Sinking Slab

I’m dealing with a house I inspected 1 1/2 years ago. No liabilities directed towards me, but trying to help figure out issue. Bungalow (semi-detached)with walkout basement. Basement floor is slab at grade level at rear, and gradually cutting into ground to a depth of approximately 5ft below grade at the front. One corner of slab (rear … at grade level) is sinking … maybe 5/8". Foundation outside shows one very normal sized vertical crack, and foundation does not appear to be sinking. This subdivision was previously a swamp. Looking into reputaion of builder (don’t know who that is as yet). My question is, how can a slab sink independent of the footings/foundation? Footings should be 4 ft below grade (I’m in northern Ontario, Canada) so the slab should rest on a ledge on the foundation wall. There is a vertical layer of Styrofoam insulation on the inside of the foundation wall, which hasn’t moved. The slab has dropped 1/2 to 5/8" and is no longer flush with top of insulation board. My only guess(s) are that sometimes there is a 1" sand cushion layer between the notch in the foundation on which the slab sits, and the bottom of the slab. Perhaps this layer has setled, or perhaps there is a breach in the foundation wall below grade, and underground water has intruded and eroded this sand layer away. The slab by the way is not cracked, and this whole thing takes place in an area approximately 4ft x 4ft in a corner.
Any thoughts or insights would be greatly appreciated. Thanks … Glenn

Floor slabs should NEVER rest on foundation walls. The sinkage probably has something to do with the soils beneath the slab…either unsuitable in the first place, or an excavation was not properly compacted when backfilled.


Was the walkout original construction or was it added later?

Retrofit walkouts may cause problems such as you have stated because the original foundation was not designed to be a walkout.

Might be underground water. A soil test might be in order


Thanks Doug. Walkout was original construction. 17 years old. Feel sure it’s an issue with soilunder the floor, but floating slabs aren’t usually found in homes … are they? So I’m trying to figure out how the floor can sink inside the foundation, and not the footing/foundation as well. Thanks. Glenn

Soil compaction… moisture… drainage, it’s there somewhere.