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Question DetailsAsked on 3/6/2015

Im in the process of buying house, Garage slab is sinking or has already sinked about 2inches

garage slab was not poured on top foundation wall, but it was below the wall with steel bars and slab sloped to drain was built in 1992. foundation wall is poured concrete.

Now I see that garage already went down about 2inch...almost all sides...two or three thin cracks on the slab. Im assuming the soil compact was not done perfect. I see a vertical crack (quarter coin Thick) on the outside foundation wall and it stopped halfway through...not sure if this has anything to do with it. seller says garage sinked even before he bought it and the crack outside... do I really need to fix the garage stop any further cracks on the foundation wall or fixing the garage has nothing to do with the foundation wall crack...I see a expansion gap between the garage slab and foundation wall...

I need someone to look at to see if this a major issue, who should I reach out ?

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2 Answers



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Answered 5 years ago by Member Services


Slabs (garage or basement) are rarely supported on top of the foundation, because as the fill settles (which it almost invariably will to at least some extent) then the slab is left suspended from the foundation, and because it is not designed to span 25-30 feet or so (for typical 2-3 car garage), it will experience severe cracking - plus as it cracks and settles, it will push out on the foundation wall and may damage it too. Therefore, the slab is NOT connected to the foundation walls (though does have internal reinforcing to control cracking) and is supported on fill that is placed inside the foundation perimeter after that is built, and because of that is commonly poorly compacted - or just levelled and not compacted at all which is not right, but without full-time inspection that is the sort of thing contractors commonly do to cut corners. To prevent the slab from pushing against the wall when it expands and contracts and to prevent bonding to the foundation wall and keep dirt out of the construction joint so it retains its expansion space, a compressible filler is used - can be compressed rubber or foam bulb, but more commonly asphalt-impregnated fiberboard on edge. And rightly should be flexible caulked at the top to keep water and dirt out but rarely is. That is almost certainly the expansion gap you see - typically 1/2-3/4 thick fiberboard forming the joint.

2 inches is a LOT of settlement - probably means the fill was just loose levelled sand, and it would be very unusual to see that much settlement without some hairline cracking in the slab. I am guessing your slab cracks are slightly meandering feathery cracks with occasional spots with 2 or 3 forks in the crack around little spalls at scattered places, and likely either roughly along centerline of slab in each direction, or possibly diagonally from center of slab to one or more corners. Whether it will settle more and crack more is anyone's guess, but usually this sort of settlement (barring a highly unlikely sinkhole situation) is mostly done after a few years, so since there since 1992 probably pretty much done settling. The cracks could just be filled with joint filler grout or even just concrete caulk in a tube, to keep water and dirt out of them and provide a nicer looking surface.

The foundation wall crack could well be from the slab pushing against the wall and wedging it outward as it settled, which would typically cause vertical cracking in the outer face of the foundation starting in the approximate center, with additional cracks near quarter points in more serious cases, and outward bowing of the foundation if significant wedging occurred. In a cinder/concrete block wall this is pretty common because their joints are pretty weak and usually does not lead to major problems - again, since 13 years in place has probably pretty much finished moving, whereas in a new house you would have less confidence it will not get worse. In a poured concrete foundation wall that would be more worrisome becdause it is far stronger so would resist normal forces like that without cracking, so could be a concern especially if there is measurable tilting or bowing out.

How serious your case is - hard to tell without looking at it in detail, but I would at least have the wall crack epoxy grouted to rebond the cracked surfaces - not just dressed over with grout, but injected or progressively packed into the crack working from bottom to top so the entire crack is filled with the grout. Likely cost - maybe $200-300 range for the foundation crack repair UNLESS the engineer feels the rebinforcing has been compromised, in which case would be likely in $500-1000 range for a jackhammering out and localized replacement of concrete at that location. Slab crack blowing out with compressed air and sealing probably about $50-100 additional (if done at same time) depending on length and width.

For a professional review, you would use a Structural Engineeer - probably about $150-200 for a site visit and verbal thoughts while there, or $250-350 range to include a letter report and written repair recommendation - which you would need in hand to provide the Seller as justification if you are going to make this item a contingency in the purchase and require the Seller repair it before closing.

If you are using a home inspector, he also could identify the cracking and recommend epoxy grouting of the wall crack - the slab cracks he might recommend patching to keep water from getting in under the slab (and thereby causing more settlement), but they are unlikely to be considered a structural flaw unless you have noticeable vertical offset between the two sides of the crack, in which either you have seerious differential settlement or the slab was not reinforced.

Answered 5 years ago by LCD

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