In southern Ontario, fence posts should sit about 1.2 m (4 ft) below grade, down toward the frost line, on a frost-stable base. A common rule of thumb is burying roughly one third of the post height. On every fence we install, we set posts that deep so freeze-thaw cannot lift or rack them.
Why frost sets the depth in southern Ontario
Depth is not really about the height of the fence first. It is about frost. Southern Ontario goes through hard freeze-thaw cycles every winter, and that is what pushes a fence around. When wet soil freezes it expands, and that expansion grabs anything buried in it and lifts. When the ground thaws, the soil drops back down. Do that a few times a season and a shallow post works its way up and out of plumb.
The fix is to get the bottom of the post below the line where the ground freezes. In the Hamilton and Halton area that frost line sits around 1.2 m (4 ft) below grade. When the base of a post sits below that depth, the freezing soil up top has nothing solid to grab down low, so the post stays put. That is why we set posts toward the frost line on a frost-stable base instead of sinking them a foot or two and hoping for the best.
The local ground does not make it any easier. Heavy clay is common up on the Mountain and around Ancaster, and clay holds water and heaves hard, so getting below frost matters even more on those lots. Sandier loam closer to the lake drains better, but the frost line is the same line everywhere, and that is the depth we build to.
The one-third rule of thumb, and why frost usually wins
There is an old rule of thumb that you should bury about one third of a post's total height. For a tall privacy fence, that puts a real amount of post in the ground and keeps the fence from leaning under its own weight. It is a good starting point, and we still use it as a sanity check.
But in this climate the one-third rule and the frost line do not always land in the same spot, and when they disagree, frost wins. We go with whichever depth is deeper. For most residential fences around Hamilton, that means digging toward that 1.2 m frost depth even when one third of the post height would have been a little shallower. Going deeper than the rule of thumb means more digging up front. It saves you from rebuilding a leaning fence two winters later.
Concrete on a gravel base, not concrete in a puddle
How a post is set matters as much as how deep it goes. We set posts in concrete on a gravel base. The few inches of gravel at the bottom of the hole give water somewhere to drain instead of collecting around the foot of the post. Standing water at the bottom of a post hole is exactly what you do not want, because that water freezes, expands, and works against everything you just dug.
The concrete then locks the post in place and spreads its load into the surrounding soil. Gravel keeps the post foot draining and dry, concrete keeps it from shifting. Skip the gravel and pour straight into a wet hole and you trap water against the post, which is how a post rots at the base and heaves from below at the same time.
Taller fences put more load on the posts
Under City of Hamilton Fence By-law 10-142, a fence in the rear or side yard can be up to 2.0 m (6 ft 6 in) tall. A taller, solid fence looks great and gives you real privacy, but it also acts like a sail. The more solid surface you stand up, the more wind it catches, and all of that wind load ends up trying to lever the posts out of the ground.
That is the other reason post setting matters so much here. A short, open fence is forgiving. A full-height privacy fence is not. The taller and more solid the fence, the more it depends on posts that are dug to depth, sitting on a frost-stable base, and locked in with concrete. We size and set the posts for the fence going on top of them, not the other way around.
Locates first: Ontario One Call before any post hole
Before any digging starts, the holes get located. In Ontario, anyone breaking ground has to request locates through Ontario One Call by law, and that holds whether you are digging a full foundation or a single fence post hole. The request is free, and it is the law. It is not a step worth skipping to save a day.
Once the request goes in, the utility owners come out and mark their buried gas, hydro, water, and telecom lines so nobody puts a post hole, or an auger, through a live service. We book the locates before we dig, every time. Then we dig to depth knowing exactly what is underground.
Frequently Asked Questions
How deep should a 6-foot fence post go in Ontario?
In southern Ontario, frost decides the depth more than the fence height does. We set posts down toward the frost line, about 1.2 m (4 ft) below grade, on a frost-stable base. The one-third rule of thumb is a useful check, but in this climate it usually means digging deeper than one third.
Do fence posts need to be set in concrete?
For a permanent fence, concrete is the most reliable way to lock a post in place. We pour it on a gravel base so water drains away instead of pooling and freezing at the bottom of the hole. The gravel keeps the post foot dry, and the concrete keeps the post from shifting or leaning.
What happens if fence posts are not deep enough?
Shallow posts are the main reason fences lean. When wet soil freezes and expands, it lifts a shallow post, then drops it when the ground thaws. After a winter or two the fence racks out of square, boards gap, and gates stop latching. Setting the posts below the frost line prevents that movement.
Want fence posts set to last through Hamilton winters?
A fence is only as straight as its posts, and the posts are only as solid as the holes underneath them. The Hammer Construction digs toward the frost line, sets every post on a gravel base in concrete, and books the locates first, so your fence stands straight season after season. We are licensed and insured. Get a free quote at our online estimator, call or text (647) 424-1148, or email hello@thehammer.construction