Expert: Daniel Humphrey - 1/24/2009

Question
Hi, I live in NY in a one story house that is on a slab.In the summer of 07 we had a concrete driveway put in after the blacktop one was removed. Last year during the winter we had no problem however this year the wall on the driveway side of the house has lifted about .75 of an inch causing cracks above the door nearest the heaving wall, also if you look under the wall that is lifting you can see the sill plate is moving with it. The house is about 50 yrs old. My question is, Is it the new driveway or can it be something else? we have never noticed this much of a heave before. Thanks for your time.

Answer
Dear Rick,

The heave you are noticing in the concrete often happens when the drainage has been altered. The other factor would be a change in the amount of heat in the slab. We see this sometimes when people remove landscaping mulch or dirt from around a building, or when snow is compacted or removed, exposing the ground directly to the cold. Or when a building has been closed up for the winter without heat.

To address the water issue, you might consider cutting a strip of the concrete driveway and installing a french drain along the house perimeter. Look for places around the house to intercept ground water that might be coming from another direction. A well designed drain has at least a 4" pipe to carry water away, covered by drain rock, a fabric, and some foam insulation, if possible, above the drain and right next to the building. Sometimes an ice lens forms under a walkway and grows beneath a building; an insulated french drain will act as a barrier.

The frost beneath the house may be eliminated by insulating the perimeter a little better, using the natural warmth from the earth to keep the ground thawed. Adding more heat inside would help too. High density extruded polystyrene foam is probably the best artificial insulation, since it does not absorb water.  

Until you can do some of these projects in spring, I recommend laying down some rigid foam along the wall and boosting the heat to the slab. It might take a week or so to settle down.

Daniel