How deep do deck footings have to go?

What frost actually does

Water in soil expands when it freezes, and the frozen layer grips whatever is buried in it and lifts. On a deck this is not uniform: the sunny south post barely moves and the shaded north post comes up 2 in. The deck racks, the ledger levers on the house, the stair risers go out of tolerance, and by spring it may or may not go back.

A footing below the frost line is below the layer that freezes, so the grip has nothing to work on. In Edmonton that is typically at least 1.2 m (47 3/16 in), but the number depends on soil and exposure, which is why the guide says typical rather than required.

Below the frost line and on undisturbed soil

Both conditions, not one. The footing has to reach soil that has not been excavated before, which on a new subdivision lot means going past the backfill around the basement. Backfill compacts for years, and a footing bearing on it settles whatever the frost does.

This is the most common reason a deck near a new house sags on the house side within three years.

The four options, and what each one costs you

Concrete pier with a pad

A pad footing below frost level and below any previously disturbed soil, carrying an integral concrete column of at least 190 mm (8 in) square that comes up at least 150 mm (6 in) above finished ground. Edmonton notes these are not commonly used today, but it is the textbook answer and it does not move.

Pier blocks on pads

The practical one, and Edmonton says so directly: pier blocks on concrete pads over well drained, undisturbed or compacted coarse grained soil free of organic material have proved generally adequate in the city for attached and freestanding decks near the ground. A typical pad is 2 ft by 2 ft and at least 4 in thick.

The condition attached to it is the important part. There has to be tolerance for seasonal movement, and a way to reach the posts and re-level them. That means either a 2 ft by 2 ft access way under the deck, where the joist bottoms are at least 24 in off the ground, or some removable decking. Plan it before you screw down the boards, because afterwards it is a saw and a Saturday.

Screw piles

They work, they go in fast, and they are not defined in the Alberta code, so they need an engineer design. On a deck more than 1.8 m (5 ft 10 13/16 in) high that design is submitted with the permit application. Most pile companies supply the stamped design as part of the job, which is the question to ask before you book them.

Concrete poured in a tube

The familiar cardboard tube pier. It is a concrete pier by another name, so the same rules apply: reach undisturbed soil below frost, get the load spread with a footing at the bottom or a bell, and keep the wood post off the concrete.

Where the post meets the ground

• Framing lumber closer than 6 in to the ground has to be preservative treated, including at every cut end.
• Separate wood from concrete that is in contact with the ground, using 0.05 mm (2 mil) polyethylene, roll roofing, a metal post base, a saddle anchor or a neoprene spacer. Concrete wicks water.
• Centre the post on the pile, pier or pad so the load actually travels down it.
• Fasten the beam to the post so the two cannot move sideways relative to each other.
• Ground anchorage is not required by the code for an uncovered single level deck up to 55 square metres (592 square feet), and Edmonton describes it as highly recommended anyway.

If a sunroom is in your future

A deck that might be enclosed later should get a permanent foundation now. Walls and a roof mean engineering whatever the deck height, and the cheap moment to put that foundation in is before the deck exists. Submit the design with the deck permit, and keep your own copy: the City is not obliged to hold drawings forever.

Before you dig

Call Alberta One Call and have the utilities marked. It is free. A deck may sit over a gas line so long as the line is not put at risk and the meter stays accessible, and the City asks for a deck over a gas line to be vented on at least two sides. That is a conversation to have with the gas utility before framing, not after.