View Full Version : Hollow space under concrete floor?

04-20-2007, 03:42 PM
I had my house built about 3 years ago and it has an attached 2 car garage. To make it, they've built a foundation around the garage floor and filled with dirt, then topped with crushed stone and concrete.

Lately, I've noticed that one corner of the floor makes a hollow sound when tapping on it. It seems to be limited to about a 3'x3' area. Should I be concerned?

Basically, I'd like to avoid calling up my builder's service if at all possible and subjecting myself to pain and self-anguish.

04-20-2007, 03:55 PM
Half you noticed this anywhere else in the garage or basement?Depending on what kind of soil they backfilled with and how it and the stone were compacted, I'd say it's very likely that the base and sub-base have settled. If it settles enough, that corner of the slab will crack and drop. The good news is that this is a repairable problem.

1. Least work is to have a slab-jacking or mud-jacking (different names for the same type of contractor - look in the yellow pages for a foundation repair company) company come in and fill the void from above. They would drill a couple of 1" holes and pump a slurry of water, sand, and cement into the holes. Once it dries, the slurry becomes a grout that supports the slab.

2. Remove the suspect slab without undermining the adjacent concrete. Make sure the soil and gravel are well compacted. Replace material including the vapor barrier and reinforcing mesh. Use 1/2" diameter x 12" rebar dowels to connect the new concrete to the old concrete. Let it cure and you're good to go.

I've seen method #1 used to fill a void 2" deep x 6' wide x 60' long without problems. My parents also had their driveway repaired that way. I want to say it was about $500 to do half of a 22' wide driveway 25' long about 7 or 8 years ago.

The other thing that I'll mention is the new home warranty that came with the home. In some states, the builder has to supply a structural warranty that is longer than the full warranty on the home. Example: Indiana requires a 2 year comprehensive warranty, 4 year roof warranty, and a 10 year structural warranty. If I was in your shoes at my house, my first call would be to my builder.

04-20-2007, 05:42 PM
It can be a serious problem. I would make sure that it is fixed properly. I had a car fall on me, when the concrete under a jack stand crumbled and the car slipped off, while I was underneath aligning the mounts during an install of an engine/transmission assembly. I was pinned under the car until the fire department was able to safely lift the car, and get me out. Not a good feeling.

04-20-2007, 06:13 PM
I was pinned under the car until the fire department was able to safely lift the car, and get me out. Not a good feeling.

Holy cow. That's a scenario I have thought about many fact, every time I am preparing to get under a car. Being "only" pinned was a blessing.
I read about a few every year that are not so lucky.

Re: the hollow-sounding concrete, I would take immediate steps to resolve the issue. It could become a large problem if ignored.

There was a section of sidewalk in front of my parents' house that was like that, It made a cool sound when we rode our skateboards over it when I was a kid. It collapsed many years later due to a leaky connection at the neighbor's water meter. Big, stinky, muddy mess.

04-25-2007, 11:01 AM
Corners always seem to drop first. This is due to the fact that in the area of the corner you have a side and a front coming together and when the footer was dug, more dirt came out of that area than on just a side or front. If you don't have your foundation painted you can usually see where the concrete first was and where it has dropped to. Anywhere from 1/2" to better than an inch. Then if you notice...any water in the garage will now pool in the corners.
BUT...your contractor should have never backfilled with dirt. The best base is undisturbed soil with just the sod removed. Any dirt that came out should have been backfilled with gravel only. Or if dirt was used, it should have been compacted or better yet, let sit in the open for about a year to get natural compaction with freezing and thawing and rain packing it down. There are a few companies that do what they call concrete jacking. They will pump concrete, using pressure, underneath the slab to fill the voids and can actually jack up the existing slab. depends on how much is needed and sometimes it is cheaper to have the slab tore out and a new on poured. Almost any garage built on a foundation with a slab poured afterwards will have voids. To check this, go around with something like a spud / pry bar and thump the floor. You will find hollow spots.


04-27-2007, 09:40 AM
Thanks a lot guys. I didn't realize it was that big of a deal until now. I'll have to get the builder service involved right away!

04-27-2007, 10:26 AM
One half of my driveway has cracked at the center control joint and is tilting to that side. I think it's due to the down spount at that corner has eroded the sandy soil below and me driving on it is causing it to tilt.

Most people don't see it, but they don't call me "Level Eye" Larry for nothing.

04-27-2007, 07:58 PM
I had my sidewalk at the front door start to drop. Broke off the expansion joint. My builder sent out a sub that drilled a couple of holes in the concrete, then he pumped in concrete into the holes. Lifted up the the whole section. I also had a hollow sound in part of my driveway. I called the guy back and he pumped concrete under the driveway that had settled.

05-11-2007, 02:16 PM
I talked to the builder and they were about as helpful as I expected. Basically, they chalk this up to 'settling' and is normal on all their houses. Moreover, he went to suggest I do nothing about it because it's not worth the money. He's saying the floor is plenty strong enough even with air pockets underneath. He went on to say the only thing they'd fix is if a crack formed with a 1/4" or greater gap and/or shifting at the crack.

It (20x23 area) is reinforced with rebar and supposedly has some kind of beam across the middle. The floor is poured into the foundation walls, which are also poured concrete, with no gap or material in between.

After talking to a neighbor about it, who built at the same time as us, he remembered finding that the builder did not tamp the backfill in the garage area. That explains it. Note to self: if you want a nice new house, get it custom built on land you own and continually check up on the subcontractors.

Should I still be concerned? The house is about 2 years old and I haven't found any cracks in the floor yet. I'm thinking of epoxying the floor, but I don't want to build up on a subpar floor structure.