A.Carl Hagstrom responds: This is a tough topic. The issue here, of course, is frost heave. When wet soil freezes, it expands, sometimes with enough force to lift and crack an otherwise well-designed, well-built foundation slab. Frost problems tend to be soil specific. Some soils tend to hold more water than others, and different soils expand and contract at different rates and to different degrees when wet. If you’re unfamiliar with the soil characteristics where you are building, I would recommend first contacting a soils engineer to evaluate the site, then use this information to solicit construction recommendations from a structural engineer or knowledgeable foundation sub.
To prevent frost heave, there are two general ways to handle a slab: Drain it or insulate it. One way is to drain beneath the slab, so there is no water in the soil to freeze. This could be challenging under a slab connected to ledge, since groundwater often follows the contour of ledge.
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