We are faced with a site for a 1,400-square-foot, story-and-a-half house in a northern climate with a 6-foot frost depth. We propose to build the house on a slab-on-grade foundation, some of which will be near ledge. Does the footing for such a slab have to extend below the 6-foot frost depth, or can it float on gravel?

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...

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