The posts are the fence. Boards and panels are what you see, but posts carry the wind load, hold the gate and decide whether the fence is still straight in year ten. In Ontario they have one more job: staying put while frozen clay tries to push them out of the ground every winter.
Why fence posts heave in Ontario
Wet clay freezes from the top down. As it freezes it expands and grips the post and the side of the concrete, and lifts. If the footing stops above the frost line, nothing below holds it down, and each winter it rises a little and does not settle all the way back. That is why a fence set at 2 to 3 ft looks fine for two winters and then starts to lean, post by post.
Three things stop it: depth below frost (about 4 ft here), a footing that is wider at the bottom than the top so frozen soil cannot get a grip on it, and drainage so there is less water around the post to freeze. A carrot-shaped footing, wide at the top where someone dumped extra concrete, is the worst shape there is. Frost grabs the wide top and lifts the whole thing.
Which fence post should you use?
| Post | Use it for | Lifespan in the ground | Watch out for |
|---|---|---|---|
| Pressure-treated 4x4, ground contact rated | Most wood fences up to 6 ft | 15 to 20 years set properly | Must be rated for ground contact, not above-ground lumber |
| Pressure-treated 6x6 | Gate posts, end and corner posts, taller fences, heavy panels | 15 to 20 years or more | Twists and checks as it dries; buy straight stock |
| Cedar 4x4 | Cedar fences where the look matters | Shorter than pressure-treated below grade | Heartwood lasts far longer than sapwood |
| Galvanized steel post (for wood fences) | Wood fences where you never want to replace a post | Decades | Needs brackets to hang wood rails; costs more |
| Aluminum or vinyl system posts | Aluminum and vinyl fence systems | Life of the fence | Must still be set below frost |
| Driven ground spike | Mailboxes, signs, trellises, short garden fences | Varies | Not for tall solid fences in clay |
Steel posts made for wood fencing, such as the Master Halco PostMaster line, are G90 galvanized and carry a 15-year limited warranty from the maker. Wood rails bolt to them with brackets, so the fence looks like wood from the yard. Expect steel to add roughly $25 to $60 per post to an installed price compared with wood. Our chain link fence guide covers chain link posts and their spacing.
Are fence post spikes any good?
For light work, yes. Home Depot Canada sells a 36 in steel ground spike for a 4x4 for $18.81 (October 2026), and the listing itself describes it as suited to light-duty projects like mailboxes and trellises. Makers of no-dig systems say their anchors can be driven into clay and re-driven if they move, but one maker of no-dig anchors says plainly it cannot guarantee they will not heave. A 6 ft solid privacy fence acts like a sail in the wind, and a 36 in spike in wet clay is well above our 4 ft frost line. Use spikes for things that can lean a little without consequence, and a proper footing for anything that has to stay straight.
Concrete or gravel?
Both camps have a point. Concrete holds the post rigidly and is what most building departments expect for pool fences. Its weakness is water: a post sitting in a concrete cup that holds water rots at the top of the footing. Gravel drains well and lets the post dry, but it does not resist wind or a heavy gate as well, and the post can work loose in soft clay.
What we do on most residential fences combines the two: gravel in the bottom of the hole so water can drain away from the end of the post, concrete around the post with a bell at the bottom, and the top of the concrete sloped away from the wood so water runs off rather than pooling against it. For gate posts we go bigger and deeper, because the gate pulls on that post all year.
How to set a fence post, step by step
- Get a locate. Request one from Ontario Dig Authority (formerly Ontario One Call). It is free, required by law before you dig, and takes up to 5 business days. Private lines, like power to a shed or gas to a pool heater, are not included and need a private locator.
- Mark the line and spacing. Run a string between the end posts and mark holes about 8 ft on centre, or to suit your panel width.
- Dig to about 4 ft. Quikrete suggests a hole about three times the width of the post, so roughly 10 to 12 in for a 4x4. Keep the sides straight, and flare the bottom wider than the top if you can.
- Add gravel. About 6 in of gravel in the bottom, tamped, so the end of the post is not sitting in water.
- Set and brace the post. Stand it on the gravel, check it for plumb on two faces, and brace it with two stakes.
- Pour the concrete. Fill around the post, rod it to remove air pockets, and finish the top sloped away from the wood. Fast-setting post mix sets in well under an hour, but let it cure before you hang heavy panels or a gate.
- Cap the post. A post cap keeps rain out of the end grain at the top, where wood starts to check and rot.
Post caps
Caps are cheap and add years to a wood post. Flat or pyramid wood caps match the fence, copper and aluminum caps add a finish, and solar caps light a path. Fence by-laws allow for them: in Hamilton a decorative cap or structural post may stand 0.15 m above the fence height limit, and in Oakville post caps may go 15 cm over.
Leaning or rotten posts
If your posts are already leaning or soft at the ground, see our fence repair cost guide. It covers bracing, resetting and replacing posts, prices for each, and the five post rule for deciding between repair and a new fence. If a whole fence has heaved, why fences rot and lean explains the causes.
Frequently asked questions
How deep should fence posts be in Ontario?
About 4 ft (1.2 m), below the frost line, and never less than 42 in. Posts set at 2 to 3 ft are the ones that lean after a wet, cold winter.
Should I use 4x4 or 6x6 fence posts?
4x4 for most wood fences up to 6 ft. 6x6 for gate posts, ends, corners, taller fences and heavy panels, where the extra size resists twisting and wind load.
How far apart should fence posts be?
About 8 ft on centre for most wood fences, or matched to the panel width for vinyl and aluminum systems. Closer spacing helps on windy, exposed lots.
Should fence posts be set in concrete or gravel?
For most residential fences in Ontario clay, concrete with gravel in the bottom of the hole for drainage, and the top of the concrete sloped away from the post. That holds the post firmly without trapping water against it.
Do fence post spikes work in Ontario?
For light-duty uses like mailboxes, trellises and short garden fences. For a tall solid fence in clay, no: the spike sits well above the 4 ft frost line and the fence catches a lot of wind.
Are steel fence posts better than wood?
They last longer, because they do not rot at the soil line, and galvanized steel posts for wood fences carry long maker warranties. They cost more, roughly $25 to $60 extra per post installed, and need brackets for the wood rails.
Do I need a locate to dig fence post holes?
Yes. Ontario law requires a utility locate before digging, including for fences. Request it free from Ontario Dig Authority (formerly Ontario One Call), allowing up to 5 business days.