What frost actually does to soil
Water expands as it freezes. In soil that holds moisture, that expansion lifts whatever rests on it or is caught in it. This is frost heave, and it is the reason a deck that was perfectly level in October can be out of true in April.
The movement is never uniform. One corner of the yard drains better, another stays soaked, a third sits in the shade of the house all winter. Each support rises by a different amount, and that difference is what twists the structure, makes stairs uneven, and pulls the deck away from the wall.
- Clay soil, common on the South Shore, holds water and moves a lot.
- Well-drained soil moves less, but still moves.
- The movement repeats and accumulates every winter.
- A support set deep enough is simply not affected.
Getting below the frost line
The solution fits in one sentence: the footing has to bear below the depth at which the soil freezes. In the greater Montreal area that depth commonly sits around 1.2 to 1.5 metres, but the number that matters is the one your municipality requires. It is something to confirm at permit stage, not to assume.
A structure supported below that line rests on soil whose temperature stays stable. Frost can lift the surface around it; the footing itself does not move. That is the difference between a deck that holds for twenty years and one that needs relevelling every three.
Helical piles, concrete tubes, or surface blocks
Three approaches turn up regularly in Quebec backyards. They are not equivalent, and the opening price often explains why the least durable one shows up in the lowest quotes.
| Type | How it works | What to know |
|---|---|---|
| Helical piles | Screwed deep below the frost line with little excavation | Fast and clean to install; capacity depends on soil and installer |
| Concrete tubes (sonotube) | Hole dug below the frost line, tube set and concrete poured on site | Proven and durable; needs excavation and curing time |
| Surface blocks | Blocks set on the ground, with no deep anchoring | Move with frost; only for small freestanding structures where local rules allow |
The right choice depends on the soil, the deck height, yard access, and what your municipality allows. A raised deck, a hot tub, or a structure attached to the house is not handled like a low platform at the back of the lot.
Why surface blocks eventually disappoint
A deck on surface blocks goes up fast and costs less. The problem does not show in year one: it shows in the third or fourth winter, once each support has moved a little, and not in the same direction.
The signs are always the same: a patio door that rubs, a bottom step that is no longer the same height as the others, a railing that leans, a gap opening between the deck and the house wall.
There are cases where blocks are acceptable: a small, low, freestanding structure detached from the house, where local rules permit it. As soon as the structure is attached to the building, raised, or carrying a significant load, depth stops being optional.
Drainage is part of the foundation
A deep footing solves frost, not water. If the soil around the deck stays saturated, the problem simply moves: wood that stays damp, soil that settles, water collecting against the house foundation.
That is why grading, downspouts, and the flow around the footings are planned at the same time as the foundation, not afterwards once the landscaping is already in.
- Maintain a slope that moves water away from the house.
- Aim downspouts away from footings and stairs.
- Avoid areas where water stands after heavy rain.
- Keep the underside ventilated so the structure can dry.
Recognising a deck that has moved
Spring is the best time to look, but autumn is the best time to record the starting point. Take a few photos and check these points every year.
- A gap opening between the deck and the house wall.
- Stair risers that are no longer equal in height.
- A surface no longer level from one corner to another.
- Railing posts with play where there was none before.
- Water now collecting in one particular spot.
A deck that has moved can be corrected, but the correction depends entirely on what is underneath it. That is the first thing to establish before talking about repair.
Questions to ask before you sign
Footings are the part of your deck you will never see again once the boards are down. That is exactly why they deserve specific questions before work starts, and an answer written into the quote.
- What type of footing is planned, and at what depth?
- How many footings, and at what spacing?
- Does that depth match what the municipality requires?
- Is a permit needed, and who handles it?
- How will the structure attach to the house, or will it be freestanding?
- What is covered if the structure moves, and for how long?
Two quotes can differ by thousands of dollars on this section alone. Comparing prices without comparing footings means comparing two decks that will not last the same length of time.
Planning a deck on the South Shore?
Send your approximate dimensions, photos, city, and preferred material. We will help you compare realistic options before construction starts.