Frost Line in Oshawa for Deck Building

If you are considering building a deck in Oshawa with proper footings (not a floating deck) then you will have to install footings.  Footings are the foundation of the deck, that transfer the weight load down to solid undisturbed ground so that the deck doesn't move.  In our area, Southern Ontario, there are two main types of footings commonly used for deck building.  They are concrete footings, and helical piles.  This article will focus on concrete footings, and we will write more about helical piles (piers) in another article.


In Southern Ontario, like many other places, we have to deal with frost in the ground.  Perhaps you have heard of 'frost heave'.  When the ground freezes, it may or may not move depending on many factors including drainage, temperature change, moisture, etc.   You will notice this mostly with fence posts. Next time you go for a drive in town or for a walk through your neighbourhood, look at the fences.  Some fence posts will be too high, making for a funny looking fence.  These fence posts weren't installed deep enough and fell victim to frost heave. The frost pushed them upwards. How does this affect deck footings?

How To Beat The Frost

To ensure that our deck footings don't move due to frost, we have to dig below the 'frost line'.  The frost line is what depth has been determined to be the lowest average depth that the frost will penetrate in the ground.  In Oshawa and surrounding area, this depth is 4 feet.  So theoretically, if our footings penetrate 4 feet or deeper in the ground, the frost will never get below the footing, and cannot push it upwards.  "But what about grabbing the sides of the footing?" you may wonder?

Sonotubes for Deck Footings

sonotube cutaway.jpg

Concrete footings are typically installed in sonotubes.  These are cardboard tubes with a waxy film on the inside and outside that the wet concrete is poured inside of (in the hole in the ground).  The waxy finish prevents the frost from grabbing hold of the outside of the footing.  The finished footing is a solid concrete column, 4 feet deep to solid ground, that has a waxy surface as to not be grabbed by frost.

In sunny, warm climates like California where there is no frost, they have no frost line. Therefore their footings can be as shallow as a few inches deep, so long as they are bearing on solid ground.  In Oshawa and area, where we can experience up to 60 degree temperature changes season to season, dealing with a deep frost line is one of the prices we pay for living in such an extreme climate.