Q & A: Solution for a sinking slab

Q: In an earlier column you offered some solutions for sloping floors on an old home with a foundation, but we are unsure what to do with the sloping floors in our home.

This subdivision was built in 1955 on former orchards, and the houses all sit on poured concrete slabs with no crawl space. Our house looks like a house that a kindergartener draws, with a peak in the middle and a main beam.

Between the orchard lands settling and the occasional earthquake, the slab has started to sink on both sides, so a marble placed in the middle right under the main beam will roll easily in either direction.

We do not want to try to raise the slab, but what about leveling the floor as it is now?

Can we just pour self-leveling concrete and lose the inch or two at one end of the slope and then place a new plywood subfloor on top of that? I would love to be able to have a level dining room table.

A: We wouldn't rule out the possibility of raising the slab. In fact, it might be the best solution, and when all is said and done, it also might be the cheapest. Our first step would be to hire a structural engineer to take a hard look at the situation and suggest a possible solution.

We see nothing but trouble with your proposed solution. Raising the floor with self-leveling concrete, which is really just a soupy slurry of high-strength cement, requires removal of all the flooring, baseboards, doors, doorjambs and trim. Then they need to be replaced.

Worse, you aren't solving the underlying problem. Without dealing with the foundation problem, the house will continue to settle, and sooner or later you'll be back in the same boat.

Your diagnosis of the cause of the sinking slab is probably right. The old orchard land is laden with decomposing roots, causing the soil to compress as the wood turns to dust. Couple that with the quakes and natural wet-and-dry cycles of the seasons and the soil is going to expand and contract. That would be OK if the slab continues to sink at the same rate.

But it doesn't, and for good reason. The weight of the roof is borne on the outside walls. The dead load of the structure is transferred from the peak, down the rafters, then down the walls to the outside edges of the slab.

The front and rear walls bear comparatively little weight. The extra weight from the roof structure results in soil under the slab compressing more than the soil at the front and rear of your "kindergartener's" house.

Generally, a single-story home built in an area where the ground doesn't freeze requires a concrete footing 12 inches below grade. That's presuming the soil is compacted and stable.

A foundation footing looks like an inverted "T" measuring 12 inches on the bottom, 6 inches on each side and 6 inches on the top. The leg of the "T" is long enough to get the top 6 inches above the finished grade, usually about 18 inches.

The perimeter of a slab foundation should also extend a minimum of 12 inches below grade with the top of the slab being 6 inches above the finished grade. Both types of foundations should contain steel reinforcing bar (rebar) to militate against cracking.

Fixing your foundation might be as simple as jacking up the low points and installing a beefier perimeter foundation or concrete piers that rest on solid soil. That's where the structural engineer comes in. He or she should be able to tell you if the foundation is deep enough, how deep it should be and exactly what type of remedial measures are required.

Finally, the engineer should be able to provide some names of licensed, bonded and reputable contractors capable of doing the repair.