Fence Posts That Don't Heave: How We Build for Thunder Bay Frost

A fence that leans, twists, or pulls out of the ground within a few winters almost always failed the same way: the posts weren't set below the frost line. It's not the wood, it's not the weather, it's the hole. Here's what frost actually does underground, how deep we go in Thunder Bay, and how to tell if a fence was built to last or built to sell.

What frost heave actually does

When the ground freezes, moisture in the soil expands and pushes upward. Anything anchored in that soil, including a fence post, gets pushed with it. If the post's base sits above the depth where the ground actually freezes, the frost grabs the post every winter and lifts it a little more each time. A few seasons of that and you've got posts leaning, rails pulling apart at the joints, and gates that no longer close square. It's slow, it's cumulative, and by the time you notice it the fence is already compromised.

How deep is the frost line here

Thunder Bay sits in plant hardiness zone 3, and our frost depth runs deeper than most of southern Ontario because of how cold and how long our winters are. We set every post below that depth, into undisturbed soil, not just deep enough to look sturdy. Skimping a foot on post depth doesn't save a meaningful amount of labour, but it's the difference between a fence that's still square in ten years and one that needs posts reset after three.

Why the concrete matters as much as the depth

Depth alone isn't the whole story. We set posts in concrete that flares slightly wider at the base than at the top, which gives frost less to grab onto as it pushes up around the post. A post set in a straight-sided hole, or one that's just backfilled with soil instead of concrete, gives frost a much easier grip. It's a small detail in how the hole is shaped, but it's one of the reasons two fences built with the same wood can age completely differently.

Signs a fence was built too shallow

• Posts that lean the same direction. Uniform leaning across multiple posts points to frost, not wind or age.
• A gate that used to close and now doesn't. Shifting posts throw off the frame it's hung on.
• Visible gaps opening at rail joints. The wood isn't moving, the post underneath it is.
• Movement that shows up every spring. If the fence seems to shift right after thaw and never fully settles back, that's frost heave doing its work.

Does wood species change any of this

No. Cedar and pressure-treated posts are both vulnerable to frost heave in exactly the same way if they're set too shallow. We cover the tradeoffs between the two woods in our cedar vs pressure-treated guide, but on the frost question, depth and concrete are what matter, not species. A cedar post set right will outlast a pressure-treated post set wrong.

What this means for your fence

If you're getting quotes, ask directly how deep the posts get set and whether they're going into concrete. It's a fair question and a legitimate contractor will have a straight answer. Every fence we build, standard $65 to $85 per linear foot pricing included, gets posts set below the frost line in properly shaped concrete footings. It costs a bit more in labour up front and saves you a rebuild later.

Get a real number for your yard

Trace your fence line on our fence cost estimator for an instant range, or book a free on-site quote and we'll walk the property with you and confirm it.