Building Homes or Extensions/void under slab


Bruce, my 1978 concrete slab (upper level) separated (and also slipped 1/2 inch) from the foundation soon after the pour (there were 2 pours), leaving a crack parallel to and 2 inches inboard from the wall on one (downhill) side of my house (in 3 bedrooms), and of course a 1/2-inch raised strip of concrete next to the wall that will interfere with flooring installation (I've just removed the 1978 carpeting and want to float laminate).  I foolishly bought the house at that time without looking under the rug.  I've been told I can insert 1/2 inch rebar horizontally from the exterior (every 2 ft) to ensure slab stability (though it doesn't seem to have settled further). I'm retired but feel I can accomplish the rebar project and fill the crack with epoxy, but haven't decided what to do about the raised concrete strip. A further problem:  The crack in one closed-off bedroom allowed ants to deposit dirt on the rug. Pushing a wire down the crack reveals a cavity of at least one foot deep.  I've no idea how far back under the slab it goes.  Is it easy enough for me to fill this void myself? Since I plan to buy a powerful drill for the rebar project, maybe I could use it to also drill down through the slab and fill with grout.  I've found a company that makes manual grout pumps (airplaco) but haven't yet found a place that rents them.  Your thoughts on any of this, especially the void would be most appreciated.  Thanks--

Hi Don, are you sure the 1/2" ledge at your wall is concrete?  It may be a piece of 1/2" expansion joint that is covered with concrete.  You may be able to cut it with a utility knife.  If it is concrete then you can take a wide chisel or rent a chipping hammer and chip it away.  As far as leveling the floor goes, you can get some self leveling underlayment and float your floor to fill in any low spots.  Drilling and doweling through the wall into the slab is a lot of work that I'm not sure is necessary especially if there isn't any evidence that it is continuing to settle.  As far as the "void" goes, you can get non shrink grout that you can mix up real thin and literally pour it down a decent sized hole of an inch or so using a funnel.  If you lived in Florida I might think you had a sinkhole going on under your house.  And in California there are seismic events that can cause dirt to shift under a foundation and if you find that the void you are trying to fill is unfillable, so to speak, you may need some geotechnical assistance to underpin your foundation and stabilize the ground your house sits on..also you may want to look for broken plumbing pipes or improper drainage around your house that may be washing out the fill under your slab..I hope this info helps, feel free to write again regarding this or other matters, sincerely Bruce Johnson ,

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Bruce E. Johnson


I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.


Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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