OBC Deck Footing Depth Calculator

Every figure on this page verified against the Ontario Building Code 2024 (amended January 16, 2025) by a Red Seal Carpenter (Ontario CofQ) — last verified July 6, 2026.

Deck Footings That Stay Put for 30 Years

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.

Why do deck footings have to go below the frost line in Ontario?

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.

According to OBC Sentence 9.12.2.2.(1) and Table 9.12.2.2, a foundation that contains no heated space — deck piers included — must bear below the depth of frost penetration. 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.

The code carves out one big exception. According to OBC Sentence 9.12.2.2.(7), a deck that is not more than one storey, not more than 55 m² (592 sq ft) in area, with the underside of the joists not more than 600 mm (24″) above finished ground, not supporting a roof, and not attached to another structure doesn't need frost-depth foundations at all — a floating ground-level deck on surface footings is code. Seasonal movement lifts the whole platform evenly instead of racking it. The moment you bolt a ledger to the house or the deck climbs above 600 mm, the exemption is gone and every pier digs to frost depth.

Sonotube, Bigfoot, poured pad or helical pier — which footing type?

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.

How do I calculate tributary area and post load?

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.

According to OBC Sentence 9.4.2.3.(1), a deck serving a single dwelling unit is designed for the greater of the specified roof snow load or 1.9 kPa (40 psf). Add a 0.5 kPa (10 psf) dead-load allowance for the framing and decking itself and you get the 2.4 kPa this tool works with. Multiply by the tributary area in m² and you get the load in kN per post; divide by the allowable soil pressure and you get the required bearing area. A 150 sq ft (13.9 m²) tributary at 2.4 kPa works out to about 33 kN — on dense sand at 150 kPa that needs 0.22 m² of bearing, a 535 mm diameter. That's not the tool being cautious; it's why real decks use more posts with smaller tributaries. In the heaviest snow belts, check whether the specified roof snow load beats 1.9 kPa — where it does, snow governs.

What soil bearing capacity does the Ontario Building Code allow?

According to Table 9.4.4.1 of the OBC 2024, the presumptive maximum allowable bearing pressures where no geotechnical report exists are: dense or compact sand or gravel — 150 kPa, dense or compact silt — 100 kPa, stiff clay — 150 kPa, firm clay — 75 kPa, loose sand or gravel — 50 kPa, soft clay — 40 kPa. If you can't prove the stiffer condition with a test hole, take the lower value. Organic topsoil and fill have zero presumptive value — footings must rest on undisturbed soil, rock or compacted granular fill (9.15.3.2), and anything on filled ground, peat or sensitive clay needs engineered design under Section 4.2 (9.15.1.1.(3)).

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 goes the other way: it doesn't heave as violently but it consolidates under sustained load, so undersized footings on soft silt sink over time — and only dense or compact silt gets a value in the table at all.

One more trap: according to OBC Sentence 9.4.4.3.(1), where a footing bears on gravel, sand or silt and the water table sits within one footing-width of the bearing surface, the allowable bearing pressure drops to 50 per cent of the table value. A hole that fills with water in spring is telling you half your bearing capacity just disappeared — size the footing for it.

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.

What are the most common deck footing mistakes?

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. According to OBC Sentence 9.23.6.2.(1), exterior posts must be anchored to resist uplift and lateral movement. Where the underside of the joists is within 600 mm of grade, Sentence (2) lets you anchor either the posts or the joists/beams directly, and Sentence (3) waives anchorage only for those same low platforms (joists within 600 mm of grade) when they are also freestanding — up to 55 m², one storey, no roof, not attached to another structure. A typical raised, house-attached deck gets none of those outs — a post sitting loose on a Sonotube is not code.

No post-to-footing connector. The adjustable post saddle — galvanized bracket, threaded rod bedded in wet concrete — is how you meet 9.23.6.2 in practice. It keeps the base up off the concrete so water drains, locks the post against twist, and gives the inspector something to look at.

When should a deck be freestanding instead of ledger-attached?

The standard deck ties to the house through a ledger bolted through the rim joist. Done right — flashed, through-bolted or structurally screwed into solid framing — it's bulletproof. Done wrong, it is the single most common cause of catastrophic deck collapse in North America. Part 9 has no prescriptive deck-ledger fastening schedule, which is exactly why the ledger detail goes on the permit drawing — our Deck Ledger Attachment tool walks through the accepted details.

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."

Keep going

Footings are step one. Size the rest of the frame with the Beam Span Calculator and the Joist & Rafter Span Calculator, look up your municipality's frost line in the Ontario Frost Depth lookup, and check guard requirements with the Guard Height tool. The full set lives on the Ontario Building Code tools hub.

If you're pricing and permitting decks for a living, ToolFluency for Ontario contractors ties these code tools into quoting, timesheets, and invoicing for your crew.