Thread: slab-on-grade floor was sunk

Needs help.
Recently inspected a two story house, structure configuration is slab-on-grade. The ground floor slab was sunk, interiorly and exteriorly. See the picture. The house was solid, no signs of movement, no wall cracks, no displacement of windows or door. Should I report as Non-structure failure settlement or report as foundation failure. Of cause, I was recommending a further investigation by structure engineer.

Thanks.

Originally Posted by Gordon Lee

Needs help.
Recently inspected a two story house, structure configuration is slab-on-grade. The ground floor slab was sunk, interiorly and exteriorly. See the picture. The house was solid, no signs of movement, no wall cracks, no displacement of windows or door. Should I report as Non-structure failure settlement or report as foundation failure. Of cause, I was recommending a further investigation by structure engineer.

Thanks.

"Interiorly and exteriorly" What does that mean?

Your picture shows a slab that has settled relative to an adjoining slab. Looks like a garage floor slab abutting a living space slab. The difference seems to be consistent. Probably the garage area was improperly compacted and has settled.

Report what you can see and the facts only. No need for labels like non-structural failure.

From your photo I would not call that a slab on grade as that has always meant (wherever I've been) to be a monolithic slab - what you have looks like an inverted tee foundation (footing below grade at some depth with a foundation wall up to above grade level) with a slab on fill.

The slab on fill is the fill within the foundation walls on which the slab sets - and, yes, absolutely yes, when the fill is not properly compacted the slab will follow the fill down as the fill compacts.

The first time I found that condition was in Hollywood, FL in a 1950s house with inverted tee footing/foundation wall with slab on fill, and I caught it because something looked strange about that 6"-9" (don't remember which it was) tile base around all the rooms ... then I notices a few pieces of the tile base were loose and when I looked behind the tile base the wall bottom plate was still attached to the slab with nails sticking up and spanning the open space between the bottom plate and the bottom of the studs ... oh, yeah, almost forgot the important part - the slab was down 3" to 4" all around the house ... no wonder the ceilings were over 8 feet high ...

I agree with Bruce - looks like a garage floor and if the foundation seems to be good then it's seldom a concern (in my experience).

This is probably a house with a spread footings and foundation walls supporting the structure with a slab-on grade floor. Assuming the slab does not support columns, etc. it does not support the structure. However, there are sometimes large voids below the slabs (especially if adjacent to a basement). I have seen the slabs fail, which can be a relatively expensive repair. I would choose your words carefully. While it does not typically support the structure, it can be considered a structural component. As to whether it is a significant concern, that depends upon how extensive the voids are and how well compacted the soil is.

Best to have a structural engineer evaluate. I often do this by drilling through the slab and probing the soil. I have occasionally removed some concrete to view below. The voids can be quite large.

Left side-facing left-rear (512x384).jpg

I don't disagree with what you're saying Mark but I'd personally be reluctant pushing for the S.E. for floors such as in Gordon's photo. Most of the floors around here are not structural and have no relationship to the building's foundation. Of course there are times in which I'm not so forgiving.

Originally Posted by Eric Barker

I don't disagree with what you're saying Mark but I'd personally be reluctant pushing for the S.E. for floors such as in Gordon's photo. Most of the floors around here are not structural and have no relationship to the building's foundation. Of course there are times in which I'm not so forgiving.

I agree with Eric's opinion, and that was what I have put in the report. But still I recommended a S.E just to cover myself.

Thank you, Bruce, Eric, Jerry and all other members, Your thoughts are greatly appreciated.

Gordon

Is that a stem wall next to the slab or another slab? From the picture is looks like a garage slab next to the wall shared between the garage and the living space. That look is pretty normal here, and will usually show the slab in the garage sloping towards the overhead door. I don't think I would remark on it at all. They usually pour the garage slab after the house foundation here.

My SWAG with the limited information.

How to Evaluate Cracks in Poured Concrete Slabs

Originally Posted by Gordon Lee

I agree with Eric's opinion, and that was what I have put in the report. But still I recommended a S.E just to cover myself.

Thank you, Bruce, Eric, Jerry and all other members, Your thoughts are greatly appreciated.

Gordon

Can you share more about where in the house the pic was taken? Is this an exterior patio, a garage floor, etc? All of us are guessing based on a very limited picture and confusing description. Would be interesting to find out more.

Originally Posted by Bruce Ramsey

Can you share more about where in the house the pic was taken? Is this an exterior patio, a garage floor, etc? All of us are guessing based on a very limited picture and confusing description. Would be interesting to find out more.

Actually, all the ground rooms' floor are sunk. But nothing happened on second floor. no cracks on interior and exterior walls. Here I up loaded more pics.
Thanks.
Gordon

Originally Posted by Eric Barker

I don't disagree with what you're saying Mark but I'd personally be reluctant pushing for the S.E. for floors such as in Gordon's photo. Most of the floors around here are not structural and have no relationship to the building's foundation. Of course there are times in which I'm not so forgiving.

Eric, I'm not saying that you should recommend a SE for every case like this. I would just be careful how you report this. Often just by stamping your foot or standing with your heels raised and dropping them to the ground suddenly you can get an idea as to whether there are large voids below the slab. Reporting that there may be a potential for additional settlement or some potential for further damage is also ok. The buyer can decide what to do with that information. Many times there is little concern with this condition, but I would not say don't worry about it. If you are talking about fine-grained soils (clay or silt) they can continue to settle for many years and will never settle to a proper level of compaction over time. Especially with garages, slabs can experience pretty large concentrated loads.