A deck footing has three jobs: get below the frost line so winter doesn't lift it, spread the load wide enough so summer doesn't sink it, and give the post a solid connection so wind and uplift don't twist it. Miss any one and the whole deck tells on you within a few seasons. Here is how to get it right the first time.
Water expands roughly 9 percent when it freezes. Soil can hold 20 to 40 percent of its volume in water, and when that freezes inside a saturated clay, the ice lenses lift the ground by 75 to 150 mm in a single winter. If the bottom of your footing sits inside that freeze zone, it rides up with the ice. Come April, the ground thaws and the footing settles — but never to exactly where it started. Repeat five winters and the far corner is an inch and a half higher than the ledger, railings are loose, stringers have pulled off the framing, and the sliding door no longer slides.
OBC Sentence 9.15.1.3 handles this — foundation footings must be placed below the depth of frost penetration, or otherwise protected against frost movement. The depth itself isn't published inside Division B; it's set municipally and referenced against Supplementary Standard SB-1 climatic data. Windsor sits on Ontario's shortest frost line at roughly 1,050 mm (42″) because of moderating Lake Erie. Toronto and the GTA come in at 1,200 mm (48″). Ottawa, Kingston, Peterborough, Barrie are on 1,500 mm (60″). Sudbury and the Sault push to 1,800 mm. Thunder Bay and the Far North can require 2,400 mm. The same permit in Thunder Bay costs twice the concrete and twice the Sonotube of the same deck in Toronto.
A Sonotube is a cardboard form — slip it in, plumb it, fill with concrete, pull the tube off once cured. Default for a typical residential deck: cheap, fast, clean. The trade-off is no flared base — the bearing area equals the tube area. Fine on good sand with a modest tributary. Not enough for 300 sq ft on silt.
When the required bearing exceeds any Sonotube size, step up to a poured pier-and-footing. Dig a wider hole, form a square pad (600–900 mm square), pour it, let it tack up, then form a smaller pier rising to grade. Two pours, more labour — but the bearing footprint is whatever you make it.
Bigfoot is the shortcut: a plastic bell the Sonotube plugs into, everything pours as one. Flared base without a second pour. Costs more than a plain tube but saves the form-up.
Helical piers are the engineered alternative — steel shafts with helical plates, driven by a torque head. The helix bears below frost without ever digging a hole. The only practical answer on tight lots with no access, on heritage properties where you can't disturb soil, or on clays where January ground is like concrete. Roughly 2–3× the cost of Sonotubes, but a week of work compresses to a morning.
Each post holds up a rectangle of deck. The rectangle a given post carries is its tributary area — half the joist span on each side times half the beam span on each side. An interior post carries a full rectangle. A corner post carries a quarter of that — it sits at the corner of its rectangle rather than the middle of it. That's why on a long narrow deck the interior beam posts are the sized-up ones, while end posts can run a tube size smaller.
Every deck in OBC Part 9 residential scope uses a combined load of roughly 1.9 kPa live (40 psf) + 0.5 kPa dead (10 psf) — the standard residential floor load under Section 9.4.2. Multiply that by the tributary area in m² and you get the load in kN per post. A 150 sq ft (14 m²) tributary at 2.4 kPa works out to about 34 kN. Divide by the allowable soil pressure and you get the required bearing area. That's the math.
OBC 9.4.2 publishes presumptive allowable bearing pressures for common soils when no geotechnical report is available. Residential-grade day-to-day numbers: dense sand or gravel — 150 kPa, stiff clay — 100 kPa, silt — 75 kPa. Loose sand drops to 50 kPa. Soft clay drops to 50 kPa or less. Organic topsoil and fill have zero presumptive value — dig through them to reach native bearing soil.
Sand drains, clay holds water, silt does neither well. Clay looks stable in summer and turns into a frost-heave monster in winter — a shallow footing on saturated clay will absolutely lift. Silt is worse the other way: it doesn't heave much but it consolidates under sustained load, so undersized footings on silt sink over time.
A soils report is worth the $1,500–$3,000 in three situations. First, whenever the lot is fill — any subdivision built in the last 40 years has compacted fill across at least the top metre. Second, whenever the presumptive number forces you into a comically big footing — a report bumping silt to 125 kPa can save half the concrete. Third, whenever the municipality requires it — some Ottawa-area lots on Champlain Sea clay will not issue a permit without geotechnical sign-off.
Footings too shallow. Dug 900 mm in the GTA because that's what the last deck called for, or because the dig hit rock and it was called "close enough." Two winters later the homeowner is on the phone. Dig to code every time.
Footings too narrow. A 200 mm tube on silt under a big tributary settles unevenly and the deck racks. Size every footing to its tributary.
Too close to the property line. Setbacks require a minimum distance for any structural element. A Sonotube half a metre off the fence is a re-do waiting to happen.
No uplift anchor. Wind on a raised deck generates lift. OBC Sentence 9.23.6.2 and the provisions of Section 9.23 require connections between footing, post, beam, joists, and ledger that resist uplift as well as gravity. A post on a Sonotube with one lag bolt is not code.
No post-to-footing connector. The adjustable post saddle — galvanized bracket, threaded rod bedded in wet concrete — isn't optional. It keeps the base up off the concrete so water drains, locks the post against twist, and gives the inspector something to look at.
The standard deck ties to the house through a ledger bolted through the rim joist. Done right with proper flashing and lag screws sized per OBC 9.23.2.3, it's bulletproof. Done wrong, it is the single most common cause of catastrophic deck collapse in North America.
Three situations where freestanding footings are the right call — a beam line on its own posts just inboard of the wall, no ledger at all. First, narrow urban lots where the house wall is a demising wall with no floor system behind it. Second, heritage properties with old brick or stone veneer never designed to carry a 300 kg ledger. Third, additions off old masonry — a 1920s two-brick wall with no cavity isn't something you hang a new deck off. In all three, the right answer is a row of footings 300–400 mm off the wall, a beam on that line, joists framed between it and the outer beam, no penetration of the existing wall. Costs more up front, sleeps better, and when the inspector asks how you attached to the 100-year-old wall, the answer is "we didn't."