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 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.

To adequately drain a slab, you need lots of compacted gravel below the slab, with sloping drains that lead to daylight. Gravel must be placed under the entire slab, extending below frost depth in an unheated building.

You don’t want one part of the slab pinned to the ledge, while other areas float on the gravel bed. If you wind up blasting into the ledge to accomplish this, make sure you have a way to drain the blasted area. I had a house last year with a full foundation hole that we had to blast out of ledge. After we blasted the hole, it was clear we would have a swimming pool on our hands if groundwater or surface drainage ever found its way into the hole, which it inevitably would. So we also blasted a trench for the footing drain that led to daylight. I don’t know what we would have done if we discovered this after the foundation had been poured, but I’d guess someone’s made that mistake before.

The other basic approach is to build an insulated "shallow foundation" using rigid foam insulation that projects out from the perimeter of the slab. How this approach is done depends on whether the house is heated or not.

Describing the construction of any shallow foundation is a book in itself. J. Crandell at the National Association of Home Builders (NAHB) Research Center has done extensive research on this topic, which has been compiled in Design Guide for a Shallow, Frost-Protected Foundation. This book is available for $20 from the NAHB Bookstore (800/223-2665); or, for more information, contact the NAHB Research Center directly (800/638-8556).

Carl Hagstrom runs Hagstrom Contracting in Montrose, Pa., and is a contributing editor to the Journal of Light Construction.