Preventing cabin foundation shift a challenge

Hey there, time traveller!
This article was published 24/09/2022 (1110 days ago), so information in it may no longer be current.

Question: We have a cabin in the Whiteshell and are looking at a re-level, after about 12 years. The cabin always moves towards the back of the property. It is built on 24-inch square pads and posts right now. We have good drainage around the cabin. We use the cabin from March to November each year. Aside from heating it all winter, to keep the ground from freezing, is there another method of dealing with the issue of shifting? Heating an empty structure for the coldest part of the year is not what we prefer.

Any input is greatly appreciated.

Thanks, Randall

Answer: Preventing shifting of a summer-use building, built on footings on grade, may be a near impossible task without spending a small fortune on wasted heat for the crawlspace in the winter. Typical drainage and water management efforts may be the only way to prevent some of the movement, which is related to moisture content and changes in the soil around the cabin.

As most of us home and cottage owners in Southern Manitoba know, typical settlement of our domiciles may be inevitable, depending on how the foundations are constructed. Some locations in our area do have sandy soils, which are much more forgiving than our primarily clay-based ones. If you are lucky enough to have a house or summer home in those areas, foundation construction is not a huge concern. With sandy soils, a cottage like yours can easily be built on concrete footings on grade, due to the forgiving nature of the sand beneath. Those structures should not be subject to expanding and contracting soils and may remain plumb and level for decades without help.

Unfortunately, it appears that your cabin is built in an area with clay-based soils, which also may have marshy patches, which may further exacerbate the movement issues. For buildings in much of lake country that is a constant concern. For decades, it was allowable to build a seasonal home on a minimal foundation, often with concrete pads and posts on grade, like yours. Because it was a seasonal building, a full basement or deep concrete piers were not required. In many cases, these structures were not heated, except with fireplaces or wood stoves. The majority had little insulation and the underside of the floor remained open above grade, which made access for adjustment of the support structure much easier. For those types of structures, seasonal movement was largely reliant on the weather and amount and frequency of precipitation.

Many of our older summer homes have been retrofitted with thermal insulation, electric baseboard or forced air heating, and enclosure of the area below the floor. Closing up that area and creating a crawlspace may extend the use of the building in the shoulder seasons, if the area is insulated and heated. That will make the floor and building more comfortable in cool weather and also prevent the plumbing pipes from freezing. It may also help prevent some movement due to frost heaving below the footings, but can also create another set of issues related to moisture.

Even if you decided to properly insulate and heat a crawlspace below the cabin floor all winter, there is no guarantee that would fully stop seasonal movement. Heating that area should keep the moisture in the soil immediately beneath the building from fully freezing, but may not succeed for the entire area. It will also dry out the soil, potentially allowing that moisture to escape into the cottage, above. Measures will have to be taken to prevent that moisture from becoming trapped inside, which could cause condensation issues in the enclosed structure, when not in use. Timed or dehumidistat controlled exhaust fans or a heat recovery ventilation (HRV) may be required for that purpose. Also, the cost of electricity for constant heating would be excessive, unless you planned to use the cabin every weekend, or more, throughout the winter months.

So, to answer your question in a more straightforward manner, there is not much you can do to prevent the movement, unless you can control Mother Nature. In a winter with lots of snow, the ground may become saturated in the spring, causing soil expansion and movement. Conversely, the heavy snow cover may help insulate the ground, limiting the depth of frost, preventing excessive heaving as it warms up. In very dry years, which we have experienced in the past decade, the soil may shrink excessively in the late summer and fall, causing more movement than usual. That could be partially mitigated by extra watering around and under the cottage, which may only have limited success. In a very rainy year, like the current one, ensuring proper drainage away from the building is critical. Either way, good moisture management under and around the cabin may somewhat minimize movement issues, which will mostly be due to environmental conditions beyond your control.

You may be able to somewhat increase or decrease the soil moisture immediately around and under your cabin to minimize seasonal movement, but it still may settle regularly. That really is at the mercy of environmental and soil conditions, which you may have no real way to control.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com