Frost Heave in London, Ontario: Why Decks, Steps and Patios Lift
Frost heave happens when water in fine-grained soil freezes into layers of ice that push the ground, and anything sitting on it, upward. In London, Ontario, clay and silt soils, wet ground and long freezing spells make deck posts, steps and patios lift and shift every winter. The fix is to break one of the three ingredients: get footings below the frost line (1.2 m in clay under the Ontario Building Code), drain the water away, and replace frost-susceptible soil with compacted granular base.
With late October here and the first hard freezes on the way, this is the last good window to fix or prevent the problems that show up in spring.
How Frost Heave Works
The National Research Council of Canada's Canadian Building Digest 182 explains that frost heave needs three things at the same time:
Frost-susceptible soil. Clays, silts and very fine sands support the growth of ice lenses. Clean sands and gravels generally do not heave.
A supply of water. As the ground freezes, water is drawn up to the freezing zone, where it forms growing layers of ice called ice lenses.
Freezing temperatures. The colder and longer the freeze, the deeper the frost goes.
Remove any one of the three and heave is controlled. An earlier NRC digest notes that ice lenses can keep growing as long as water reaches the freezing plane, and that heave is usually prevented by replacing fine-grained soil with coarse granular material. Drainage helps, but on its own it rarely eliminates heave.
There is a second force too: adfreezing. Wet soil can freeze to the sides of a post or concrete pier and lift it, even when the bottom of the footing is deep. That is why smooth, properly backfilled piers matter.
Why London Is Prone to Heave
Much of London sits on fine-grained glacial soils. For example, a geotechnical report filed with the City for a site on Wonderland Road South found clayey silt and till to depths of about 4 to 7 m and recommended frost protection for exterior footings. Every lot is different, but heavy, slow-draining soil like that is exactly what ice lenses love.
Add our freeze-thaw cycles, snow melt that pools against houses, and downspouts that dump next to steps, and you get the classic spring complaints:
Deck posts that rise and leave the deck out of level
Front steps that tilt toward or away from the house
Interlock and patio stones that heave in one corner
Fence posts that push up a little more every year
Gates and doors that stick in winter and free up in spring
What the Ontario Building Code Requires
The Ontario Building Code sets minimum depths for footings in Table 9.12.2.2. For clay or soils that are not clearly defined, the minimum depth is 1.2 m. Where there is no heated space next to the foundation, it is 1.2 m but not less than the actual depth of frost penetration.
Some details that matter for homeowners:
Steps. Concrete steps with one or two risers are permitted to be laid at ground level. Steps with more than two risers must meet the full footing depth requirements.
Decks. Deck footings generally must go 1.2 m below grade. Municipal deck guides based on the 2024 Code also describe an exemption for certain small, freestanding, low decks, and warn that their posts may need seasonal adjustment because of frost heave.
Inspections in London. The City of London says attached decks need an excavation or footing inspection before concrete is poured. Attached decks under 600 mm high still need a permit, while freestanding decks under 600 mm do not. Decks must be at least 1.2 m from the property line.
Always confirm requirements for your project with the City's building division before you dig.
Preventing Heave Under Patios and Walkways
Patios and walkways do not get 1.2 m footings, so they rely on a good base instead. A properly built interlock or flagstone patio:
Removes topsoil and soft material. Organic soil holds water and compresses.
Grades the subgrade to drain. The native clay is shaped so water runs away from the house and out from under the patio.
Uses a thick granular base. Crushed stone such as Granular A is placed and compacted in thin lifts. Municipal specifications commonly call for lifts of around 100 mm compacted to a high standard Proctor density.
Adds edge restraint. Edging stops the pavers from creeping outward.
Manages roof water. Downspouts are directed away from the patio and steps.
A Canadian home inspection magazine put it simply: as moisture in the soil or base increases, its load-bearing capacity decreases. Base depth and drainage are where most residential patios fail. For more detail, read our guide to interlock base prep that survives freeze-thaw. You can order the right base material from our gravel products.
Insulation: Another Way to Stop Frost
Where digging deep is not practical, insulation can keep frost away from a footing. A CMHC design guide for frost-protected shallow foundations notes that 50 mm (2 in) of insulation can serve the same function as 1.2 m (4 ft) of soil, with horizontal insulation "wings" pushing the frost line away from the footing. These systems must be designed properly, so treat them as an engineered solution rather than a DIY fix.
Frost Heave Checklist: Fall Prevention
Extend downspouts so roof water lands well away from steps, patios and deck posts.
Fix low spots where water pools next to the house; see our guide to lot grading in London.
Clear leaves from swales and drains so meltwater can escape.
Check deck posts and fence posts for rot or movement before snow arrives.
Note which steps or pavers move this winter so they can be rebuilt properly in spring.
Avoid piling plowed snow on patios and walkways where melt will soak in.
Fixing Heave That Has Already Happened
Deck posts: Re-set on footings that go down to the required depth, or switch to a design the Code allows for your situation.
Steps: Rebuild on a proper footing if they have more than two risers, or on a well-drained compacted base if they qualify as ground-level steps.
Patios: Lift the pavers, dig out wet or soft base, correct the grading, install new compacted granular base and relay.
Retaining walls: Walls that lean every spring usually need better drainage behind them; see our article on retaining walls in London.
FAQ
How deep is the frost line in London, Ontario?
The Ontario Building Code sets a minimum footing depth of 1.2 m for clay or soils not clearly defined, and footings without heated space beside them must also be below the actual depth of frost penetration. Check with the City of London for your project.
Why do my deck posts rise every winter?
Most likely the footings are too shallow, the soil around them is wet clay, or soil is freezing to the sides of the posts. Deeper footings, better drainage and granular backfill usually solve it.
Do my front steps need deep footings?
Under the Ontario Building Code, concrete steps with one or two risers may be laid at ground level. Steps with more than two risers must meet the full footing depth rules.
Can interlock patios be built to resist frost?
Yes. Removing soft soil, grading for drainage and building a thick, compacted granular base in thin lifts keeps water out of the base so it does not heave.
Is it too late to fix drainage before winter?
Not usually. Late October and November are a good time to extend downspouts, clear swales and correct small grading problems before the ground freezes.
Get Ahead of Frost Damage
SPM Landscape & Excavation has built bases, footings, patios and drainage in London since 2006. We are WSIB covered with $1 million in liability insurance. See our photo gallery, then call (519) 474-0008 or email spmlandscape@gmail.com for a free quote.
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