Two failure points decide whether a deck survives many winters: the connection to the house, where meltwater can get behind the ledger, and the footings, where freezing ground can lift the structure. Both are addressed before the surface boards ever go down.
The ledger connection
The ledger board is the horizontal member fastened to the house frame that carries the deck's floor joists. Where it meets the wall, water from rain, and from snow melting against the house, will find any gap. Guidance for Part 9 deck construction requires the ledger to be flashed so precipitation cannot get in behind it, and any fastener holes that pass through the flashing or the weather-resistant membrane are sealed.
Flashing is layered in shingle fashion: the upper edge tucks behind the wall's water-resistive barrier so water runs down over the flashing and onto the deck rather than into the wall cavity. A drip edge at the bottom of the flashing carries that water clear of the ledger.
Fastener and flashing compatibility
Use corrosion-resistant connectors throughout, and do not place aluminum in direct contact with lumber treated with copper-based preservatives such as ACQ or copper azole. Galvanized steel with a heavy zinc coating, stainless steel, copper, or a UV-resistant plastic recommended for the use are the compatible flashing choices.
Footings below the frost line
When soil freezes it expands, and that expansion pushes upward on anything resting in it. A footing placed above the frost line can heave through the winter and settle in spring, tearing an attached deck away from its ledger. The standard answer is to carry non-floating footings down below the local frost line.
Part 9 deck guidance gives a common minimum depth of 1.2 m below grade to the underside of the footing where the foundation is not bearing on sound bedrock. Footings must sit on undisturbed soil, cleared of topsoil, loose material and standing water. Where a footing can bear directly on sound bedrock that is not susceptible to frost movement, it may be supported at that depth instead.
Frost depth is local
The required depth tracks how deep the ground freezes, which differs sharply across the country. The values below illustrate that range; the figure for your site comes from your municipality.
| City | Approx. required depth |
|---|---|
| Victoria | About 18 in (0.46 m) |
| Calgary | About 48 in (1.2 m) |
| Ottawa | About 60 in (1.5 m) |
| Winnipeg | About 72 in (1.8 m) |
| Saskatoon | About 96 in (2.4 m) |
Floating decks are the exception
A deck classified as floating, low to the ground, not attached to the house and not supporting a roof, is permitted to be built on grade without the frost-depth excavation. Pier blocks suit a freestanding deck, but using them on a deck attached to a house tends to let the deck rise and fall against the fixed ledger as the ground heaves.
Detailing that lasts
- Set footings to bear on undisturbed soil, free of organic material and standing water.
- Integrate ledger flashing behind the wall's weather-resistive barrier and seal penetrations.
- Provide a drainage path so moisture trapped behind the ledger can escape rather than sit against the framing.
- Keep all connectors corrosion-resistant and compatible with the treated lumber in contact with them.
Frost-depth values are illustrative. Confirm the required footing depth and flashing details with your local building authority before construction.
References
- Town of Carleton Place, Code Guideline – Part 9 Decks (PDF).
- Municipality of Port Hope, Deck Design Guide (PDF).
- National Research Council Canada, National Building Code of Canada 2020.