Why Your Sunroom Floor Slopes: Frost Depth and Shallow Piers
Winnipeg's design frost depth runs 1.8 to 2.4 metres. Most addition piers stop well short of it — and that is the single most common structural fault we find.
The most common structural fault in Winnipeg crawlspaces
If we had to name one defect that accounts for more crawlspace structural work in this city than any other, it would be this: support founded above the frost line.
Winnipeg’s design frost depth is roughly 1.8 to 2.4 metres depending on exposure and soil conditions. A foundation founded below that depth is sitting in ground that does not freeze, so it stays put. A pier founded at 600 millimetres is sitting in ground that freezes solid every winter — and freezing ground expands.
How frost heave actually works
Frost heave is not simply water expanding as it turns to ice. The larger mechanism is ice lensing: as the freezing front moves down through the soil, it draws additional water upward from below through capillary action, and that water freezes in layers. Those lenses can lift substantial loads.
Winnipeg clay is particularly good at this, because fine-grained soils transport water to the freezing front efficiently. Sandy, well-drained soil heaves far less.
So a shallow pier does not just get cold. It gets pushed up, often by a measurable amount, then released in spring when the lenses melt.
Twenty years of a one-centimetre cycle
A single season of movement rarely breaks anything. The damage is cumulative.
Picture a sunroom built in 1992 on six sonotube piers set 700 millimetres deep. Each winter, some piers lift more than others, depending on exposure, snow cover and soil moisture. The structure racks slightly. Each spring it settles back, but never quite to where it started.
By 2026 that sunroom has been through roughly thirty cycles. The floor slopes. The patio door binds every January and works fine in July. The grout along the tile line has cracked and been repaired twice. The junction where the addition meets the house has opened enough that you can feel air moving through it on a windy day.
Homeowners usually describe this as “the addition settling.” It is not settling. Settlement goes one direction. This is cycling, and it will continue until the support reaches stable ground.
Why it shows up in additions specifically
Original house foundations in Winnipeg are generally fine, because they were built with a full basement at proper depth. Additions are where the shortcuts happened:
- Sunrooms and three-season rooms converted for year-round use
- Rear family room extensions
- Enclosed porches and mudrooms
- Decks converted to enclosed space
- Attached garages with crawlspace areas beneath
These were built to add space economically, often without a permit requiring frost-depth footings, and the consequences take a decade or two to become obvious.
How it gets corrected
There are three common approaches, and the right one depends on load, access and soil:
Underpinning the existing piers. Excavating beneath them and extending the bearing down below frost depth. Effective but labour-intensive, and awkward in a tight crawlspace.
Replacement footings and adjustable steel posts. Where the structure allows shoring, new footings sized for the actual load with properly installed adjustable posts. This is the standard fix for interior support.
Helical piles. Steel shafts with helical plates, driven to stable bearing. Their advantage in a crawlspace is significant: compact equipment, minimal excavation, and a verifiable installation torque that correlates with capacity.
What to expect from the repair
Correcting support stops further movement. Recovering the floor to level is a separate question, and the honest answer depends on how long it has been out.
Lifting is done gradually — days to weeks, in small increments — because raising a settled structure quickly cracks drywall, plaster, tile and sometimes plumbing. A floor that has been out of level for thirty years may come back most of the way. It may not come back entirely, and forcing it can do more damage than it repairs. Stabilization plus partial correction is often the right goal.
Do this before you seal anything
If you are planning an encapsulation and the space has moving support, correct the structure first. Footing work involves excavation, shoring and cutting; doing it after a barrier is installed means opening the barrier back up. Sequence matters more than any single product in this trade.
Checking your own house
You do not need instruments for the first pass. Put a ball on the floor of the addition and see whether it rolls. Note whether doors bind in winter and release in summer — seasonal binding is the clearest signal. Look at where the addition meets the main house for a crack that has been filled more than once.
If any of that matches, the question worth answering is how deep the piers go. That is a straightforward thing to establish during an inspection, and it determines everything else.