House slab cracks

Jul 1, 2009 by

A home built in 1989 was offered for sale in 2009 and purchased by a young couple.   The house was on a relatively flat lot, approximately two feet above the street.   It was a single story, wood framed home and was stuccoed on a concrete slab-on-grade foundation.  Roof drainage was to the surrounding yards and planters which were located below the roof eaves.

The interior floors were carpeted and the interior walls had been freshly painted.   There were no significant wall cracks noted.  After the sale, but before moving in, the buyers decided to replace the old carpet with new.   When the carpet contractor pulled up the old carpet, many serious cracks in the concrete slab-on-grade floor were exposed.

The buyers had a number of contractors look at the problem as well as a structural engineer, who recommended a soil investigation.  At that point, GEI was called to inspect the insured’s site to 1) identify the cause and 2) determine the approximate age of cracks in the slab.

Our site investigation revealed many problems. The cracked concrete floor slabs sloped slightly toward the exterior walls, in the few places where they were still attached to the footings.   While the windowsills and interior counters were relatively level, the interior doors were misaligned in doorframes and someone had chiseled out the wood of the doorframes (to allow the doors to close).   The front door strike plate had been readjusted so that the lock bolt would fit into the hole.   The slab foundation was vertically separated from the front north foundation along the wall east of the front door.   Water ponding had occurred against the house foundations, particularly in the back yard.

These damages were typical of those caused by differential movements of concrete slabs subjected to expansive soil shrink-swell vertical movements. Expansive soils contain clay and exhibit volume changes with changes in moisture content; i.e., such soils shrink and crack when dry and swell and expand when wet.   Soil moisture moves from moister soil to drier soil in expansive soils.

Houses built on expansive soils, where foundations and slabs were not designed and/or constructed to current standards of depth, thickness, reinforcement, and horizontal ties and soil pre-moistening, experience moisture changes in the soil that commonly cause slabs and footings to move differentially (at different rates).   Instead of the structure rising and falling as a single unit, different parts of the house rise and fall independently of each other.   The living room exterior wall may rise by .35 inch, while the kitchen floor may sink .25 inch.   The floors, walls, and ceilings then slope and crack, the door and window frames go out of plumb, causing the doors to stick (and require trimming), and lock strike plates may require many adjustments to function properly.   This was what was happening to this home.

The second question of the age of the damage was interesting. Our automatic “conflict check” showed that we inspected this same structure in the year 2000. The 2009 seller was the 2000 owner.   In our 2000 inspection, we noted cracking and mounding to the interior concrete slab, the front door frame was raked, and a separation of .25 inch was under the front entry ceramic tile.

This crack continued in the slab under the carpet, and there were numerous interior wall cracks throughout the house with some sheetrock nail pops as well. 

The previous owner spackled and painted the wall cracks, and reinstalled the ceramic tiles at the front door, covering the 2000 crack that ran across the tile threshold.

The root cause of the slab damage was not corrected.   The grout under the new tiles filled in a portion of the old crack, which was documented by comparing the 2000 and 2009 photographs.

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