Expert: Ted Barnhill (Principal, Design45 LLC) - 12/30/2007

Question
My mother recently employed a builder to dig out her lounge floor and lay a concrete one to a depth of about four inches.I now notice that in certain areas it has a hollow sound and has begun to hairline crack towards a part that seems to moveup and down fractionally. I contacted the builder and he said that this does happen sometimes but it is nothing to worry about as it will withstand quite a weight.     Could yoiu possibly give me your advice ?  Thanks so much.       Peter

Answer
Peter,

Without seeing your floor it's hard to tell how bad it is.  All concrete can experience hairline cracks, even if properly reinforced, set, vibrated, and cured.  This is part of its character.

However, it should definitely not move up and down.  The hollow sound is a concern as well; in my experience, only very thin slabs of 1"-2" have this issue.  

There are many ways that your mother's builder could have messed up; I recommend that you have a concrete expert take a look at your floor.

Good luck, Ted

(Sorry for the delayed response, I was out for the holidays.)