Know the soil under your home


By CCHRC Staff

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Q: My home is on a postand- pad foundation because I live on spongy ground. I notice some changes in the floor in my home over the winter and summer each year. Is there anything I can do to prevent this?

Most Fairbanks homes using post-and-pad construction were built in anticipation of some degree of seasonal movement or in an attempt to cope with ground instability because of permafrost.

A home’s foundation changes with the seasons as the soils underneath the pads expand and contract in reaction to yearly freeze-thaw cycles. For example, entry doors may consistently stick all winter, and then work fine in the summers, year after year. This can happen regardless of whether your home is situated on permafrost or not.

The layer of ground directly under a home that freezes and thaws with the seasons is called the active layer. In some cases, the active layer can extend down 10 feet or more depending on the types of soils and seasonal conditions. Autumns with heavy rain followed by cold winters with little snowfall particularly affect soils, sometimes creating what is known as a “frost heave.” For this to happen, soils have to be of a fine enough particle size to trap or “wick” water and enough moisture must be present to cause the soil to expand when it freezes.

Unfortunately, much of Interior Alaska is covered with fine silts which tend to drain poorly and can expand aggressively if they contain too much water when winter hits.

Removing these soils and replacing them with non-frost susceptible material is typically unrealistic for an existing house.

However, there are a few relatively minor improvements, which can help curb the severity of the problem. Good site drainage can make a big difference. The ground should be sloped away from the post-andpad foundation, and large roof overhangs with gutters will help divert water away as well.

The downspouts on the gutters should extend along the ground horizontally to divert water away from the pads and soil that bears weight.

Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, e-mail us at can also call the CCHRC at (907) 457-3454.