material. Crack monitoring devices exist, but they often are not practical on slab cracks and they usually require monitoring over a long period of time.
You should not be concerned about cracks in most residential concrete slabs. Wide cracks and those with vertical displacement should be brought to the attention of a qualified professional for evaluation and recommendation
Construction defects and mistakes put your family’s health and safety at risk and cost you money. Everybody’s Building Code helps you avoid construction defects and mistakes, whether you do the work yourself or hire a contractor. Everybody’s Building Code explains the International Residential Code in plain language and illustrates it with numerous drawings and pictures. Learn more about doing the job right the first time at