Do deck supports shift with ground frost?

Creating a solid base is key when you are getting ready to build your new deck – regardless if it’s wood, composite or a stone deck. For those of us who live in Canada, winter and ground frost is a real concern.  So, do deck supports shift with ground frost? The answer to this question is no – IF you choose the appropriate deck support for your job and you install them correctly.

To properly answer the question of do deck supports shift with ground frost we should quickly look at what ground frost is. As a high level assessment of ground frost – moisture is absorbed and held within the ground materials and as this moisture freezes it expands.  As this material expands, it is displaced and can displace objects within, around or on its surface.

These are some points to consider in determining what deck supports are right for your job; what do local building codes require?  What is the size, weight and elevation of your deck?  What are the soil conditions that the deck is being built on? Besides the building code aspects, there are other factors that will determine whether deck supports shift with ground frost.

(Please keep in mind these points are for purposes of discussion and any actual building plans should be validated by a qualified source.) 

If you are dealing with sandy soil conditions, you will already have an advantage. Sandy soil will hold less moisture and will be less affected than any freeze-thaw cycles. If compacted properly, sand compacts well, with little to no settling.  In this case, it is entirely possible to create floating deck footings, which are adequately sized concrete blocks or pads, placed where required as per the design of the deck.

Clay soil is on the other end of the soil spectrum, and is the most problematic as it can cause deck supports to shift with ground frost when using floating footings.  Clay will hold a lot of moisture and as a result of freeze-thaw cycles, will cause it to move and shift.

By far the best and most reliable way to prevent your deck supports from shifting with ground frost is to use in-ground footings. These are either concrete footings or mechanical screw piles, both of which are installed below the frost line.  Your geographical location will dictate the depth of the frost line, dependent the average cold temperatures and how long they last.

You need to determine the diameter of the footing required when installing concrete footings, and it is important to bell the bottom.  This is to create a base that will act as an attached base pad that is wider than the footing itself. This accomplishes two things – provides more support to the bottom of the concrete footing, but more importantly prevents the deck support to shift with ground frost. Have you ever seen a fence installed and after one winter all the posts are pushed up and sideways?  This is caused by the installer digging straight holes down and pouring cylindrical footings that are easily pushed up by ground frost. You will find ground frost over 36” deep in some regions of Canada. When designing your concrete footings, it is important to determine the load requirements for each to ensure they are constructed appropriately.

So, to prevent deck supports shifting with ground frost mechanical screw piles are the best choice.  With mechanical screw piles, there is no guess work in ensuring you have the correct footings for your deck.  Most reputable screw pile companies are able to recommend the appropriate size of pile, depth and torque it should be driven to, based on the size and weight of the deck and snow loads where it is being installed. By virtue of the design and nature of a screw pile, it is prevented from being pushed up. There are several choices when it comes to choosing the option of supports for you deck.  If you select the appropriate one for your job, you will have no issues with your deck support shifting due to ground frost.