Does a cracked slab mean the house is in danger?

Question

The house was built in the 70's (central CA) when I removed the carpet in the hall I found a crack about 1/4 in wide and 10 1/2 ft long. There does appear to be a small crack running in the patio, about even with this other crack. Is this going to make my house fail?

Answer 1

Oh, right on the San Andreas fault. The ground is probably shifting. Your house won't fall down (yet), but you probably want someone to take a look at it.

Answer 2

Concrete slab cracks are par for the course. However, a 1/4" crack is something else. In order to answer you question, I will assume the following: you are living in a ranch style home; or have a finished basement (which you did not mention) with interior partition walls since you mentioned it was a hall where you saw the crack. So, with these assumptions in mind, here is my answer:

First off, the slab is not an integral part of the foundation walls or foundation footings. That is, it is not a monolithic pour with the foundation wall. It would have been poured separately and expansion joint material placed along the edges against the wall. With that said, the probable cause of the crack is due to an eroded sub-base that collapsed the slab and made it crack. This is not a structural issue and your house will stand up just fine (until you get to my next answer). So since the house is very old, the slab is also very old and will not move anymore. However, if there is a vertical deviation from the top of the slab on side of the crack to the other side of the crack, then in order to have an even floor if you plan to finish it with a sticky back parquet flooring for instance, you will need to either remove the offending crack by saw cutting it out and grouting the saw cut in order to level it out. You can also use a leveling compound before apply a finish if you plan to use tile instead. Leveling compounds are a godsend with poorly trowed concrete.

Now, to address your concern if the house will fall, the answer lies with the foundation wall and footer. The only way a novice can tell if the footer has settled is if there are visible cracks in the concrete wall (for basements) or in the gypsum board walls (for the main level of a ranch home). If you see noticable cracks in the walls, then yes the house has either fully settled or is still settling. Since settling is common with homes over 30 years old due to poor drainage and compaction techniques, more than likely the house has fully settled to a 0 tolerance. But, if you are watering your flower bed along the foundation wall, or your gutter and downspout system is not transfering water away from the foundation by 3 feet or more, then the foundation/footer system will continue to settle.

Per code, even back that far in time, required what is called a perimeter drain (aka, french drain) to be installed along the perimeter footer. In the early days it was a clay perforated pipe placed in gravel bed to collect and daylight water away from the foundation. Nowadays, the pipe is plastic with perforations, but the idea is the same.

Also, per code, the foundation walls are to be coated with dampproofing to keep the hydrostatic head from penetrating the foundation wall.

Overall, it would be in your best interest to divert all potential water away from the foundation wall.

If you do the water diversion thing, you will have no problems with house falling blues, unless you are on a fault line or the house was built atop a horizontal mine shaft which is the case where I live. That is why we have foundation settling insurance written exclusively for abandoned mine shaft collapse.

Regarding the patio slab, this, again, is not an intergral part of the foundation. It is poured separately much like the interior floor slab.

So, to feel better about your new digs and the wonders of home ownership, you should feel better that you house will not fall down because of a cracked floor slab.

Hope this is helpful and have a good house warming.

Oh, btw, if you still don't feel comfortable about the crack, wait about one year to see if the crack changes in size or new cracks evolve. If this does not occur, then you can have a bigger house warming with the feeling your quests will not fall through the floor slab.

Answer 3

It would be an unusual house that didn't have some crack in the concrete slab after that many years. Look at the walls for cracks that might threaten your house. That is a much more likely spot to find evidence of too much settling.

Answer 4

it properly cracked about right after that concrete dried just stick some membrane over it

Answer 5

only if your near the fault line