Correction for concrete floor may be slab-jacking

Q: A large crack has developed in the concrete floor of the daylight basement in my home in West Seattle. (Or maybe it's just become more apparent — it's hard to say, since the area is covered by wall-to-wall carpeting.)

The house was built in 1952; my husband and I bought it in 1990. The house sits on level ground but the back yard slopes downward to the north. The crack runs north and south.

The pre-purchase inspection revealed settlement in the walkway, driveway and rear patio of the house but the inspector concluded the settlement was "due to concrete being placed on marginally compacted fill material and probably occurred within the first few years after construction."

He noted a minor crack in the uncovered floor of a workshop in the southwestern corner of the basement but again concluded that this occurred because of "its placement on marginally compacted bearing soil." He found no evidence of shifting or settlement anywhere else in the basement.

The area of concern is in the northeastern part of the basement. We use this part of the house as our master bedroom.

The floor is covered by a firmly anchored carpet and pad. The crack has definitely become more noticeable in recent months. Should we brace ourselves for jackhammers and hoists, and tens of thousands of dollars in costs?

A: Your concrete floor is not structural. The foundation carries the weight of the home, not the floor slab. It doesn't appear that you or the inspector note(d) any difficulties with the foundation itself.

That said, floor cracking can be disconcerting and possibly a sign of foundation problems that may occur in the future.

If this cracking and settlement are limited to the perimeter of the basement and the floor tilts toward the exterior, leaking sewer/downspout drain pipes around the exterior is the main suspect.

But this does not seem to be the case for you, lacking foundation damage and your description of floor cracking locations.

With no foundation issues, and cracking walks and drive, I would agree with your inspector that this may be a compaction issue, and not washout or undermining.

To repair, you could tear it out with a jackhammer and suffer the resultant disruption and expense. Consider slab-jacking, or mud-jacking as it is sometimes called. Small holes can be drilled in the floor and cement slurry injected beneath, helping to raise the floor level.

Slab-jacking is not perfect, but will greatly improve things and help prevent further settlement in the floor.

As cheap insurance before proceeding, I would suggest having a video survey of the sewer and buried downspout drain lines performed so you can make sure you have no breaks or fractures that may be contributing to the problem.

Darrell Hay is a local home inspector and manages several rental properties.