Freeze-thaw damage happens when wet soil freezes, expands, and lifts anything sitting too shallow, then drops it on the thaw. Across Ontario this frost heave racks decks, tilts fences, and cracks concrete. Two rules stop it, and both start with how we excavate and grade the ground: build below the frost line, and move water away.
How freeze-thaw actually damages a backyard
Soil holds water. When the temperature drops below freezing, that water turns to ice and expands. The wet ground swells and pushes up on anything resting in it. When the thaw comes, the ice melts, the ground settles, and whatever it lifted drops back down, rarely to the same spot. That single cycle is frost heave. Run it over and over through an Ontario winter and the movement adds up.
Here it does the most damage to the things people build shallow: deck posts set in a foot of dirt, fence posts tamped into wet clay, a concrete pad poured straight on grade, a patio laid on loose fill. Hamilton's heavy clay, common up on the Mountain and around Ancaster, holds more water than sandy ground, so it heaves the hardest. The same lot can lift a few centimetres one winter and a few more the next, until the structure is visibly out of level. Nothing on the surface is strong enough to fight it. A heaving lot will lift a loaded deck or a poured slab without effort, so the only real defence is to build so the heave cannot reach it.
Decks and fences: get below the frost line
A deck or fence only stays put if its footings sit on ground that does not move. In the Hamilton and Halton area the frost line is about 1.2 m (4 ft) below grade. Ontario Building Code Part 9 requires deck footings and posts to bear on undisturbed soil below that depth, and it is not a formality. A footing that stops short sits inside the freeze zone, so every winter the frost grabs it and lifts.
The signs of a shallow build show up fast:
- Decks: the frame racks, the ledger pulls, stairs separate from the landing, and the surface stops sitting flat.
- Fences: posts tilt, gates stop latching, and the line of the fence goes wavy.
We set fence posts down toward frost depth for the same reason we dig deck footings to it: a post on a frost-stable base does not lift and rack over the winter. Reach undisturbed ground below the frost line and the freeze-thaw cycle has nothing to push against.
Concrete: the right mix and the right joints
Exterior concrete fails two ways in this climate. It heaves if it is poured on ground that moves, and it spalls and crumbles if the mix is not built for freeze-thaw and road salt. Both are preventable.
For flatwork that lasts through Ontario winters we pour an air-entrained mix: 5 to 8 percent air, CSA Class C-2, around 32 MPa. The tiny air bubbles give freezing water somewhere to go instead of blowing the surface apart. We place it on a compacted granular base so the slab is sitting on something stable, and we cut control joints no more than 2.5 m apart. Concrete will crack; the joints decide where, steering the crack into a clean line instead of a random break across the slab.
Interlock and patios: the base does the work
With interlock, the stone people see is the last thing that matters. The base under it is the whole job. We excavate to depth and lay compacted granular, deeper on clay and deeper again for anything a vehicle drives on, then set the pavers with edge restraint locking the perimeter and jointing sand between them.
Skip that and freeze-thaw takes the pavers apart. A thin or poorly compacted base holds water, the water freezes and lifts, and the field of pavers heaves, ruts, and spreads at the edges because nothing is holding them in. A proper base drains and stays put, so the surface stays flat.
Retaining walls: drain the water out
A retaining wall has a second enemy on top of frost: the water trapped in the soil behind it. When that water freezes, or just builds up with nowhere to go, it pushes on the back of the wall and bows or topples it. The fix is drainage, every time.
Behind our walls we build a drainage path: clear stone against the wall and a weeping tile at the base so water collects and escapes instead of freezing solid behind the block. On taller walls we add geogrid, layers of reinforcement tied back into the soil so the wall holds the load. A wall without drainage can look fine the day it goes up and lean out a winter or two later once the water behind it starts to freeze and push. Base, drainage, and reinforcement are what keep a wall standing through the freeze-thaw years.
The two rules that beat freeze-thaw
Every fix above comes down to the same two rules, and they hold for every backyard build in this climate:
- Build below the frost line. Footings and posts bear on undisturbed soil below frost, about 1.2 m (4 ft) here, so the freeze has nothing to lift.
- Move water away. Compacted bases that drain, control joints, drainage behind walls, and grading that carries water off the structure keep the soil from holding the water that does the damage.
Get those two right and a deck, fence, slab, patio, or wall rides out the Ontario winters instead of fighting them. Get either one wrong and the ground wins, usually within a season or two.
Frequently Asked Questions
What causes freeze-thaw damage in a backyard?
Water. When wet soil freezes it expands, and that expansion lifts anything resting above the frost line. On the thaw the ground drops and so does the structure. Repeat that cycle every winter and a deck racks, a fence tilts, and slabs crack. Hamilton's heavy clay holds the most water, so it heaves hardest.
How deep do footings and posts need to go here?
In the Hamilton and Halton area the frost line sits about 1.2 m (4 ft) down. Ontario Building Code Part 9 requires deck footings and posts to bear on undisturbed soil below that line. Fence posts follow the same logic. Reach frost-stable ground and the freeze-thaw cycle can no longer lift the structure.
Can I just pour concrete or lay pavers over the existing ground?
Not if you want it to last. Exterior concrete needs an air-entrained, freeze-thaw-rated mix on a compacted granular base, with control joints to steer the cracking. Interlock needs the same compacted base plus edge restraint. Skip the base and the slab or pavers heave, lift, and crack the first hard winter.
Build it to last through the winters
Freeze-thaw is the one thing every backyard build in this area has to beat, and beating it starts underground with the footings, the base, and the drainage. The Hammer Construction builds decks, fences, concrete, interlock, and retaining walls to stand up to Ontario winters. We are licensed and insured. Get a free quote at our online estimator, call or text (647) 424-1148, or email hello@thehammer.construction.