Our new house is just complete and we are about to close. Today during the
final checkup we found the 2-car garage slab has a hairline crack in the
middle, running from the front to back for 3/4 of the depth, ending at a
point about 6' from the inside wall. We put a 6' straight edge (steel ruler)
across the crack and found the two halves of the slab are at an angle. When
one end of the ruler sits on the slab at one half, the other end is at least
1/4" about the floor. When the ruler is entirely on either half, it sits
straight and square on the slab. So it is apparent that one side of the slab
has settled more. It appears that the front corner that is on the far
outside (away from the other part of the house) has sunk. When looking for
more evidence, we checked the foundation wall around the garage. The wall is
about 1' tall around the slab. We found at three places has 2" long hairline
crack at the top edge. They do not run all the way to the slab, though. The
locations of these small cracks confirmed it is that corner that has settled
more. Subsequently we carefully inspected all foundation walls in the
crawlspace. The other part (around the heated space) look fine.
We know many houses will eventually have some fine cracks due to
differential settlement. But this appeared before we move in. It happened so
soon. The concrete was poured in October, only four months old. When the
concrete was poured, we asked GC if he had compacted soil in the ditch he
said no need because that's undisturbed soil. We have clay soil here and it
is as hard as rock at the back of the house but at the front it appears not
as hard. But we trusted GC.
We just found it today and will speak to the GC tomorrow but we'd like some
opinion from poeple here. Is it a concern big enough to stop the closing
process? What the GC can do to remedy it? The city inspectors OK'ed the
house last Friday. Don't know if they noticed the slab crack. We like the
house and want it but if the cracks grow larger and larger in the coming
years it is scary.
Some source (not an engineer) told us that the garage slab may not be
structural. In the south they use slab foundation thus the slab is what the
house sits on. We are in the northwest. The foundation is foundation wall
and the studs sit on the foundation concrete wall not the slab. Slab is just
to cover the garage floor. The slab is somehow not well connected to the
foundation walls. Our foundation walls were poured early October but the
slab was poured one month later after the framing and roof were built up.
With this info we went to check again in the evening. The crack IS higher
and the steel ruler sits across it bit like a seesaw. With 3' (half of the
6' ruler) sitting on one side of the crack, the other end is about 1/4"
about the floor. We extrapolate this 1/4" from 3' to 11' (half of the garage
width), we get about 0.9". That would be the amount the far end slab has
settled. We checked where the foundation wall and slab meet all around the
garage. There is NO sign that there is shift between the slab and the wall.
It looks more like there has been some clay soil heaving near the front
middle part of the slab so the crack open up and is a bit higher than either
side - the soil under the gravel is clay, and the house has gone through
freezing temperatures of Dec and Jan without garage door in place. The three
2" long cracks on top edge of foundation wall are probably not uncommon. If
the walls had settled down together with the slab at one corner for 1" or
so, we'd have larger cracks in wall.
We know the builder put rebar in the slab. He told us he put rebar
everywhere: driveway and patio.
So it doesn't sound as bad as it did at the beginning.
Your concrete possibly was not originally finished flatter than 1/4 inch in
6 feet so I would not asume the cracks are due to settling. Hairline
shrinkage cracks are very common in concrete and if stable should cause no
I would immediately get opinion of a commericial or residential structural
engineer, and not one selected by the GC. Hairline cracks are not uncommon,
but differential shifting could be serious on a four month job, as you
suspect, and may be due any one of many causes, including incorrect soil
treatment, insufficient thickness of compacted ballast gravel pad under the
slab, concrete's too thin, improperly mixed, or undersized or missing
re-bars, or incorrectly linked rebars. Onsite hands-on inspection by a
consulting specialist, paid by the visit or hour, is the way to go, as the
most you'll get from a newsgroup is armchair speculation as to root cause,
including this opinion..
If it were my house, I would be concerned. I think your
suggestions are right on the mark.
My 2-car garage has a crack in it as does the drive way, but
they developed over time. And it is clear now that that the
fault was in the initial prep. My foundation walls also
have a couple of cracks (crawl space) which shouldn't be
there but have caused no problems. But cracks in four
months? Contrary to others, I've never seen cracks, small
or even fine hairline cracks, in well placed concrete. I
have never seen it any any of my concrete whether
wheelbarrow, small mixing machine, or big truck stuff. And,
I have never seen it in old industrial type stuff. Don't
know about new industrial stuff, but I'll bet hairline
cracks are due to poor mix or poor placement.
I've watched foundations poured around here and have been
appalled at the way new concrete structures are treated.
Speed is of essence for contractors and sometimes wall forms
are removed in less than 24 hours and usually removed in
36-48 hours. The fresh concrete is usually neither covered
and nor kept damp, and never kept warm in freezing
temperatures, so the concrete strength is very poor compared
to what it could be. Neither is there any steel
reinforcement, and initial ground prep often looks rather
We also have lots of clay which expands and contracts with
water content. From October until now, the moisture content
of the clay base could have radically changed. So, you can
expect some movement. That means that good ground
preparation is needed to minimize the movement. If one
corner of the garage has sunk, then ground preparation must
have been poor in regard to compaction and base prep or
I would have to really like the house to accept it and would
demand that the problems found by the consultant (hopefully
not the brotherinlaw of the GC) be fixed to my satisfaction
and guaranteed for a 10 year period.
opinions from people in this newsgroup would be the blind leading the blind,
visual inspection and other procedures are necessary to answer your
hire an independent structural engineer to determine if the slab is a
problem, otherwise you'll be worrying about it until you get an expert
opinion, whether it is a problem or not
The only way your going to know for sure is to hire an engineer and inspect
The way that homes are being done in my area the garage is part of the
God help you if the builder is KB.
While cracks are not at all unusual and most are not cause of alarm or
an indication of a problem or fault, I would be worried about the one you
are describing. It will take an engineer to be sure, I would hold off until
I was sure on this one.
I doubt if it is going to turn out a big problem, but if it is a big
problem it can be a really big problem.
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